Large Aperture solar telescope using a modded Lunt etalon?

Post by **Merlin66** » Wed Jan 18, 2012 7:15 am

I haven't heard of anyone trying it as yet, but it seems from looking at the drawings of the Lunt internal etalon assembly that it's close to the PST etalon design. i.e. it has a Barlow lens up front and a re-imaging positive lens at the rear.
The 60mm version is 500mm focal length. If Lunt (??) could confirm the focal lengths of the lenses used it would be an easy job to build a much larger modded telescope a la the current PST mods.
(If someone has a 60mm, can they measure the distance from the centre of the objective to the front of the etalon body and also the distance from the rear of the etalon body to the front of the blocking filter diagonal - this will allow me to calculate the focal lengths and look at some options..)

Bill E. · Post by **Bill E.** » Wed Jan 18, 2012 9:47 pm

Interesting topic. I wish I could read the Lunt info , Vanter posted. It is to small to read. Where did you find them?

Post by **Merlin66** » Thu Jan 19, 2012 8:43 am

Vanter,
Thanks for that!
Bill, the detail in the parts list is illegible, but the overview and dimensional drawings can be enlarged.
This data represents a tilt adjusted etalon 60mm f8.6 system.

Working from the 60mm objective @ f8.6
The distance to the barlow element is about 150mm, this then means that the barlow is a -350mm focal length.
The etalon appears to be about 50mm diameter (much larger than the PST), and the lenses around 42mm diameter.
The rear element to bring back the focus at f8.6 needs to be 42mm diameter and focal length 350mm. (compared with the 200mm of the PST)
A couple of questions/ comments:
1. Who is Astro Technik??
2. What's the "heat reflecting" coating on the front of the barlow??
3. There's no ERF coatings on the objective - full light/ energy in.
4. The UV-IR protection is behind the etalon (like the PST)
5. The finesse/ bandwidth of the etalon is similar to the PST.
6. That being the case the blocking filter from this unit would work well with the PST Stage II mod.
7. If Jean-Pierre is correct in his measurement of the objective - ie 60mm f5, then the front barlow would be only -150mm focal length. This is feasible.
Summary:
No technical issues in using the Lunt etalon assembly in a large aperture mod. The critical "majic" distance seems to be around 350mm (or -150mm if Jean-Pierre is correct)

Any further measurements or additional data would be useful to confirm the above.

swisswalter · Post by **swisswalter** » Thu Jan 19, 2012 4:50 pm

Hi Ken

great infos, good work out and summary. thank you very much

Photonist · Post by **Photonist** » Fri Jan 20, 2012 2:22 pm

Hi Ken

Here is a French thread on this topic with a good drawing with many measurements
http://www.astrosurf.com/ubb/Forum2/HTML/034966.html

I hope it helps.

I collected a bunch of posts on this Lunt mod from French and German sites last summer, but I got a bit pessimistic and went for the folded two telescope idea in the style of Rogerio and resembling the marktownley "crazy" mod. Anyhow I will try to find the posts again.

Regards,
Viljo

Post by **Merlin66** » Fri Jan 20, 2012 3:30 pm

Viljo,
Any particular reason you didn't go ahead??

I've read through the french stuff and the main area of difference is in the measurements of the front objective focal length...
They have the objective as being a 64mm clear, 263mm focal length which gives f4.1 - This doesn't seem to match up with the Lunt docs(!????)Can anyone re-measure to verify?
The etalon and refocusing lens make sense and give a 280mm back focus distance which sounds reasonable.
Unfortunately this gives a final focal length of only 444mm and an f ratio of 6.9 for the final assembly - again at odds with the Lunt docs.
Summary:
The front element could be -100mm barlow, functioning in an f4 cone
Back focal length seems to be definately +/- 444mm
Any further input/ data vey welcome!

Photonist · Post by **Photonist** » Mon Jan 23, 2012 8:12 am

Ken,

The reason I went for the folded two telescope design using the DS front filter only was that Rogerio had proven it works with the finest images seen and he had enough information and details for me to do it. In the French thread that I referred to, there were three attempts where the resulting Solar images suffered from low contrast, halos etc. The same problem on a few German threads. There seemed to be a fundamental flaw in the design (perhaps a focal ratio mismatch, wrong specs or …).

The folded system works well, but it is cumbersome to set up and align the components properly without permanent installation. The straight through design would be nice, but I am afraid it becomes quite long.

