80mm Triplet

[kimo]

Hi,

I am intending to get an 80mm F/3._ Triplet or ED Refractor. is there any such kind of telescope?
Thanks
Kimo

[orly_andico]

The Tak FSQ 106 can be brought down to f/3.6 with the dedicated super reducer ($$$$$). Same for the FSQ85. The Vixen VSD 100 is similar, I believe it's f/4 native and can take a reducer which brings it down to f/2.8 or so. Costs even more than the FSQ106. Borg 125SD is f/6 natively and there's an available 0.66X reducer which gives you f/4. Still ($$$$) but less than the Vixen or Tak.

I've taken my regular f/6.7 triplet (AT90EDT) and attached the Altair Astro Lightwave 0.6X reducer/flattener. As the math suggests, this takes it down to f/4. This particular reducer/flattener is not recommended for scopes below f/7 so my f/6.7 is already marginal. I can try to dig out an image I took with the AT90EDT + 0.6X reducer/flattener. Stars aren't that round at the corners and there's obvious vignetting on a DSLR sensor (but can be fixed with flats).

IMHO if you want fast and not-too-expensive, you need a newtonian. Stay above f/4 (say an f/4 newtonian with a Paracorr).

I just sold my ASA Keller reducer... it takes my f/4 newt down to f/3 (!) but it's almost impossible to use, focusing and collimation tolerances are in the tens-of-microns range. I'd been fighting it for 8 months now and frankly.. life's too short to spend collimating. Now waiting for a Paracorr. I get 912mm FL @ f/4.6 but hopefully far less headaches. What's the use of f/3 if your stars are eggy.

[kimo]

Thanks for info. I am using 80mm DIY scope. it does not have quality objective lens but doing great job for me. if I could have good refractor with reducer about F/3._ then should be fine.

[orly_andico]

Like I just said.. to get down to f/3.x with a refractor requires either the Takahashi, Vixen, or Borg. I'm not aware of any other options for refractors under f4. MooEy or Stuart are the real luxo refractor experts here, they can confirm or give more suggestions.

FSQ106 - f5 native, f3.65 with the reducer

http://www.optcorp.com/takahashi-fsq-10 ... 016nu.html
http://www.optcorp.com/tk-tsq0106rd-73x ... 106ed.html

Vixen VSD100 - f3.8 native, and I believe a reducer is also available to make it faster

http://www.optcorp.com/vixen-vsd100-f-3 ... 26145.html

Borg 125SD - f6.4 and f3.9 with the 7704

http://www.sciencecenter.net/hutech/bor ... /index.htm
http://www.sciencecenter.net/hutech/pri ... l/7704.htm

[Bergkamp_]

Will FSQ 85 interests you? If yes, kindly msg me. Thanks! =)

[orly_andico]

Wee Ern is correct, while the FSQ-85 is f5.3 (compared to flat f5 for the -106) it is also compatible with the 0.73X Reducer-QE. On the baby Q the final focal ratio would be f3.87.

But I would point out that the reduced FSQ-85 is 329mm focal length, f3.87. This is very close to a 300mm f4 camera lens such as the Canon 300/4L (which has some pretty good results on astrobin - http://www.astrobin.com/gear/3265/canon ... -l-is-usm/) I suspect the Canon is not as sharp as the Tak, and it won't be able to cover a larger-than-Full Frame imaging circle like the Tak, but it's almost 1/3 the price of the Tak + Reducer-QE (and makes a decent birding lens).

The only challenge with the Canon is that it has very minimal back-focus; some systems with filter wheel will not work (but the SBIG STF8300 with the FW8 will work, since that system has a total back-focus of 38mm, and Canon system has 44mm of back-focus). Use of an OAG is out, though, unless you use a QSI WSG camera which integrates the filter wheel and OAG in the body.


Edit: there is another, much more low-rent solution, but I suspect the results will be mediocre at best. A lot of vendors sell 0.5X 1.25" reducers for Mallincam use. Slap one of these on an f/7 refractor and you'll magically get an f/3.5 refractor. However, the coverage is dismal (reports are that even on ICX285 chip is not fully illuminated) but if you're using a small sensor camera like an ICX274 (e.g. Atik 320E) then these will work. I actually have one of these low-end 0.5X reducers, maybe I should try it and see how bad it is.

You don't want to put these cheap 0.5X reducers on a refractor faster than f/7 - because these reducers don't flatten, and the field curvature gets pretty bad below f/7.