Focal Reducers and Newtonians

[Scooot]

There's a limit to the low power /wide field eyepieces you can use in a newtonian due to exit pupil etc and it made me wonder about focal reducers. I've read they don't tend to be used in Newtonians for some reason and I wondered why? Do they introduce other drawbacks or is it just simpler to use a different scope?

[YKSE]

Most Newtonians are already very fast (f5 or under), especially the dobs, to have a big aperture in a portable size. These fast reflectors has already quite some coma in the edge, a reducer will make it worse or much worse.

[Scooot]

Thanks for the reply Yong, presumamby a coma corrector would help with that if you can use the two together.

[bambuko]

Well... there is for example TAL-120 with built in focal reducer, which is short tube version of TAL-1.

[Scooot]

Well... there is for example TAL-120 with built in focal reducer, which is short tube version of TAL-1.

Thanks, thats interesting, maybe there isn't a problem using them with Dobs after all.

[bambuko]

What is telling, is that they very soon abandoned production of them   and nowadays you can only get straight version (i.e. TAL-1).

I think that with TAL-120 they have overcome the problems by designing the focal reducer/corrector specifically for this particular scope.

If you buy off the shelf reducer, how do you know it will be OK with your particular mirror/scope?

[Scooot]

What is telling, is that they very soon abandoned production of them   and nowadays you can only get straight version (i.e. TAL-1).

 

I think that with TAL-120 they have overcome the problems by designing the focal reducer/corrector specifically for this particular scope.

If you buy off the shelf reducer, how do you know it will be OK with your particular mirror/scope?

I don't but I was hoping to find out what possible problems could arise, not necessarily because I want one, but because its still raining and I'm just curious. :-)

[bambuko]

Understand

and yes it is still chucking it here, so you have few more hours before the best of it gets to you ...

[John]

I'm not sure a reducer would add any coma to the optical system and that, as I understand it, comes from the properties of the primary mirror. What you might well get though is somewhat more visible coma as you would be seeing a lot more off axis. There might be the possibility of some vignetting of light at the outer field edges depending where exactly you position the reducer.

Maybe you just have to try it and see   

[Scooot]

Thanks John, I've read something similar on cloudy nights since I started the post. I'm not really sure what vignetting is, but I think it dims the edges. I wonder whether the reducers have to be positioned in a specific place like coma correctors or whether you just screw them on.

[alan potts]

Reading between the lines here, I would have thought that if such a reducer works someone, for sure Teleskop Services would market and sell a reducer as such. I have never seen a reducer for what you are asking.

I have a SC reducer for my Meade LX and that increases the FOV a little and allows the use of smaller wide field eyepieces, by that I mean a 28mm instaed of a 41mm to achieve more or less the same. You cannot just bang a massive 40mm eyepieces of the 2 inch type into a reducer and expect it to work, it will vignette very badly indeed.

I also tried the same reducer on my 180mm Mak and thought it would work, it don't, this leads me to believe they are designed for a specific foacl speed, i.e. F10, my Mak being F15.

I would be very careful just buying and trying, I believe you would stand a very good chance of getting it wrong.

Alan.

[SnakeyJ]

The skywatcher standard coma corrector is also a 0.9 focal reducer - this does give a little extra fov, but the half stop makes an appreciable difference for exposure times.

typed on my mobile with Tapatalk

[Scooot]

Many thanks Alan, great info.

[laser_jock99]

 These fast reflectors has already quite some coma in the edge, a reducer will make it worse or much worse.

Not strictly true,  you can buy a very highly corrected  0.7x coma corrector reducer which is optimised for AP (so might be okay for visual?)

Trouble is they are not cheap (in fact probably more than the scope!)

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p4685_ASA-2--Newton-Korrektor-und-Reducer-0-73x.html

I use one for AP as it turns any of my three F4 Newts into F2.9 astrographs. I should try looking through it with an eyepiece one day?

[alan potts]

Lovely piece of equipment I am sure but I bet it will not work for visual like the Meade F3.3 reducer , sometimes it is cheaper to buy another scope but it would be nice to hear after a test. F 2.9 is really quick.

Alan

[YKSE]

I use one for AP as it turns any of my three F4 Newts into F2.9 astrographs. I should try looking through it with an eyepiece one day?

I'm very interested in your test, it's always better to know than just speculate (as I did).

[Scooot]

Thanks for all the comments. I've been doing some more googling as well and it seems the main problems would be either focussing or vignetting. The critical point being the spacing between the reducer and the eyepiece. If the spacing is too little it will cause vignetting because the light cone won't fill the eyepiece and if it's too large it won't focus. That's my understanding anyway.

I found this link in an older thread that I found quite useful in learning how they work.

http://homepage.isomedia.com/~cvedeler/scope/focal_reducers.htm

It seems the only way to find out it is to try one but reading between the lines it might be possible to get a small reducer to work, 0.8 or maybe 0.7. This would bring my f4.8 down to f3.84 or less. Having said that I don't really want to spend the money just to try :-)

[laser_jock99]

Thinking about it, using the ASA Keller 0,7x Komacorr won't work unless you get the spacing between the corrector and the eyepiece exactly right. I know from using them for AP that the backfocus distance is almost sub-millimetre critical. So you might need to spacers of some kind. The other certainty is that for each eyepiece this distance will be different!

I'll update on this thread when I have done some testing.