The Charlie Bates Solar Astronomy Project is proudly sponsored by the following fine companies:
ImageImageImageImage
Image
Image
Transit of Mercury in the Spicule Layer. Phil took this great image with his Quantum 6 Scope, Solar Spectrum etalon and PGR Camera.

Welcome to Solar Chat! The best forum for Solar Astronomy
Moving forward from solar cycle 24 to 25

Is a C8 and a PST mod really usable?

Frankenscope? Let's see it!***be advised that NOTHING in this forum has been safety tested and you are reading and using these posts at your own peril. blah, blah, blah... dont mess around with your eyesight when it comes to solar astronomy. Use appropriate filtration at all times...
Post Reply
langleif2
Oh, I get it now!
Oh, I get it now!
Posts: 47
Joined: Sat Jun 30, 2018 8:32 am

Is a C8 and a PST mod really usable?

Post by langleif2 » Sun Aug 18, 2019 9:21 am

Can a C8 SCT work with a PST mod? Yes, and it works just fine is the short answer, but not without a few issues that can have a limiting impact. First, with the big DERF in place and the mod focused with a binoviewer ( + 1,6x GPC) or a camera ASI290MM, you will find that the light cone at the exit of the C8 is about 30mm. As is well known, the apparent diameter of the etalon is only 20mm and therefore aperture loss? or vignetting is taking place. But not that it can be seen at the magnifications I use. Please correct me, if I'm wrong.

In terms of backfocus, it is advantageous to place the etalon as close to the exit as possible, but require an adapter that I do not have. My etalon is now located about 50mm from the exit and even here the light cone is still around 30mm. But to me there seem to be back focus enough without any aperture loss The most important thing is that I can now see protuberances and surface details with a bino and at a large magnification, that'll be above 200-250x even at my latitude.
P1070728 (2).jpg
P1070728 (2).jpg (198.06 KiB) Viewed 816 times
Due to the small etalon the sweet spot is tiny at low magnification but just right with bino ( + 1,6x GPC) or camera ASI290MM. To reach the needed magnification with a bino an 1,6x GPC is doable and also necessary to shorten the light path a bit. I purchased an adapter so it's possible to mount the GPC without the nosepiece on the bino and so shorten the light path a little further. And it works, but not without a change to the ITF filter in the Coronado BF10 diagonal which is the other and second limiting factor.

The ITF filter in a Coronado BF10 is the culprit here because with the magnification potential the light gets dimmer. So the ITF had to be replaced with a UV / IR filter on kg3 glass but here it came to my surprise that the ITF obviously also has an H-alpha or some kind of red layer which is necessary for rendering. The 5mm block filter on the PST has this layer integrated, but apparently not on the Coronado BF diagonals.

Well, a 610nm long pass filter got out the protuberances in matchless detail, but in return missed a better representation of the surface, so a 35nm H-alpha filter I think will do just that, but then again I was a little worried about reflexes,as it has a reflective layer. And so it was seen at the edge of the Sun, but the surface had a better resolution than with the long pass filter.
P1070725 (2).jpg
P1070725 (2).jpg (137.94 KiB) Viewed 816 times
Therefore, right now I am relegated to switch between these filters depending on what I want to see or record. I know the H-alpha filters is available at a closer bandwidth, but the light is also getting weaker. Of course, I can put the 35nm filter up front instead of the kg3 filter. Now, the beam from the DERF is rather hot at the exit and I'm a little concerned putting the 35nm filter up front, because of the possible heat build up in the tube. The filter is reflective, you know. Or is there no concern regarding this?

I don't know if a telecentric setup can avoid the loss of aperture? or vignetting together with a PST etalon, but please tell if you know. And any suggestions for a better SCT f10 mod with the PST etalon is welcome.