However, now I believe the design problems can be finally solved as a result of your new project. Thanks for starting it. I am eager to build the mod when a promising solution is found.

I measured roughly the focal length of the Lunt 60 objective by projecting a light bulb on a screen only 7.5 meters away (no Sun option here ). The rear surface of the objective was 275.5 mm from the screen. Adding half of the lens thickness, 4.5 mm makes 280 mm, a nice number but I doubt it is exact (the short distance, principal point and mathematics stuff).

I will post more measurements soon.

Best regards,
Viljo

Post by **Merlin66** » Mon Jan 23, 2012 8:25 am

Viljo,
Thanks for that!
The focal length of the lens, based on a projected sun image, I think would be slightly less than the 280mm.
Let me play with some numbers. ( If any one has a better focal length measurement let me know...)
The positioning of the front barlow element is critical for performance, within 4-5mm max. Also the focal ratio of the donor scope.....
It could be that the European "trials" were of limited success due to incorrect spacing/focal ratio.
I know from my own experience and the feedback from many successful PST mods, that if the spacings are correct the performance is excellent.

Photonist · Post by **Photonist** » Mon Jan 23, 2012 8:58 am

Ken,

I made calculations on the focal length (which I really should not) using the numbers available and guessing the principal point being 5.7 mm in front of the rear surface of the lens (9 mm thick) and got a FL about 270.7 mm. Is there any sense.

Viljo

Post by **Merlin66** » Mon Jan 23, 2012 9:05 am

Viljo,
That makes sense!
OK for the purposes of checking the spacings let's agree on 275mm focal length.
(This means that the 64mm clear objective is f4.3 - not the f8.6 quoted by Lunt)

Photonist · Post by **Photonist** » Mon Jan 23, 2012 9:36 am

Ken,

I made a few quick measurements of my Lunt 60 telescope (unfortunately not complete and with no guarantee).

There seem to be some consensus of one or two measurement, the objective clear aperture diameter 64 mm (or 64.08 mm) and the clear aperture of the collimating lens and etalon, 34 mm (some say 35 mm).

There seems to be a tilted red ERF in front of the collimating lens. The diameter of the ERF is 40 mm and it is tilted 1.5 mm from edge to edge. A collimating lens cannot be tilted, I think. Anyhow, I don’t see an ERF in the Lunt diagram.

For now, I did not remove the ERF in front of the collimating lens, so the thickness of the collimating lens must be guessed. The distance from the ERF front center to the rear of the collimating lens is 16 mm, so there is plenty of room for an ERF and a collimating lens.

The distance from the objective front surface to the rear surface of the collimating lens is 140 mm. The objective lens is 9 mm thick. When half of the objective thickness (4,5 mm) and the collimating lens full thickness (guessing 3.5 -5,5 mm) is subtracted from 140 mm, the distance from the center of the objective to the front surface of the collimating lens is about 130-132 mm. I am not sure how much knowing the principal point of the objective could affect this.

The distance from the rear surface of the correcting lens to the rear end of the Lunt standard focuser body is 102 mm. The Lunt diagonal is 90 mm, the nosepiece collar 3 mm and the front plate to CCD of the DMK31 is 12 mm?, in total 105 mm.

Now I can’t exactly remember how far out the focuser and/or the diagonal tube is when focused and I am not able to check it on the Sun before April here up in the North (N60.1). But I think the distance from the focuser body to the front plate of the diagonal is about 55 mm when focused for DMK31. Some lucky one under Sunny skies could perhaps check this out?

The measurement from the center of the objective to the rear end of the focuser body is 301,5 mm.

If the focuser out distance is correct, then the measurement from the center of the objective to the CCD in focus would be 301.5+55+105= 461,5 mm. The distance from the corrector lens to the CCD would be 102+55+105=262 mm.

I hope even one of my numbers would be useful at this point. Once again, my results differ a bit from the specifications and also the previous measurements. I just wonder why.

