Newtonian Focal Reducer / Relay Lens Idea

Started by atracht, 11/03/2004 07:10PM
Posted 11/03/2004 07:10PM Opening Post
Conventional focal reducers rarely work with Newtonians because they lack sufficient in-focus. Still, I'd like to increase the field of view of my CCD, which is about 10mm corner-to-corner. Three-fold would be nice.

So, I propose putting two eyepieces front-to-front, with their eye lenses separated to make their exit pupils coincide. One, say a 35 mm would be in the normal position in the telescope, converting the real image formed by the primary into a virtual image at infinity. The second, say 12 mm, would focus the image at infinity down to a new, real image of 12/35 the size of the original, focused on the CCD which would be placed at the position of the 2nd eyepiece's field stop, which must be accessible.

With a 10mm final field diagonal, the eyepieces would need a good 45-degree field of view.

I hope that using both eyepieces at their infinity focus will minimize image defects.

Can anyone suggest the (probably obvious) defects in this idea? Will there likely be problems with vignetting or field flatness?

If this is a good idea, does anyone make and sell them?
Posted 11/03/2004 08:05PM #1
I am not sure if the double eyepiece idea will work at all, and it will certainly be fairly difficult to get them lined up properly. But the more serious issue is vignetting. Unless you have a fairly large secondary, you are likely to have significant vignetting across a 30mm focal plane (which you are trying to shrink into 10mm for your CCD). You would probably be better off by replacing your secondary with a larger one, moving the secondary/focuser down a bit closer to the primary to bring out the focal plane, and using a normal focal reducer.

This is one of the reasons that newt's are not the best choice for astrophotography. If you want wide field, you're real best bet is to get a small refractor. An ED-80 doesn't cost much more than 2 high-quality eyepieces....

Jarad
Posted 11/04/2004 07:05AM | Edited 11/04/2004 07:07AM #2
Allen,

Sounds very interesting. There is a special tube (tele-extender) that Meade/Celestron etc. used to sell that you could place an eyepiece in it for doing eyepiece projection. I suppose you could clamp two eyepieces into this tube to try it out.

Here is the Orion version:

http://www.telescope.com/shopping/product/detailmain.jsp?itemID=52&itemType=PRODUCT&iMainCat=6&iSubCat=29&iProductID=52



Also, once the light was collimated by the first ep., I suppose you could just put a finder scope up to the exit light of the first ep, and see the image with the new focal ratio of the finders objective. Maybe not too useful, but might show a proof of concept.

Here is a test I did by holding up a "relay" lens to the focuser, and took a picture with my digital camera. It shows some field curvature, so I suspect this will be a problem.

http://www.astromart.com/forums/viewpost.asp?forum_post_id=232923&poll_id=&news_id=&page=


David Simons




David Simons