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Focal Reducer Questions / Recommendations

Started by Peter Glus, 08/08/2006 08:37AM
Posted 08/08/2006 08:37AM Opening Post
I have a old orange C8 and bought a relatively inexpensive GSO/KO 30mm 80 degree UWA eyepiece. As I understand it, my actual FOV is about 1.2 degrees (80 degrees divided by the magnification, which is 66x).

Anyway, I am interested in buying a focal reducer to expand the FOV even more. My understanding is that a f/6.3 reducer would drop my magnification to about 40x, thereby increasing my actual FOV to about 2 degrees. My questions:

1. Am I right, or confused regarding my above numbers?
2. Is there any downside to using a reducer?
3. I have seen 6.3, 5, and even smaller ratios. Seems like the 6.3 is the most common. Any real differences besides the numbers?
4. I have also seem a lot of sub 50 dollar reducers, and a recent batch of APO reducers for 100 or so. Is there any review or recommendation for these on AM that I have missed?

THANKS!

- Peter
Posted 08/08/2006 09:18AM #1
>>>Anyway, I am interested in buying a focal reducer to expand the FOV even more. My understanding is that a f/6.3 reducer would drop my magnification to about 40x, thereby increasing my actual FOV to about 2 degrees. My questions:

1. Am I right, or confused regarding my above numbers?
2. Is there any downside to using a reducer?
3. I have seen 6.3, 5, and even smaller ratios. Seems like the 6.3 is the most common. Any real differences besides the numbers?
4. I have also seem a lot of sub 50 dollar reducers, and a recent batch of APO reducers for 100 or so. Is there any review or recommendation for these on AM that I have missed?
-----
This is my understanding:

Normally the field of view is determined by the field stop in the eyepiece. In the case of the 30mm 80 degree eyepiece, this is about 42mm.

But this is not the only thing that can limit the FOV, in the case of an SCT, the hole in the back of the scope also acts like a field stop. To take advantage of that 30mm 80 degree eyepiece, you need to have a rear port that was 66mm in diameter. The end result is that you are essentially limited to either using a 2 inch widefield eyepiece at F/10 or a 1.25 inch eyepiece at F/6.3.

Bottom line is that you might get a slightly wider field of view with a focal reducer but it will be vignetted and no where near 2 degrees.

As far as which focal reducer to choose, there are lots out there. Many are designed for use with CCDs where the chips are small so they are not big enough for use visually. The F/6.3 is the standard focal reducer/CORRECTOR and can be used both visually and photographically with good results. These not only are focal reducers but also correct for coma and flatten the field as well.

Jon