Viljo

Post by **Merlin66** » Mon Jan 23, 2012 9:44 am

Viljo,
The critical measurements are:
1. The focal length and focal ratio of the objective
2. The distance from the objective to the front barlow (collimating element)

The rest are nice to know but not as important, when considering the mod.
Let me see what I can make of the data so far.
Appreciate your input.
ken

swisswalter · Post by **swisswalter** » Mon Jan 23, 2012 9:32 pm

Gruezi all

thank you very much , very interesting discussion, I follow that thread with great interest

Photonist · Post by **Photonist** » Wed Jan 25, 2012 5:19 pm

Ken,

I found this cut drawing (new for me) that seem to be closer to reality than those mentioned earlier. The ERF and collimating lens are now visible and the their tube is longer. The collimating lens seemed to be thicker than in my guess so I scaled the image and measured the center thickness to be about 7 mm. If the drawing is correct, then the distance from the collimating lens front to the objective center would be around 128.5mm.
http://www.apm-telescopes.net/de/Sonnen ... uszug-2692

Viljo

swisswalter · Post by **swisswalter** » Wed Jan 25, 2012 5:22 pm

Ah viljo

new facts, thank you very much

Post by **Merlin66** » Wed Jan 25, 2012 6:06 pm

Viljo,
Unfortunately I can't get the link to work for me....
can you tray again??

Photonist · Post by **Photonist** » Wed Jan 25, 2012 7:00 pm

Ken,

Unbelievable, but they changed totally the layout and also the link sometime today. Here is the new link. Click on the second icon from left.
http://www.apm-telescopes.net/de/Sonnen ... uszug-2692

http://tinyurl.com/7womhpl

Viljo

Photonist · Post by **Photonist** » Wed Jan 25, 2012 7:10 pm

Now the new link does not work for me by klicking (too long?) but it works if I paint it, copy and paste into the address bar.

Viljo

Now the above links should work by clicking, sorry.

Viljo

Photonist · Post by **Photonist** » Thu Jan 26, 2012 2:16 pm

Ken and all,

I finally found one of the German forum threads I mentioned earlier: http://www.astrotreff.de/topic.asp?TOPIC_ID=111084

It is a building report with many images of the mod using Lunt 60 and a 127/1200 mm refractor, but unfortunately very little technical information. The guy first cut the tube by 70 mm but then made a much longer extension tube. Hard to tell how much the original tube was lengthened. I have emailed the guy about the technical details and measurements, so let’s hope he answers.

Here are a few links to Solar images taken with the mod:
http://www.astrotreff.de/topic.asp?ARCH ... _ID=111078
and here a few images and his statement that the mod works:
http://forum.astronomie.de/phpapps/ubbt ... e_20110102
and 3 images here http://www.astrotreff.de/topic.asp?TOPIC_ID=112366
and here some images which have a very uneven field http://www.astrotreff.de/topic.asp?ARCH ... _ID=111208

Viljo

Post by **Merlin66** » Thu Jan 26, 2012 3:22 pm

Hmmm
Lack of detail...
It really doesn't make sense to add such an extension...

Photonist · Post by **Photonist** » Thu Jan 26, 2012 5:08 pm

Ken,

Here is an interesting link to a German thread where the guy gives a simple idea for the Lunt mod.
http://213.193.123.178/topic.asp?TOPIC_ ... hichpage=1

See the text just under the image of a mod using threaded rods and the Lunt front filter.

The idea sounds a bit like the magic 200 mm you produced for the PST mod. But I am skeptical (until proved) it would be a general one size fits all type of number.

He says: Unscrew the Lunt objective with the white tube and measure the focal point. It is 70 mm behind the white tube ( my projection method gave a focal point 77.5 mm behind the white tube). Now just install the Lunt without its objective so that the black body shoulder (the point where the white tube connects to the body) is 70 mm (or my 77.5 mm) in front of the focal point of the new big telescope.

I hope I understood his idea correctly.

Note that 2 mm of the white tube goes into the black body, so my measurement 77.5 is to the white tube end and 79.5 mm to front of the body shoulder. So again a difference in measurements, now 7 mm.

The idea sounds too good to be true. What do you think?

I have emailed the guy for more details. However there are some language problems. I understand some German if I read a chapter five times (Google translation does not help much here), but writing is more difficult (40 years after school). Do you write German? You could formulate the correct questions better, if you think there is any potential in his idea.

