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Two Inch Diagonal and Focal Reducer

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#1 John Kocijanski

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Posted 21 July 2005 - 08:14 AM

I just picked up a Celestron two inch SCT diagonal for my C8. I wanted to use it with my f/6.3 focal reducer and a BW Optik 30mm ep to get an almost 2 degree field of view. It works but I almost run out of focus and with higher power eps (5mm) I can not focus. When I use my 1.25 inch diagonal with the focal reducer I have no problems focusing (obviously I don't use my BW with that setup). Any suggestions or is this just the way it is? The collimation seems to be pretty good on the scope.

#2 southmike

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Posted 21 July 2005 - 12:11 PM

i winder if an extension works..do you have a barlow..try removing the front ..and using it as a hollow tube see if that makes a difference...but be careful when removing the front..my celestron is good because the element is trapped between rings but some may not be

#3 Starman1

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Posted 22 July 2005 - 02:38 PM

I just picked up a Celestron two inch SCT diagonal for my C8. I wanted to use it with my f/6.3 focal reducer and a BW Optik 30mm ep to get an almost 2 degree field of view. It works but I almost run out of focus and with higher power eps (5mm) I can not focus. When I use my 1.25 inch diagonal with the focal reducer I have no problems focusing (obviously I don't use my BW with that setup). Any suggestions or is this just the way it is? The collimation seems to be pretty good on the scope.


You might note that the maximum unvignetted field of view on an 8" SCT is about 1.3-1.4 degrees. Though it's possible to come up with eyepiece combinations that exceed this, severe vignetting will appear because the opening in the back of the scope is only about 35mm (38mm max).

When you add a focal reducer, edge illumination is reduced even further, and vignetting becomes even more severe (It's the nature of the focal reducer) and the illuminated opening shrinks from the 35-38mm to as little as 30-31, effectively.
Using a focal reducer to shorten the f/ratio and flatten the field (a secondary benefit) is a cool thing to do, but don't expect to bypass the laws of physics.

One last thing: the opening in a thread-on SCT 2" diagonal actually is substantially smaller than a thread-on adapter and an insertion-type (refractor-type) diagonal. Vignetting will be worse with this type than it would have been with a refractor-type diagonal.

I'm surprised that you come near to the end of focuser throw with your high-power eyepieces. The standard C8 focuser allows a huge range of focus. What you notice obviously describes a difference between Meades and Celestrons. The Celestron must form its final focal plane closer to the back of the tube than does Meade. So, unless you use different eyepieces that require less infocusing, you are probably out of luck.

But I have a question: If you desire higher powers than a 5mm eyepiece gives you, why not remove the focal reducer?
I used a focal reducer for many years on an 8" SCT, but I don't recall using an eyepiece shorter than 8.8mm when the focal reducer was in place.

#4 sixela

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Posted 22 July 2005 - 02:48 PM

It works but I almost run out of focus and with higher power eps (5mm) I can not focus.


Not uncommon - a focal reducer often requires focuser in-travel, and a 2" diagonal lengthens the optical path (compared to a 1.25" diagonal).

There *are* some "negative height" 1.25" to 2" adapters like the one on the left of a picture at http://www.astrosystems.biz/focm1.htm. (but noly for eyepieces with a barrel that's not too long and that are narrow enough at the base to fit in a 2" barrel).

If your current one is very high, even a simple flat one (like the flat top Televue one) could well be enough.

But as already stated, C8s with a focal reducer have an unvignetted field pretty much maxed out by even a 1.25" Panoptics 24, so using a focal reducer *and* a 2" eyepiece (which is presumably the driver for using a 2" diagonal) isn't optimal even if you fix all of these issues.


#5 jrcrilly

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Posted 22 July 2005 - 02:50 PM

I'm surprised that you come near to the end of focuser throw with your high-power eyepieces. The standard C8 focuser allows a huge range of focus. What you notice obviously describes a difference between Meades and Celestrons.


It's fairly common for SCT's to run out of focus range when focal reducers are used with large diagonals. The worst offenders are the C9.25 and the Meade 14" - but a long enough diagonal can do it to any of them. It's partially because the reducers are designed for use with a shorter light path than the 2" diagonals.

#6 John Kocijanski

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Posted 22 July 2005 - 03:51 PM

But I have a question: If you desire higher powers than a 5mm eyepiece gives you, why not remove the focal reducer?
I used a focal reducer for many years on an 8" SCT, but I don't recall using an eyepiece shorter than 8.8mm when the focal reducer was in place.


I could but putting on and taking it off constantly would be a pain. I was just toying with the idea of having a larger field of view that would help me locate objects when I use my C8 on my Unistar mount. Actually using either a 1.25 inch diagonal with the focal reducer at f/6.3 with a 32mm ep or not using the reducer at f/10 with the BW Optik 30mm wide field work fine as far as ease in hunting down objects. I just wanted to give the two inch diagonal and the focal reducer a try. I tend to use my 7mm Xl with the C8 at f/6.3 for higher power views. The 5mm came out as I experimented with the setup using various eps. The 5mm doesn't really do well with my C8. Thanks for the advice.

#7 Joe Ogiba

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Posted 23 July 2005 - 02:38 PM

When I want a wide FOV with my C9.25 I use my 2" Televue Everbright with 40mm Rodenstock 70° AFOV eyepiece or my Rini 2" 35mm 68° AFOV short barrel eyepiece with Celestron f6.3 FR with any of my three 2" diagonals (TV,WO SCT,WO Prism) and still reach infinty focus.

Joe


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