Focal Reducer

[Guest CodnorPaul]

Is there any benefit/downside to using a focal reducer in an SCT?  I asked before and as I understand it from Kim's kind answer it would just let me see more of the light cone but back then I was using a 1 1/4" visual back, however I have now upgraded to a 2" diagonal and use a 2" EP for DSOs so would a focal reducer add anything, or would it in fact just add more glass and maybe degrade viewing?  Or have I got it completely wrong?

 

I am purely visual, no AP

Edited May 5, 2013 by CodnorPaul

[philjay]

From my experience with a c11 and a 0.63 reducer the benefits for visual are well worth it. The reducer acts like its reducing the fl of the scope so you get lower mags using the same eyepieces and wider field of views. Yes there will be a slight reduction in light transmission but you will need a photometer really to notice any difference, visualy you wont be able to detect it

[Guest CodnorPaul]

Thanks 

[dawson]

Philjay, with reducers on catadioptric scopes, do you multiply the focal length by the reducing factor or the f/ value?

Also, with regards the increased FoV, do you get the all of the increase in FoV that the calculations would suggest, or do the tubes and innards of the scope limit the true increases in this?

And one last question, sorry, does a reducer cause any problems or benefits at the edge of the FoV, which might be noticeable visually or when imaging?

James

[Brantuk]

Reducers do the opposite of barlows, they reduce the f-ratio. So a .63 reducer will bring the f-ratio of an f-10 scope down to 6.3. IIRC it "effectively" reduces the focal length, though "physically" the focal length doesn't "actually" change (i.e it will always focus in the same focal plane).

 

The fov of the scope will be limited by the aperture of the visual back (e.g. Sct's and Mak's). I've never noticed the effects at the edges like I have done with a barlow - depends on the quality of the glass and the figuring I guess - good question!

[dawson]

I've always wondered if the tube inside the telescope which channels the light from the "secondary" to the eye piece restricts any widening of the FoV.

James

[philjay]

James, basically yes ish to the multiplying the reducer no by the scope fl, I say ish because the sct has moveable primary and it messes with the calcs a bit but rule of thumb is yes.

As for stuff getting in the way, eell the original question was regard to visual and most of the time with normal fov eyepieces you wont notice much but start putting the monster fov eyepieces in and yes I used to get some vignetting

As for imaging, it depends on thechip size of your camera. A dinky ccd chip wont show owt but stick a camera witha big chip on it and you willget vignetting, I used to with my old 300d and c11 plus 0.63 reducer. But its simple to fix with flats. However if yourthinking of using it on your mak 127 it willbe worse because you only have an 1.25" hole for your camera to look through as discussed the other night