See also the link just under the mod image about layout for Ha Solar telecentric systems, OSLO software and even a PST simulation. Oh, how I regret I was not serious enough about the German and also French etc. classes in school and did not practice them even later. Mastering those languages would open a tremendous amount of knowledge useful in the astronomy hobby, especially modding.
http://www.astrotreff.de/topic.asp?TOPI ... hichpage=1

Viljo

Post by **Merlin66** » Thu Jan 26, 2012 5:26 pm

Viljo,
Thanks for the additional info. Unfortunately my German is even worse then my French, but google translate helps a lot.
In none of the example is there a demonstrated understanding of the barlow/ re-imaging lenses used on either the Lunt or the PST. The designs may work (an interesting double stack Lunt/PST!)but more by luck than judgement.
Using a PDT/Lunt etalon in a faster beam than the original design objective is just asking for problems.
I'm 100% sure when we get to the bottom of the two key questions - see posting above, then we can optimise a design solution which would certainly perform as well as any of the existing PST mods.

Photonist · Post by **Photonist** » Thu Jan 26, 2012 8:49 pm

Ken,

I measured the Lunt objective radius (126 mm) but it does not help much without knowing the lens glass material or lens refraction index. Using N-BK7 material gives EFL 243.8 mm. Correcting the radius with 1 mm changes the EFL only around 2 mm, different materials changes the result very much. Fused Silica happens to give 275.3 mm, close to the previous projection method (270.8 mm and your estimate 275 mm). Well, fun to play but hardly very reliable.

What material (or lens index) could the Lunt chromat lens probable be?

What would be a more reliable and exact method to measure the FL in the kitchen table lab? Could the Ronchi screen perhaps be used in this case?

Viljo

Post by **Merlin66** » Fri Jan 27, 2012 6:15 am

Viljo,
Under the circumstances, I believe the 275mm is a good number to use.
I'll sketch up my thoughts and upload later today for further discussion.

Post by **Merlin66** » Fri Jan 27, 2012 8:27 am

Viljo,
Can you measure the thread size/pitch of the tube fitment to the etalon assembly/ rear body?
Is there any possible interference between the tilt ajusting screw and the exterior surface of the etalon block ( I'm considering, like the example in Germany, that it may be easier to mount the Lunt etalon assembly by clamping/ tight fit onto the outside of the etalon block - the part which usually sits inside the tube - if that makes sense. The sketches I'm preparing will make things clearer)
Ken

Photonist · Post by **Photonist** » Fri Jan 27, 2012 11:11 am

Ken,

The white tube connects to the black body on threads with the following measurements: Diameter is 86.45 mm on the outside of the thread tops and the pitch is 1 mm. (However, I’m no thread standard expert). The threads are short, only 5 mm and the white tube is sitting sturdy only because the tube goes 2 mm into a slot in the black body.

When the white tube is removed, there are no parts visible belonging to the tilt wheel system (if that is what you meant). I have been in high fever a few days, so my head is even more fuzzy than normal.

The diameter of the black tube (that holds the ERF and collimating lens in their smaller tube) is 83.30 mm and its length to the white tube threads is 53 mm. The etalon module itself is revealed only when this tube with the ERF is unscrewed.

Did you plan to use the threads used by the white tube (like the German guy) or only clamp on the black tube holding the ERF and collimation lens?

Do you plan to replace the original ERF with a larger one? The German guy seemed to keep the original and added still a large ERF in front of the big scope.

I dreamt of the simple idea to only remove the objective cell and somehow clamp the white tube inside the larger telescope tube. Of course it depends on the new magic number you produce (I can’t wait to see the number).

Viljo

Post by **Merlin66** » Fri Jan 27, 2012 12:57 pm

Viljo,
Thanks for the continued support and helpful data.
Last question for the day...
Looking at the 83mm diameter "shell" around the etalon assembly, do you think it would make a secure fixing point? i.e. if the donor OTA was cut back (say) and a spacer used to hold the Lunt on this surface -it would be OK??
I'm finding around 1-2mm difference in the consolidated data - less than 1% - looking good.
I'll upload soon.
Ken

Photonist · Post by **Photonist** » Fri Jan 27, 2012 1:35 pm

Ken,

No problem with more questions, let them come. In my current condition, I’m not able to shovel snow or anything so I just lay on the couch, make measurements and take a nap in between.

The 83 mm shell is quite thin and light, only 1.6 mm. It threads on 13 mm long threads and when tightened in place it feels quite sturdy. I would certainly use it (or the white tube) at least in my prototype instead of making an expensive threaded adapter.

Viljo

Photonist · Post by **Photonist** » Fri Jan 27, 2012 2:07 pm

Ken,

Just for the sake of completeness the following info:.

I hope I’m not stressing you or distracting you from the Lunt mod calculations with all my links and bits and pieces. As a complete amateur I’m not always sure where the critical key information may be hiding.