Pos.
• does not require power
• the convenience of a SCT big aperture
• less expensive than a Daystar or Solar Spectrum solution
• relatively short light path compared to a telecentric one
• works fine with BF10 and a binoviewer ( + 1,6x GPC) or camera ASI290MM

Neg.
• big ERF mandatory
• aperture reduction or vignetting
• block filter alteration needed

Conclusion:

Works fine even with mentioned shortcomings and the SCT can be used for nighttime without the ERF and also cheaper than a dedicated H-alpha coated SCT which cannot be used as normal. Regarding the DERF from Valery, thickness is 15 mm! I think 12 mm would be suitable for a C8 and save some weight. It's a heavy chunk of glass. Because of its thickness and as a side benefit I believe it's also doable as an optical flat for DPAC (Double Pass Auto Collimation) testing optics! So nothing wrong without it being good for something else, I guess.
Last edited by langleif2 on Wed Sep 11, 2019 7:16 am, edited 1 time in total.

User avatar
marktownley
Librarian
Librarian
Posts: 26460
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Contact:

Re: Is a C8 and a PST mod really usable?

Post by marktownley » Sun Aug 18, 2019 9:59 am

Great looking mod there!
Image
http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!

bart1805
Almost There...
Almost There...
Posts: 631
Joined: Mon Feb 06, 2017 9:04 pm
Location: The Netherlands

Re: Is a C8 and a PST mod really usable?

Post by bart1805 » Sun Aug 18, 2019 8:12 pm

Hi langleif, looks nice! Don't understand completely what you are saying.
- there is no difference between a 180mm f/10 refractor and a C8 when you look at the light cone. That the light cone is bigger than the etalon means that you are not able to see the whole of the sun. Not that you are using just 6 inch when using a magnification of 200 times or a ASI290MM.
- when imaging you could try without the ITF but with a UV/IR block or the Baader 35nm H-alpha.
- what do you mean with the H-alpha layer on a BF of a PST? Don't you mean the other filter you cab find on top of the black box?
AOK made me an adapter to mount the etalon directly to an Intes M703: http://solarchatforum.com/viewtopic.php ... 64#p238164
If asked he would surely be willing to make one for you.
Looking forward to images you made with the ASI! Bart.

langleif2
Oh, I get it now!
Oh, I get it now!
Posts: 47
Joined: Sat Jun 30, 2018 8:32 am

Re: Is a C8 and a PST mod really usable?

Post by langleif2 » Tue Aug 20, 2019 8:06 am

Thanks Mark, any thoughts on my questions asked?

langleif2
Oh, I get it now!
Oh, I get it now!
Posts: 47
Joined: Sat Jun 30, 2018 8:32 am

Re: Is a C8 and a PST mod really usable?

Post by langleif2 » Tue Aug 20, 2019 8:27 am

Hey Bart.
There may not be a difference between the refractor and a C8 when looking at the light cone, but the smaller etalon will vignette the light cone as the cone is bigger. Hence the aperture loss. So I understand the arguments in various posts.

There is a layer of H-alpha coating integrated in PST block filter nearest your eyes, not in the hidden ITF as with the BF10 etc. When you look at the two together without the ITFs, you will find the PST block trim dimmer than the BF10. I can see H-alpha details without the ITF with the PST block alone. Its not possible with the BF5-BF10-BF15 diagonals. Hence the H-alpha layer is a part of the BF ITFs.

Any other thoughts on my questions asked?


Regards

AndiesHandyHandies
Ohhhhhh My!
Ohhhhhh My!
Posts: 116
Joined: Fri Mar 30, 2018 6:46 am

Re: Is a C8 and a PST mod really usable?

Post by AndiesHandyHandies » Tue Aug 20, 2019 5:46 pm

Hi,

Does the PST with the original black box on the back reach focus on the C8? If so how far is the eyepiece field stop back from the C8?

The optimum back focus on the C8 for no spherical abberation is 5" 127mm. The PST Collimator needs to be 200mm inside the original telescopes focus to collimate properly. You may need to move the Collimator out of the PST and up inside the focus tube or diagonal. So that the PST etalon can go after the Celestron output port with a short adapter. The lens fits in a Celestron/Skywatcher plastic Moon filter. So that the C8 and PST are used optimally. Whilst you can get a focus where its not in the optimal position there will be abberations from the C8 and/or PST if both not used optimally.

