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?
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.
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..
Samyang and Rokinon are DEFINITELY no good at quality control. I've not only experienced it with a new lens, I've read so much about the inability of many of their lenses to obtain focus at infinity.
Unfortunately I have an 85mm f1.4 I bought used from eBay and when I couldn't achieve focus at infinity the seller told me I had to use live view and p*ssed in my ear about how to obtain focus via some setting I knew nothing about because I didn't have live view and he knew that.
I should have returned it for an immediate full refund. It may focus stopped down to f4 or f5.6 but that's not why I bought an f1.4 lens.....
Yep, but new, return if focus can not be achieved at infinity.
I got my new 14mm f2.8 ED replaced immediately, the new one will achieve good focus but it's nowhere near the infinity mark on the lens and it's considerable fidgeting to obtain focus. I now have a red nail polish mark on the focus ring where infinity is sharp for astro. I spent hours out adjusting, running inside to look at the image on a screen I can actually see and back out to adjust focus again, and again, and again.. Four hours in one night running in and out after every 2 or 3 shots to see where it was closest.
Don't ask me about my new PK lens to Nikon F mount adapter from K&F with the glass element, it's a nightmare, infinity is at around 3 feet on the scale and far from clear. It works with a lens locked on macro at 80mm but nothing else and I have several old Pentax lenses, PK and M42.
... and when I couldn't achieve focus at infinity....
Leo,
Inability to reach infinity focus probably could be corrected by moving the mechanical stops.. I did this on my FD Canon lenses when converting them to EOS..
I know I am spending too much time with this lens (one of my friends on other forum suspects I am obsessed with cats..).
Since I have two specimens now (Opteka DG and recently acquired Roninon DX brands), I started to swap elements to determine which one is contributing to Opteka DG coma.
First, I tried to vary the distance between two elements of Opteka FF assembly (drawing is in the first post of this thread), but the results were inconclusive, so the next step was to swap only individual elements of the FF group.
The easiest was to swap the 2nd element of the FF group (negative doublet, accessible from the back without dismantling the rest of the lens), and the improvement was much more dramatic - almost acceptable, shown on attached image (visible CA may be atmospheric? Arcturus (imaged) is quite low above horizon..).
The next step I am planning is quite unorthodox - I will try to replace the whole FF group with Meade 2x Barlow, it will fit nicely in place with the help of adapter (to be machined from Delrin ø40mm rod). I am not expecting a lot but since the optical designs are similar, barlow should work somehow (provided I can reach focus with Canon camera).
Barlow 2x is in place..
Hopefully I will manage to get focus, there should be enough space for adjustment with lens focusing mechanism and barlow could be moved +/- 10mm as well.
I know I am spending too much time with this lens (one of my friends on other forum suspects I am obsessed with cats..).
Since I have two specimens now
Only 2?
I have 5 cats!
Not obsessed, neighbour died some years back and his little jet black Bombay decided she lived at my place, at first just in the yard in somewhere I made comfortable, dry and warm (na dfed her twice daily, fresh water at all times, spoiled), then she had 2 litters of kittens, 3 got stolen (desexed and chipped) but I kept the lot, plus already had one.
She's a big hearted tiny little thing, big teeth too.
I'll look into the focus with the stops but not always easy to find, I've had the 85mm Rokinon stripped as far as I was prepared to go (and that's further than most people) but I couldn't find an answer.
With the Pentax lens it's the cheap K&F adapter. I believe if I spend the higher money and get a good branded adapter (around $150) it will be fine but I've decided I may as well just put that into a Nikon lens itself.
If I could just find the AIS 28mm f2.8 Nikkor I'd snap it up in a heart beat BUT, I won't pay over $500 for it and I've seen them for that from Japan on eBay, a lens that didn't cost that new I believe (I'm talking older 80's models).....
I'll find one one day or a 20mm f1.8 (or 1.4 I can't remember).
If I had the finances I'd have the 14mm Nikkor in a heart beat too, I need a faster get away car, my old 92 Falcon just won't cut it against these fancy BMW's, LOL
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