The guy who helped (gave the measurement idea) to build the German Lunt mod answered my email a while ago (in German but I think I got the idea). He says: measure from the Lunt body shoulder (where it meets the white tube) to the Lunt focal point which the modder measured to be 275 mm. Then just install the Lunt body so the shoulder is 275 mm inside of the focal plane of the big scope. Actually, all the information was already in the Image 12 of the building report but it is hard to see clearly.

Simple, but a bit confusing when you remember the long extension tube seen in the building report. After all, the guy had a nice turned and threaded adapter which should have been enough. However, again this is quite different from the previous “magic” number (75 mm) I reported earlier.

Again the 275 mm is a bit more than mine but I was quite unsure of the correct focuser out distance when focused.

Glad to see you find the consolidated data to be quite consistent and within 1%. I have been a bit nervous about making measurement or other errors in my fever delirium.

Viljo

Post by **Merlin66** » Fri Jan 27, 2012 3:06 pm

Viljo,
Rest easy...all will be revealed.....
The stuff behind the etalon..the bodywork, focuser, blocking filter etc etc is of no consequence to the mod. The critical "key" is finding the focal ratio of the objective and the position of the front barlow element from the prime focus. Almost there....

Post by **marktownley** » Fri Jan 27, 2012 9:34 pm

this will be interesting to see how Lunt mods compare to PST mods...

Thanks for your hard work chaps, keep it up

Post by **Merlin66** » Sat Jan 28, 2012 5:24 am

Hmmm,
Doesn't work exactly like that...
without going into all the details of solar image sizes and aperture stops, an external etalon would need a barlow before the etalon placed in the optical path to generate a parallel collimated beam which would then be directed to and through the etalon.
At the rear of the etalon you need a refocusing lens to bring the beam from the etalon to a focus.
The size of the external etalon body and access to the tilt tuning needs to be considered.

Post by **Merlin66** » Sat Jan 28, 2012 6:34 am

Rusty,
You're overlooking the point that the etalon is up front.
The light from the sun is basically already collimated and parallel when it hits the front of the etalon, and the telescope objective behind the etalon refocuses the image through the blocking filter to the eyepiece/CCD.
If you relocate the etalon behind an objective/ERF then you need to also supply some means of providing a collimated parallel beam, hence the barlow.
make any sense?

Post by **marktownley** » Sat Jan 28, 2012 9:12 am

Yup, Ken is right. Light from the sun reaches us an effective f-ratio of f108.

Post by **Merlin66** » Mon Jan 30, 2012 9:21 am

I can't seem to upload images from this Win7 laptop...when I get to the office machine I'll upload a couple of sketches to show the various measurements I have so far.
As I've said previously the "key" to a successful Mod is knowing the effective focal length of the collimating barlow and the f ratio of the original objective.

Summary:
For the Lunt 60 system, we seem to have an objective which is 275mm focal length. This gives a f ratio of approx f4.5. Any donor OTA should therefore be f4.5 or greater.

The size of the solar disk at the etalon = 24.5+ 2.75 = 27.53mm

Measured from the "flange" - where the original white tube fits the etalon/ focuser body, the barlow sits close to 54.7mm in front. This gives an effective focal length of -111.5mm.
It also means that the "majic" number - based on this flange position - would be 56.8mm; call it 57mm. i.e. the flange has to sit 57mm INSIDE the prime focus of the donor OTA.

Issues/ problems
The "body" of the etalon assembly is 83mm diameter.
The position of the "flange" relative to the prime focus really means a spacer of about 150mm is required.( based on my measurements of a couple of refractors - the length of the OTA seems to be about 200mm shorter than the focal length.)
Finding/ cutting and fixing such an "adaptor tube" may be problematic.
This is not as easy as mounting a PST etalon in a 2" nosepiece!

Treat these numbers with caution until further measured data becomes available, but I'm confident that they can be used to assess the "degree of difficulty" in using a Lunt etalon module for a Modded Ha scope.
Any questions/ comments - just let me know.

Photonist · Post by **Photonist** » Mon Jan 30, 2012 5:21 pm

Ken,

Thanks for the calculations. I’m waiting for the sketches before commenting too much, so I’m sure we are talking about the same critical measurement points. I used “shoulder” for the point where the black body meets the white tube as I could not get the better word “flange” in my head.