The 30mm is the whole beam and probably vignetted. I asked what the fully illuminated field was for a similar Meade LX200 on CN and Don Pensack said 23mm, and thats at the output. I worked out for someone on CN the fully illuminated field with a diagonal on and I said he only needed to use 0.96" old eyepieces. Say 20mm field stop? Daystar say the clear aperture of the PST is 18mm.

The focal length of the C8 is 2035mm so the suns size at focus is about 21mm so you will not quiet have a full Sun.

You need to have the PST Collimator lens 200mm in front of the telescopes focus to use the full F10. But it will be vignetted as the beam there is the focal plane size of 21mm plus the light cone needed for full illumination. 200mm x 200mm / 2035mm = 20mm. So at 40mm you will have some vignetting for a 30mm output. Only the centre of the image will be fully illuminated.

Most front DERFs seem to pass about 40% of the light through at best.
Astrograph uses 2" Baader 35nm filters as an internal ERF up to 6" on refractors. He says the light coming through is 'cold'.
So a 1.25" one will work up to 4" and I use one on a 110mm Maksutov without a problem. It should then work on the C8 with the DERF you have. I use bits of flocking to tilt the filter in its holder, double for the 2". Two on one side about 15mm apart and one opposite so it sits in a triangle.
Put the Baader up the focusser till its fully illuminated. I use a 48mm to 43mm step down ring on the front of a 2" Baader 35nm in my 127mm refractor to act as a baffle for the rest of the focus tube beyond it.
The Baader 35nm on my 110mm has the ambient temperature on the outside of its holder.

I use a Beloptik UV-IR KG3 in the eyepiece for visual to block the far IR the Baader 35nm lets through. For imaging you could put the KG3 after the Baader.

Andrew

bart1805
Almost There...
Almost There...
Posts: 631
Joined: Mon Feb 06, 2017 9:04 pm
Location: The Netherlands

Re: Is a C8 and a PST mod really usable?

Post by bart1805 » Tue Aug 20, 2019 7:16 pm

Hi langleif, luckily vignetting and aperture loss are not the same. If you use the etalon in a 100mm f5 telescope and position the etalon at minus 20cm of the focal point, then there is aperture loss. The systems functions as a 50mm f/10. When using a small etalon on a big f/10 scope, yes there is vignetting, but no aperture loss. You can see lots of images here that are made with bigger telescopes than a C8 and the resolution is a lot better. If your way of thinking was right there would be no difference.
Can't answer your other questions. The C8 is a great instrument to use as a mod, look for example at the pictures allhoest posted recently. I use a Intes Alter M703. It has the advantage that the back focus is a lot bigger than that of a C8.
CS! Bart.

langleif2
Oh, I get it now!
Oh, I get it now!
Posts: 47
Joined: Sat Jun 30, 2018 8:32 am

Re: Is a C8 and a PST mod really usable?

Post by langleif2 » Tue Aug 20, 2019 7:55 pm

Hello Bart, thank you for the answer. Now I understand.

langleif2
Oh, I get it now!
Oh, I get it now!
Posts: 47
Joined: Sat Jun 30, 2018 8:32 am

Re: Is a C8 and a PST mod really usable?

Post by langleif2 » Tue Aug 20, 2019 8:45 pm

Hello Andrew, yes, its not quite in the optimal position but that'll do. Frankly speaking, I don't see the aberrations or I don't know what to look for. Will there be overcorrection or undercorrection? If overcorrection when optimized in green then its good, I suppose, because we are looking at the red end of the spectrum. If undercorrection directed against the blue end of the spectrum, not so good. But the view at around 250x is sharp at the edge of the Sun when seeing allows.

I measured the temperature at the exit with the DERF and it reached 57 deg. C and the Sun was not even high in sky as in midsummer. So my concern about the 35nm filter is when the hot beam is reflected, will the temperature rise in the tube? Will it focus inside or outside the tube? Will it pass the DERF at all?

I have no intension with this setup to see the whole Sun. For viewing full disk I have another setup, an Oracle 3" refractor and adapted PST mod. A 2" Baader 35nm filter as ERF and the PST block filter in a diagonal + ND filter. With bino it's working remarkable well.

