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New to EAA - Focal Reducer question - Celestron f/6.3 diameter 2"?

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#1 tedc99

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Posted 18 January 2021 - 11:52 PM

I just started reading about EAA a few weeks ago and specifically this forum. Thanks to all for the great information here!

 

Even though I have been observing for almost 35 years (my first telescope was a Meade SCT 4" I bought in 1986), I am still mostly a beginner as I haven't had time to do more than occasionally set up my scope and also to show to friends. I bought a new Celestron NexStar 9.25" three years ago, standard equipment including alt-azimuth mount. I've really enjoyed it but I am still just an occasional and inexperienced observer.

 

After spending much time reading in this forum, I have decided on getting an ASI385MC camera and also a focus reducer and using them on my NexStar. I think this is all I need to get started and, though I use an iMac as my primary PC, I have an old Dell laptop that I plan to use for EAA, at least to begin with. I thought I would get the more expensive (also considered the GSO 0.5x reducer) Celestron f/6.3 reducer but I am now confused by the listed diameter size for the Celestron. As my star diagonal and eyepieces are all 1.25", would I now need any other equipment to work with the Celestron reducer as it is listed at 2" diameter? The GSO is 1.25". I feel like this is something I should now as I have owned a telescope for 35 years but I am use to just attaching the 1.25" star diagonal (for both of my telescopes) and then using 1.25" eyepieces. 

 

Any info is greatly appreciated,

Ted



#2 jallbery

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Posted 19 January 2021 - 04:49 AM

The 2" on the f/6.3 reducer/corrector refers to its 2" SCT threads-- the same threads on the back of a C8.    To use the r/c, you take off the 1.25" visual back and screw the r/c directly to the threads on the back of the SCT, and then screw the visual back on the male threads at the back of the r/c, and use your 1.25" diagonal and eyepieces.

 

To use the r/c with the asi camera, you replace the visual back with the necessary adapters to create the optimal spacing.  I use an asi to Nikon adapter and then a Nikon t-ring on the celestron t-adapter, since I also want to use Nikon lenses with my camera.  


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#3 alphatripleplus

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Posted 19 January 2021 - 09:27 AM

Yes, I also use a T-thread adapter on the rear of the f/6.3 reducer to conver from 2" SCT thread to a M42 T-thread. Many CMOS cameras will directly attach to a M42 T-thread, and I believe the 385MC does.


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#4 tedc99

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Posted 19 January 2021 - 12:49 PM

Thank you both for the info! That makes more sense to me now.



#5 jallbery

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Posted 19 January 2021 - 01:41 PM

Yes, I also use a T-thread adapter on the rear of the f/6.3 reducer to conver from 2" SCT thread to a M42 T-thread. Many CMOS cameras will directly attach to a M42 T-thread, and I believe the 385MC does.

It does have t threads, but I believe it you attach it directly to the SCT prime focus adapter you won't have the optimal spacing and will also get less of a reduction.


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#6 ZepHead

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Posted 19 January 2021 - 02:07 PM

It does have t threads, but I believe it you attach it directly to the SCT prime focus adapter you won't have the optimal spacing and will also get less of a reduction.

I believe that is correct.  In order to use the Celestron F/6.3 reducer, you must have 105mm of back focus.  The 105mm is the distance from the FR's lens to the camera sensor and is typically obtained by a combination of the Celestron T-Adapter (50mm), plus 2 additional spacers (16.5mm and 21mm) that come with some ZWO cameras (like my ASI533MC).  The camera itself has a 17.5mm distance from the front to the sensor plate, so the total is 50+16.5+21+17.5 = 105mm.  This provides the optimal spacing and the correct reduction.


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#7 alphatripleplus

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Posted 19 January 2021 - 02:26 PM

Yes, if you want the f/6.3 reducer operating at exactly f/6.3 you'll need 105mm of backfocus and you'll need extra T-spacers to get there, and yes 105mm backfocus will minimize aberrations for the reducer. However, many EAA users will choose to operate at either more or less focal reduction than f/6.3, by varying the distance from the sensor to the reducer. Depending on the size of your sensor, you may notice more aberrations towards the edge of the FOV if you choose to do this. With a small sensor camera like the 385MC, you will have some latitude though before those aberrations are noticeable if you "play" with the nominal 105mm  backfocus. 


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#8 sparky103

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Posted 19 January 2021 - 06:06 PM

I sometimes use both the .63 and a .5 with a 385 with and without a diagonal on my cpc 11. It works fine


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#9 aiken999

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Posted 19 January 2021 - 07:02 PM

I have the evolution mount and 9.25 scope for about 4 years now. I also have the asi 385 with nightowl .4 focal reducer.

Connection of the camera is : celestron 2 inch visual back, nighowl, 25 MM spacer ,385. Never tried the .5 focal reducer.  I have the .63 reducer and used it with the 1.25 diagonal without issue as well. I wanted a wider field of view and shorter focal length and simpler straight through connection.

You need the 2 inch configuration to get the widest field of view. With this setup i just get enough clearance to prevent bumping into the mount. I went this route rather than a 2 inch diagonal to simplify the setup. Works great.


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