For now I think your distance 54.7 mm (my measurement 55 mm) is from the rear of the etalon assembly tube (diameter 83 mm) to the rear of the Barlow lens. From the Lunt black focuser body flange (which meets the white tube) to the Barlow rear is a around 10 mm more (65 mm). Plus what you add for the Barlow (thickness around 7 mm).

Now I see we would have needed a Lunt sketch to fill in from the beginning.

Viljo

Post by **Merlin66** » Tue Jan 31, 2012 9:32 am

OK here are some sketchs.
The first is the Lunt drawing..
The second is the "Summary" sheet
and the third is the spacer requirement.
I looked at Pedro's 152 module (thank's Rusty for the link) and you can see the benefit of having a longer OTA on the donor, basically what Lunt have done here....
Any questions let me know,

Lunt60mmhalphatelescopedimensions.pdf

Photonist · Post by **Photonist** » Tue Jan 31, 2012 6:20 pm

Ken,

Thanks for the sketches. There are a few confusing points in them (at least for me).

Quote from your previous post: “Measured from the "flange" - where the original white tube fits the etalon/ focuser body, the Barlow sits close to 54.7mm in front. This gives an effective focal length of -111.5mm.”

I think there is a misunderstanding with the number 54.7 mm. See also my previous post.

From the Lunt black focuser body flange (where it meets the white tube) to the Barlow rear is a around 10 mm more (= 65 mm). Plus what you add for the Barlow (thickness around 7 mm)? So the distance is between 65 and 72 mm, depending on to which point of the lens you measure?

The tube assembly with the ERF (diameter 83 mm) does not extend to the Lunt body flange. It extends only to the front edge of the white tube threads (diameter 86.45 mm) on the body, so there is still 10 mm to the flange. The ERF assembly tube threads are 80.2 mm in diameter.

My measurement of 53 mm is only the length of the larger diameter tube part of the ERF assembly tube (the smaller tube in front with the ERF and Barlow is 24 mm long). I measured it in connection to your question about using it for adapting the Lunt body to a telescope tube.

I’m also confused with total length (56.8 mm) of the Barlow, etalon and refocusing lens assembly in your sketch top center. My measurement of the lens/etalon assembly is 71 mm in total front to end if the thickness of the Barlow lens is 7 mm (64 mm from the rear of the Barlow to the rear of the refocusing lens). The rear of the refocusing lens is 1 mm in front of the flange.

Viljo

Photonist · Post by **Photonist** » Wed Feb 01, 2012 2:36 pm

Hi Rusty and all,

Thanks for the focuser adapter tip. The adapter could be useful for the Lunt mod or some other one. However, an extension tube is also needed (150-200 mm long) for this mod. My biggest problem in modding have been to find aluminium tubes with a diameter more than 50 mm. Many dealers don’t sell to private persons or they want to sell the full 6 meter long tube plus shipping, so the total price for a short length of tube needed is too much. I haven’t had much luck visiting junkyards and metal workshops quite frequently. Ordering an adapter this big from a lathe company is way too expensive here, at least for a prototype.

Perhaps the cheapest and easiest way would be to buy a bunch of used or even new telescopes only for the tube. Any other tube tips anyone?

Does anyone know of a dealer in Europe who would want to sell tubes 80-150 mm in diameter in a length of 1 meter for example.

Viljo

Post by **Merlin66** » Wed Feb 01, 2012 3:13 pm

Viljo, et al,
I think the problem would be resolved in two or three minutes sitting together with a beer - Oh and the Lunt 60!!
The issue is really trying to reconcile the data given in both the LUNT PDF and the LUNT sectional drawing with the sizes your measuring.
According to the Lunt drawing the total length of the etalon assembly, from the front barlow to the rear re-focusing lens is only 56.8mm
Anyway, to maybe resolve the dimensions we have, can you confirm my understand of your last message.
The attached sketch shows the etalon assembly relative to the "shoulder/flange". If I read you correctly the distance to the front of the barlow is 85mm?? Yes/No

If this is correct can you then confirm the length of the white optical tube from rear of front cell to end of thread is 200mm?

Attached files

Photonist · Post by **Photonist** » Wed Feb 01, 2012 6:27 pm

Ken,

Yes, the 3 minute mini-symposium and a beer is a great idea. After that I would be ready to continue to talk through all other possible mods and have a few more beers. Then I would go out and buy 6 meters of aluminium tube. But perhaps later as I am still in bed with fever and all. Now that we have your sketches, it is a bit easier to proceed. I tried to draw a sketch laying in bed but it was a mess.