Finally I would like to understand why it's good enough to coat a simple SCT corrector plate triple band and yet if one might choose a DERF instead it's thick and heavy. But I don't complain because it apparently also will do as an optical flat for testing optics or am I wrong here?

AndiesHandyHandies
Ohhhhhh My!
Ohhhhhh My!
Posts: 116
Joined: Fri Mar 30, 2018 6:46 am

Re: Is a C8 and a PST mod really usable?

Post by AndiesHandyHandies » Wed Aug 21, 2019 4:01 pm

Hi

Well theory and practice may not differ so much.
Many people run Telecentric Daystars at F16 to F20 for seeing when F30 is optimum for the telecentric.

How did you measure the temperature? A thermometer in the beam?

The Baader will reflect most of the energy outside of its 35nm passband, and let some long IR through, over 1100nm.

What it reflects has passed through the DERF once so it should pass back through.

Because the Baader is reflecting a narrowing cone it will focus it back up the telescope to a point as far in front of it as the focus is behind it. Need to avoid its reflected focus being in corrector lenses or the DERF.

Interesting to know the temperature after the Baader is added, and also if the Beloptik KG3 is than added after the Baader.

Cheers. Andrew.

langleif2
Oh, I get it now!
Oh, I get it now!
Posts: 47
Joined: Sat Jun 30, 2018 8:32 am

Re: Is a C8 and a PST mod really usable?

Post by langleif2 » Thu Aug 22, 2019 1:49 pm

Hi Andrew, today I made some interesting measurements. The result I got yesterday doesn't count, that is I did the measurement at the edge and not at the center of the Sun and not at high noon. Today on the other hand I did just that at my latitude of 55 deg. north.

The measurement took place with the surroundings at 26 deg. at 50 mm from the SCT exit where my etalon is located with 1.25 tube hole and with a lab thermometer in C deg and the core pointing just at the beam in 5 minutes and 5 minutes with each filter in place. The focus is equivalent to the PST mod and binoviewer in place, that is, it will also match the focus roughly of the camera.

With no filter in place temperature rises to 70 deg. C :shock: With the Baader 35nm in place the temperature lowered by 30 deg. to 40 deg. C. Now, one could think, that the UV/IR on KG3 filter will surpass that and be even lower, but no, on the contrary the temperature rises to 45 deg. Ok, that's not significant but it took me by surprise anyway. So in short term the most effective filter is the Baader 35nm H-alpha filter. But I don't know in the long run, of course.

By the way, I did measure the distance from focus with the Baader filter in place and it will focus somewhere in the middle between the exit and the secondary mirror. I don't know how to interpret that, but by all means I do believe that in the long term it will build up heat in the telescope. So in my opinion it's better using the KG3 filter up front as it is a heat absorbent filter despite of also having an UV/IR cut layer. :cool:
Last edited by langleif2 on Thu Aug 22, 2019 6:34 pm, edited 1 time in total.

bart1805
Almost There...
Almost There...
Posts: 631
Joined: Mon Feb 06, 2017 9:04 pm
Location: The Netherlands

Re: Is a C8 and a PST mod really usable?

Post by bart1805 » Thu Aug 22, 2019 2:24 pm

If you can measure the distance of the focus of the incoming lightcone that is reflected in the 35nm filter you know how far you are from the real focus point of the C8. Optimal distance to the focus point for the front lens of the etalon is 20cm, anything less means that there is substantially more heat entering the etalon.

langleif2
Oh, I get it now!
Oh, I get it now!
Posts: 47
Joined: Sat Jun 30, 2018 8:32 am

Re: Is a C8 and a PST mod really usable?

Post by langleif2 » Thu Aug 22, 2019 6:32 pm

What I did, the Baader filter was in place up front the PST mod at the exit of the C8 (The focus is measured when I see a sharp image with the PST mod and binoviewer in place, because that's where it's usable). I then removed the mod and measured the distance to focus point (the sharp image) albeit it was not exact. To my logic that must be the distance (heat) reflected by the filter from where it is located and then entering the telescope. But math isn't really my thing ;-) or I do not quite understand optics.

