Tinkering with Opteka 500mm f6.3 DG mirror lens

[bojan]

I am trying to improve coma affecting one of my catadioptric telephoto lenses (Opteka 500mm DG), by adjusting the distance between two glass elements (FF group.. I have a crazy feeling the problem could be there because I know of good specimens out there and maybe Opteka decided to cut cost by omitting spacers..).
The idea is to glue the right lens to threaded retaining ring, so the distance can de varied.

I was thinking of using shellac or similar, because it is easily removed by isopropil alcohol.. but not sure where to get it from
Is there a better stuff for the purpose?
Perhaps PVP-based glue stick is OK?


Thanks in advance for any suggestion!

[iborg]

Hi Bojun


Do you need glue? I would first consider if heavy duty silicone vacuum grease would hold it in well enough, if not, a thin smear of silastic.


I may have some shellac you could have if you want it.



Good luck


Philip

[Leo.G]

I believe Bunnings sell Shellac, or used to because I used to mix it with methylated spirits as a timber finish (French polish).


https://www.bunnings.com.au/feast-watson-250g-shellac-flakes_p1567019


It used to be a lot cheaper and in a big plastic bag......

[bojan]

Thank you guys for help, I am going to Bunnings now :-)

[bojan]

Gluing the lens to retaining ring was successful, now I can vary the position of the assembly and see if coma can be improved by increasing the distance between elements.. If it gets worse, I will try to reduce their distance by machining the barrel by ~1mm or so..

In the meantime I tried the lens w/o field assembly on the Moon last night..
The focal length of the system is reduced from 500m to 278mm (from f/6.3 to f/3.5).
This could be useful for guiding.. maybe.

[bojan]

This weekend I combined the FF from Rokinon and the rest of Opteka..
Initial results strongly suggests the coma at Opteka is indeed caused by FF group, because this combination doesn't have coma any more.

It seems they (Samyang and Co.) are not very good in terms of assembly and quality control..