I think the reason for the many unsuccessful mod attempts is the many poor and incorrect drawings floating around the web and also not understanding the focal ratio/spacing idea.

In short: I would skip all the technical drawings I know about on the web.

First your new detail sketch: There is a small flaw in the flange area. I have used the front edge of the flange as I find it easier to use (the point where the white tube meet the black body, when the scope is assembled). So it is 10 mm to the flange edge, not 8 mm as in your sketch. The white tube goes then 2 mm into the body as in your sketch, so it is 12 mm to the bottom of the pocket. Sorry, if my understanding of the term “flange” is a bit fuzzy. Well, quite many words for this detail.

I quote myself from the previous post:
“From the Lunt black focuser body flange (where it meets the white tube) to the Barlow rear is a around 10 mm more (= 65 mm). Plus what you add for the Barlow (thickness around 7 mm)? So the distance is between 65 and 72 mm, depending on to which point of the lens you measure?”

So if the Barlow thickness is 7 mm, then the distance from the Lunt black body flange point described above to the front of the Barlow is 72 mm (65 mm to the rear). You have the Barlow too far in front . The rear of the Barlow is 2 mm into the tube. The ERF is where your Barlow is now. I gave a link in some of the later posts to the “better” Lunt drawing where the design can be seen (it is very dark and needs heavy Photoshopping to be visible).

The distance 85 mm is not correct as stated above (perhaps you added 77 and 8 mm?).

To avoid confusion, I would perhaps add the ERF in front of the Barlow into the sketch and add a comment that it is removed (if you think it should be removed?) as there is already a front ERF in the mod.

The white tube length of the assembled scope from rear of the lens cell to the famous flange is 200 mm.

I return to the number 56.8 mm in the next post (after a nap) as I have also a few comments to the front end numbers of the scope relating to the drawing in the French forum post mentioned earlier.

Viljo

Photonist · Post by **Photonist** » Wed Feb 01, 2012 9:07 pm

Ken,

Concerning the number 56.8 mm, I quote myself from a few posts back (still relevant except the typo, 58.3 should be 56.8 mm) and now easier to understand in the light of your new detail sketch and my comment to it:

“I’m also confused with total length (58.3 mm) of the Barlow, etalon and refocusing lens assembly in your sketch top center. My measurement of the lens/etalon assembly is 71 mm in total front to end if the thickness of the Barlow lens is 7 mm (64 mm from the rear of the Barlow to the rear of the refocusing lens). The rear of the refocusing lens is 1 mm in front of the flange.”

So the number is 71 mm, not 56.8 mm but note that the thickness of the Barlow (7 mm) is a bit uncertain as I could measure it only from a scaled cut drawing mentioned in the previous post.

To be continued….

Viljo

Photonist · Post by **Photonist** » Thu Feb 02, 2012 9:31 am

Ken,

There is a problem with two numbers in the front part of the scope sketch. The distance 163.5 mm from the objective to front of the Barlow, and also the distance 267.2 mm.

The same numbers appear also in the French forum post drawing here http://www.astrosurf.com/ubb/Forum2/HTML/034966.html
By the way the guy also said that the objective FL is 263 mm and f/4.1?

However, my Lunt 60 is quite different for some reason, as my measured distance from Barlow front to front of objective is 133 mm, about 30 mm shorter than in the “French” drawing (origin unknown or is it a Lunt prototype sketch?).

Also the distance 267.2 mm leads on my scope from the objective front to no specific usable point (flange etc.). The measurement ends somewhere on the focuser part of the Lunt body. Needless to say but the body/focuser part in the French drawing doesn’t resemble my Lunt 60 at all. I don’t know how much different the Lunt PT version is and of course there are replacement focusers in use. I have the standard original version. So the number is useless (my two cents, or Euros). Anyhow my alternate version to this distance (if needed at all for the mod) is the distance from the objective front to the famous flange edge, that is 205 mm (lens cell front to flange is 216 mm minus 11 mm to the recessed objective front = 205 mm).

On the other hand, the French guy states that the collimating lens intercepts the light cone at 123 mm from the objective, again quite different from the drawing below his text. If he measured from the collimating lens front to the rear surface of the objective, then it is very near my measurement 124 mm (see below). So it seems that he did not use the drawing in his post for this distance.

