85mm lens ordered for CCD.

[ollypenrice]

Hi,

I've decided to go for one of these 85mm primes from Telescope Service;

TS Aspheric 85mm f/1.4 - Telephoto Lens for Canon EOS - Teleskop-Express: Astro-Shop + Fotografie + Naturbeobachtung

I was tempted by Canon's 85mm prime at a bit more money but on this lens you can deal with the aperture manually and can't on the Cannon, according to the technician I spoke to at a camera shop. It seems this Samyang lens falls behind on resolution once stopped down but when wide open out performs the optical greats.

I aim to use it with the Geoptik adapter I bought from Altair Astro and our Atik 4000s. I will try getting the colour in one go in the OSC and then Ha in the mono. The scale will cover constellations in, say, 4 panel mosaics which at around f2 should be moderately quick.

Anyway, let's see what happens!

It will also be good for the many Cannon owners we see here, I hope.

Our Tak EM200 mount is getting nearer. Maybe Monday! Clear skies, clear skies, clear skies...

Olly

[FraserClarke]

Sounds like it will be a nice system.

I have a 135mm f/2 Canon prime in an instrument, and the performance is stunningly good. I over-designed the rest of the instrument to accommodate some potential performance issues with the lens; but there weren't any!

The only thing to watch for is defocus in the extreme blue (<420nm) -- the lens I have goes off very quickly below that. Might be worth adding a filter to cut the extreme blue out if you see this problem.

[ollypenrice]

Thanks for the tip. I'm wondering what filter that might be? A minus violet or similar?

Olly

[FraserClarke]

You can get 'long pass filters' (e.g. Schott GG435 glass, which transmits red of 435nm), but I'm not sure if you can get them ready mounted into a convenient size of cell. A Yellow filter would probably do the job too...

Might not be a problem for you of course; just something I noticed in my setup.

[ollypenrice]

Thanks, we can see. I will have a UV/IR in the system so maybe that will do it.

Olly

[Dangerous-Dave]

That looks like the Samyang 85mm/f1.4. I've got no experience of the lens in question, but I've just ordered their 8mm fisheye which is supposed to be an outstanding lens for the price. Fully manual, hence much cheaper than the rest of the field.

Register/back-focus is the main problem using 35mm lenses with your CCD, you are limited to one shot colour cameras or breaking the whole thing apart to change filters. I had a custom adaptor with built in filter holder made for my Leitz 180mm apo, but changing filters was a major operation - I had to remove the lens, refit it and refocus. There were a couple of occasions I nearly dropped the lens or filter, fiddling about with cold fingers.

In the end I bought a pair of Pentax 6x7 lenses off Jan McLare on UKAI - 200mm f4 and 300mm f4 - and got the same company (SRB Griturn) to make me a 6x7:T2 adaptor out of a 6x7 extension tube. A much better solution and, being medium format, have plenty back focus to get a filter wheel in there. I'm not sure how they would perform for LRGB, but they certainly do the biz for narrow band, which is what I bought them for. Also a beautiful, perfectly flat field which is the other big drawback with 35mm lenses.

[ollypenrice]

Thanks Dave,

Yes, I see the filters issue. I have OSC and Mono versions of the same camera so I hope to do Ha(L)RGB.

Thanks for the input.

Olly