mod project - ideas welcomed

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Catalin Fus

mod project - ideas welcomed

Post by Catalin Fus »

Hi,

finally after a very busy 2013, I've found myself in the position to start building a h-alpha scope, based on the mods I've read here.
I'm contemplating (as a first choice) on installing a Lunt 60 PT back on 105mm f6 or 140mm 7.5 refractor.
Besides Viljo, who's thread on building the scope was read a couple of times, there is anyone else who tried such an 'endeavor', based on the same unit or bigger one from Lunt (like the 80mm or 100mm DSII internal units)?

My questions to everybody who uses such a mod would be the following :

- have you encountered any issues with getting on band or sweetspot on the Lunt PT back?
- have you seen real improvement in visual/photo, over the smaller aperture used originally (i.e. - 60mm)?
- what BF do you recommend, stay with the 1200 from Lunt or go to a Coronado 10/15mm?
- would that tri-band filter made by Oliver be enough if placed in a mechanical assembly before the 'Lunt back'? I'm thinking it is better and more logical to buy a multi purpose filter that can be used internally and just swap the 'back' from H-a to CaK.

Now, my second option, which hunts me from some time, would be to place a Daystar H-alpha filter within a telecentric assembly. Unfortunately, although I have a confirmation that it would work, from Jen (actual owner of Daystar), I see that there is little info around, anywhere, not just around here.
Is there anybody who tried something like this and can give a feedback?

Thanks in advance to all of you who will take their time to respond to this. Any ideas and help over my raised questions, is really appreciated!!


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Re: mod project - ideas welcomed

Post by marktownley »

Hi Catalin! Happy new year! :)

You're thinking the exact same thing as me. I've been thinking about getting the lunt 60 PT for this very purpose. The questions I have on this are has anyone actually measured the focal length of the Lunt objective? Is it the 500mm(?) they quote for the objective alone, or a cumulative figure based on a summation of the objective and the collimating / refocusing assembly? This figure is crucial for the maximum success of the mod in terms of matching it to a donor OTA.

Regards the tri-band, It all depends what the coatings and hence bandpass of the objective on the Lunt are. maybe if we asked nicely walter would be able to measure this on his lunt 100 if is possible to get the objective in the spectroscope - it may be a bit big?

Regards the BF, if you're imaging I would just try them all and see which is best. This could be done on the Lunt before modding.

Regards the daystar, this should work...

mark


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Catalin Fus

Re: mod project - ideas welcomed

Post by Catalin Fus »

Hi Mark,

Happy New Year to you too !! :)

I was thinking to measure the objective of the Lunt and even make a Zemax simulation to see the aberrations without the asphere. Surely that singlet is aspherized but without the real numbers (conical constant), one can at least make an idea what that 1.85x barlow corrects.

Will try to see if I can buy one 60 PT at a reasonable price here in EU, otherwise, will get one from USA (more down to earth prices). I'll solve this in the following weeks as I want to have the unit before the visit I'll make to one of my opticians, to measure everything correctly.

Visually, I have seen the Lunt 60 but I simply didn't looked at the lens with too much attention.

Regarding the 100 Lunt, the question from me would be the following :
- has anybody seen side by side the 100mm Lunt and one of the generic 102mm f7 ED doublets?
The 80mm Lunt case is the same. The basic ED doublets are sold in Europe under different labels, Astro Professional as example. You can find them as 102/714 and 80/560.

May I ask, why is this primary lens coating important? I would like to understand it :)
You'll remove it anyway and use an internal or external ERF.

Yes, the Daystar should work. Indeed. I'll try to see if I can get on with both projects in parallel, besides my usual work.


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Re: mod project - ideas welcomed

Post by swisswalter »

Hi Catalin, Hi MArk

no problem to measure the front lens of the LUNT 100. Will do it that week and post the plot


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Catalin Fus

Re: mod project - ideas welcomed

Post by Catalin Fus »

Thanks in advance!


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Re: mod project - ideas welcomed

Post by marktownley »

Hi Mark,

Happy New Year to you too !! :)

I was thinking to measure the objective of the Lunt and even make a Zemax simulation to see the aberrations without the asphere. Surely that singlet is aspherized but without the real numbers (conical constant), one can at least make an idea what that 1.85x barlow corrects.

Will try to see if I can buy one 60 PT at a reasonable price here in EU, otherwise, will get one from USA (more down to earth prices). I'll solve this in the following weeks as I want to have the unit before the visit I'll make to one of my opticians, to measure everything correctly.

