How to Choose a Donor Telescope for PST Mod

[kornfeld1]

I'm almost positive I've read the answer to this question somewhere, but I can't find it to save my life now. I'm trying to find what criteria are used to identify a good donor scope for a PST mod. I know it needs to be f/10 or greater and have an aperture that makes all of the work worthwhile (i.e., ~70mm at the very least), but does the optical quality of the telescope matter? I feel like I read that the scope doesn't have to have super high-quality optics because it's effectively only focusing one wavelength of light, but now I can't find that anywhere to confirm.

Here's why I ask: I can find a pretty cheap 80 mm telescope on craigslist and then I think I can add a 2" focuser to it for less than many of the refractors that already come with a 2" focuser. Is this worth doing, or will the optics limit the quality of the final result? Here's the scope I found on craigslist for only $100:

http://www.celestron.com/astronomy/tele ... -80eq.html

So it isn't a totally worthless toy, but at that price, it seems like the optics are probably not the best in the world. What do you all think?

(And if you happen to know of a refractor that already comes with a 2" focuser that isn't terribly expensive, I'm all ears!)

[Merlin66]

You have most of the criteria covered...
One of the best PST mods I've done was using a "basic" Meade achromat 90/880 Nextscope (I think)It works very well and gives great results. You certainly don't need a three element APO for this mod.
(It's covered in my PST Mod write-Up)

[kornfeld1]

Why is the Vixen A80M so regularly mentioned for this? Is it that it's already the right length and has the 2" focuser?

[Merlin66]

Basically yes.
It has the f11 focal ratio and was one of the first donor OTA's used in the Mods.
Very successfully.
I have one sitting on the bench as I type just waiting on the knife......

[keithatrochdale]

The TAL100RS makes a good donor scope and gives cracking images. The DERF (and PST) can be easily removed to use as a 'normal' scope too.

[Merlin66]

There has also been some great successes with the LyraOptics 102mm f11 OTA......

[peter drew]

Best not to confuse "cheap" with inexpensive. Good quality optics always give best results, a good quality achromat is ideal for Ha work, the F10 minimum requirement for a PST mod pretty much exempts APO's as they are usually much faster optically. There is little difference in cost overall between modding a good 80mm and a 100mm, I have both and the 100mm is always better.

[keithatrochdale]

This link takes you to one of Peter's conversions, neat and works very well (uses a 75mm DERF part way down the tube to keep costs down)

PST conversion with TAL100RS

[Merlin66]

As they say - There's many ways of skinning a cat, the hard part is catching it!
Same for PST Mods...over the past few years the PST mod has developed and grown...the only fixed "point" in the mod, is the "majic" -200mm from prime focus.....
I hope, moving forward, we will see even more successful mods!! (maybe including the Lunt etalons!!??)

[sullij1]

Here is anither way to skin the cat. Thanks to Merlin and others here on solaechat I am already modding an old Meade DS 90 for use with my frakentilter and other solar mods. I have scrapped the original focuser for the GSO 10:1 from Agena here: http://agenaastro.com/gso-crayford-focu ... -86mm.html

Here is the scope data:
http://www.mediacollege.com/equipment/m ... s90ec.html

Right now the scope is at work as I have to drill and tap new holes in the tube to accommodate the new focuser. I have been pushing my fast scopes to test with for now but that requires stopping them down to 25mm or using the 3 and 4x Powermates. The DS will not require me to stop down so hard for the full disk view and deliver the slow F number needed to have the filters work. This mod will be my dedicated solar scope until I hit a lotto or something and can get a real solar scope.

Best

[colinsk]

[quote]As they say - There's many ways of skinning a cat, the hard part is catching it!
Same for PST Mods...over the past few years the PST mod has developed and grown...the only fixed "point" in the mod, is the "majic" -200mm from prime focus.....
I hope, moving forward, we will see even more successful mods!! (maybe including the Lunt etalons!!??)[/quote

I think the DS50 module would be perfect for a custom designed collimator or telecentric. I have one that I would like to replace with the new DS60 module. I also have a couple of 5" achromats. Perhaps I should try it sooner rather than later.