But I don't use the Baader filter up front any more because of the possible heat development. And I'll use what I think would be safe for the etalon and block filter and that's the UV/IR cut on KG3 glass. I will not allow 70 deg C entering the etalon device. By the way, I have used this setup for a while now and never had any issue with heat build up in the PST mod. Not so sure about the 35nm filter I used in a short period. I recall some heat tube currents or was it just bad seeing. Whatever, I don't use it up front anymore. I'll do otherwise if told, though.

User avatar
Valery
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 3350
Joined: Sun Nov 18, 2012 3:13 pm

Re: Is a C8 and a PST mod really usable?

Post by Valery » Thu Aug 22, 2019 7:54 pm

langleif2 wrote:
Thu Aug 22, 2019 6:32 pm
What I did, the Baader filter was in place up front the PST mod at the exit of the C8 (The focus is measured when I see a sharp image with the PST mod and binoviewer in place, because that's where it's usable). I then removed the mod and measured the distance to focus point (the sharp image) albeit it was not exact. To my logic that must be the distance (heat) reflected by the filter from where it is located and then entering the telescope. But math isn't really my thing ;-) or I do not quite understand optics.

But I don't use the Baader filter up front any more because of the possible heat development. And I'll use what I think would be safe for the etalon and block filter and that's the UV/IR cut on KG3 glass. I will not allow 70 deg C entering the etalon device. By the way, I have used this setup for a while now and never had any issue with heat build up in the PST mod. Not so sure about the 35nm filter I used in a short period. I recall some heat tube currents or was it just bad seeing. Whatever, I don't use it up front anymore. I'll do otherwise if told, though.
Use a 35nm Baader or UV/IR in front of the PST etalon for absolute safeness and the BF10 at the end. No more is required.

Valery
"Solar H alpha activity is the most dynamic and compelling thing you can see in a telescope, so spend accordingly." (c) Bob Yoesle.

Largest full size 185 - 356mm Dielectric Energy Rejection Filters (D-ERF) by ARIES Instruments.

langleif2
Oh, I get it now!
Oh, I get it now!
Posts: 47
Joined: Sat Jun 30, 2018 8:32 am

Re: Is a C8 and a PST mod really usable?

Post by langleif2 » Fri Aug 23, 2019 7:10 am

Hi Valery, thank you for chiming in. I'm aware of using some kind of UV/IR but I'm the type of guy who want's to understand. Otherwise I will be left ignorant and the forum too.

So what happens to the heat reflected by the 35nm Baader back into the telescope? Will it do nothing and why? Will it develop heat build up in the telescope or what?

Langleif

langleif2
Oh, I get it now!
Oh, I get it now!
Posts: 47
Joined: Sat Jun 30, 2018 8:32 am

Re: Is a C8 and a PST mod really usable?

Post by langleif2 » Fri Aug 30, 2019 8:49 am

Okay then, I consulted a German solar expert that was willing explain it to me. Not the solar heat is in reflection, only the solar radiation! You get a heat problem if you concentrate this solar radiation power on a point with a absorber material. So the Baader 35nm is safe - as long as the filter is not broken under stress, because a little radiation behind the coating goes in the red glass absorber substrate. The optical density of this interference coating is between OD3...OD4 without specific Laser damage threshold.

Regarding the aberrations taking place because of the -200 collimating lens. There isn't really any. The SC is a nearly color free optical design. A tiny bit of color comes from the Schmidt plate, but many times lesser than an apo. The C8 optic is designed for use with a visual back and a 1.25 prism diagonal and is +100mm light path to get it at f10. So there's plenty of back focus left without noteworthy aberrations and aperture loss. I'm also using my bino with a GPC. In other words - just enjoy!

langleif2
Oh, I get it now!
Oh, I get it now!
Posts: 47
Joined: Sat Jun 30, 2018 8:32 am

Re: Is a C8 and a PST mod really usable?

Post by langleif2 » Fri Aug 30, 2019 9:16 pm

do.

User avatar
marktownley
Librarian
Librarian
Posts: 26460
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Contact:

Re: Is a C8 and a PST mod really usable?

Post by marktownley » Sat Aug 31, 2019 3:12 am

Thanks for the update Leif
Image
http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!

Post Reply