Visually, I have seen the Lunt 60 but I simply didn't looked at the lens with too much attention.

Regarding the 100 Lunt, the question from me would be the following :
- has anybody seen side by side the 100mm Lunt and one of the generic 102mm f7 ED doublets?
The 80mm Lunt case is the same. The basic ED doublets are sold in Europe under different labels, Astro Professional as example. You can find them as 102/714 and 80/560.

May I ask, why is this primary lens coating important? I would like to understand it :)
You'll remove it anyway and use an internal or external ERF.

Yes, the Daystar should work. Indeed. I'll try to see if I can get on with both projects in parallel, besides my usual work.

Hi there,

When I looked at Walters 100mm Lunt it looked a bog standard 102mm f7 ED objective - nothing special.

I have a feeling that Lunt may have some kind of coating on their objective lenses, maybe, Walter will kindly confirm or disprove this for us :)


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Re: mod project - ideas welcomed

Post by Merlin66 »

Catalin, welcome back!
On the French site they said 64mm diameter, 263mm focal length fr=4.1 for the Lunt 60 objective....
Viljo measured 270mm fl

Some comments from Viljo:

Now that we have the new “Lunt FL factor” of 1.8725x, doesn’t it also explain the strange Lunt 60 telescope specs problem of the 500 mm FL. My measurement of the Lunt objective FL in February was 269.5 mm and that multiplied with the Lunt factor 1.8725 is 504.6 mm, quite close to specs.
I have tried to test the “magic number” of the Lunt 60 mod by sliding the extension tube in and out around the calculated distance, 141.5 mm in front of the FP of the donor scope. That translates to 255.5 mm from the SW150 black tube rear to the Lunt 60 body flange (SW150 black tube to FP 325 mm – Lunt flange to Lunt FP 69.5 mm = 255.5 mm).

The Lunt focuser adjustment range of 34 mm allows for extension tube adjustment from about 248 mm to 265 mm. When using the Lunt 60 telescope as such, my focuser out distance is 18 mm + adapter, total out 52.8 mm). So the calculated 255.5 mm seem to fit quite well for the Lunt focuser (15.5 mm out + adapter, total out 50.3 mm). The extension tube adjustment is very sensitive; 1 mm movement requires a 3-4 mm adjustment of the focuser.

So the focuser out distance of the mod is almost the same as in the original Lunt 60 telescope (for the calculated distance).

I wonder, does this indicate something about the magic number, is this just obvious, good luck or is the magic number close or far from correct?



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Catalin Fus

Re: mod project - ideas welcomed

Post by Catalin Fus »

Hi Merlin,

I'll surely post here the results of our measurements, for comparison.

The advantage in my case is the following :
- I know exactly for the donor scope, the focal length behind the last lens surface
- I can simulate correctly the light cone
- I can measure exactly where the Lunt back will be and can see the exact numbers regarding diameter of light cone at best position and back-focus length remaining.

The small differences in Viljo's measurements might come from small differences in focal lengths, which can be measured empirical, but with some 2-3mm error, at all times. Types of glass used, their thickness in the center, light cone diameter at exit should be known precisely for a correct simulation, in my opinion.
I.E. I know that for the 105mm f6 light cone at exit is 103mm and BF behind last optical surface is 605mm. These are a little different numbers than the theoretical 105mm and 630mm.

Also, each lens has a different focal length, which may vary in the +/- 5mm range from the stated one, which usually is rounded up or down as convenient.

Hopefully my explanation makes some sense.


Catalin Fus

Re: mod project - ideas welcomed

Post by Catalin Fus »

So, here are some updates as the situation has developed in the meantime :

- 105mm ERF (BelOptik) from Beloptik was bought, to serve as internal ERF
- will have a Lunt 100 DSII unit to make some 'research' and see if it needs Telecentric lenses to work or not (any input here, is truly welcomed. Maybe somebody already checked the DS internal units from Lunt)
- the above will be adapted to 140mm f7.5 refractor
- I'm closing a deal on a Lunt 60mm but it seems I'll have the OTA to start modding, only in March.

More than the above, I've acquired Lunt Ca-K, B1200 version module. Can't wait some sunny days and some free time to put it to work. I shall be present in the gallery section soon (hopefully I'll have something worth posting).
I'll post some question in a different thread.

Thank You for all your help!


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