Here are again a few of my measurements as a summary:

The objective thickness is 9 mm and the Barlow lens thickness is about 7 mm (measured/estimated from the more realistic version of the Lunt cut drawing). The FL of the objective is at least 270 mm, probably around 275 mm and the diameter is 64 mm.

Barlow rear to front of objective: 140 mm (measured to the rear of Barlow because it is visible and I found only later the cut drawing with the correct ERF and Barlow visible and I didn’t want to remove the ERF for now).

Barlow front to front of objective: 140 mm – 7 mm = 133 mm
Barlow front to center of objective: 140 mm – 4.5 mm = 128.5 mm
Barlow front to rear of objective: 140 mm – 9 mm = 124 mm

The numbers can then of course be corrected for the Barlow thickness if needed. I wonder if it’s correct to measure from the front surface of the objective (or use it in calculations) like in the drawing of the French post?

Perhaps the remaining measurements needing refinement is the focal length of the objective and the Barlow thickness in case we want on the spot numbers for the calculations of the “Magic” number (I would like that).

Does someone know or guess what the text in the French post means (above the image) “Le FB fait 28mm de diameter”. It brings to mind the Blocking Filter, etalon or ERF, but he uses ERF in the text above and the diameter doesn’t fit anything I know of.

Oh, what a terrible amount of words in this thread, very few images and not a single Solar image. Perhaps I should have used those yellow “solar” blobs available in the editor to cheer up the visual impression (perhaps there are a also few mild “tongue in cheek” comments). I hope there isn’t a rule for maximum words per post here.

Anyhow, I’m sure all this can be cooked down soon to a single and simple sketch with a short text like “An excellent mind-blowing mod, see the sketch”.

Image 1. Two sunspots and a filament.

Viljo

swisswalter · Post by **swisswalter** » Thu Feb 02, 2012 5:57 pm

Hi Viljo

just lead me to the place where you need translation. I'll do whatever I can

Photonist · Post by **Photonist** » Thu Feb 02, 2012 6:05 pm

Hi Vanter and Walter,

The problem text is above the image and also several times later in the thread here:
www.astrosurf.com/ubb/Forum2/HTML/034966.html

Perhaps FB could be the size of the light cone at the etalon but why so small? By the way 28 mm happens to be near the calculation in Ken’s first sketch in a previous post. However, I can’t figure out the French FB abbreviation.

Reading the whole French thread gives the impression that it means the etalon module or block (Fabry…Bloc?).

If FB here means etalon, I wonder what is the etalon design philosophy. Why use an expensive 35 mm clear aperture etalon in Lunt 60 if only 28 mm of it is used. If I remember correctly, the PST etalon diameter is 20 mm and the light cone is even a bit larger than that. Do I miss something here?

Viljo

swisswalter · Post by **swisswalter** » Thu Feb 02, 2012 7:11 pm

Hi Viljo

thank you, I'll visite the site and try to translate it as far as I can

Photonist · Post by **Photonist** » Sat Feb 04, 2012 8:59 am

Rusty,

Thanks for the fine tips. I started thinking along these same lines in my post on Feb 01. The tubes seem to be very affordable but perhaps I try more local (European) resources first to avoid double taxes and all the hassle with the customs office.

Viljo

swisswalter · Post by **swisswalter** » Sun Feb 05, 2012 3:34 pm

Hi Viljo

I have read the french article you mentioned. Indeed it is a little bit puzzling. But by the end of the day what he writes is, that the numbers in mm on the drawing are wrong, he means maybe by intention. The front single lens is has f/4.1 and not f/8,3. f/8,3 is the resulting f/ with the two lenses in front and on the back of the etalon.

However I could not work out all he wrote. That with 20th of a centimeters did not make any sense to mee, because that would be just 0,5 mm distance from the refocusing lense to the etalon.

I'm sorry for not beeing able to be of more help

Post by **Merlin66** » Sun Feb 05, 2012 3:39 pm

Walter,
That's OK!
You're not the only one confused....
All the drawings and data I've seen so far are in total conflict.
Give me a L60 for a couple of hours - and I'll have the data - FOR THAT PARTICULAR TELESCOPE
Not sure they are all the same.
Anyway, the journey has just begun.
Onwards and Upwards.

Photonist · Post by **Photonist** » Sun Feb 05, 2012 4:48 pm

Hi Walter,

Thank you very much for your effort. Now we have turned that stone. A few still left.

Viljo