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Some Useability Improvements for Celestron 8” EdgeHD and Similar Telescopes

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#1 tom_fowler


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Posted 22 June 2020 - 10:19 AM

Everyone who has worked with the Celestron 8” EdgeHD telescope, and I’m sure other 8” SCTs, knows their useability problems: (1) the scope is difficult to focus, and (2) the screw-on diagonal provided by Celestron is difficult to tighten enough to prevent eyepiece rotation, and when it is tight enough, very difficult to loosen. I worked on these problems for some time, and have come up with what I believe are very worthwhile improvements.

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#2 sanbai



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Posted 22 June 2020 - 01:18 PM

My own improvements:
Baader Clicklock visual back
Baader UFC filter slide
Baader 2" diagonals with clicklock
Celestron shield
Reflectix on both OTA and shield
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#3 demorcef



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Posted 23 June 2020 - 10:43 AM

Another suggestion. Get a inexpensive astro camera (low resolution is fine) and plug it up to your laptop and place it on a table to the side of the telescope. Then you can sit on the other side facing the tube and very conveniently adjust your secondary collimation screws and watch the results in real time. It takes all the guess work out of it!

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#4 AJK 547

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Posted 24 June 2020 - 11:39 AM

Nice review Tom!


I recently added the FeatherTouch focuser to my C8-A and C6 Evo, and along with the ability for easy ‘best’ focus... the image shift during focusing was significantly reduced!  Very Happy!  Starlight Industries also sent me machined spacers (free of charge) for both installations to ensure enough back focus for my Denk II BV.


i went a similar route as Sanbai with a Baader T2 VB and Clicklock, Baader Zeiss BBHS prism and BBHS Sitall Mirror diags.  The Astrozap (heated) dew shield with a 4 channel Thousand Oaks controller (pulsed) has also been a a real treat in eliminating dew on the correctors of my SCT’s during these humid summer nights in Chicago...

#5 RichA


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Posted 25 June 2020 - 11:18 PM

I don't know anyone who has been around in amateur astronomy from the 1980's onward who would use a screw-on 2" diagonal on an SCT.  They are and always have been, nightmares.  Not only do they unscrew at a touch, they are a pure pain to use if you rotate the diagonal much to improve your viewing angle.  With heavy eyepieces, they are an accident waiting to happen.  The normal practice is to use a 2" tube, permanently threaded onto the SCT and a standard, smooth tube diagonal so that the rotation is easily achieve by loosening the 2" bore-tube set-screws or twist-lock. 

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#6 earlyriser


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Posted 26 June 2020 - 08:37 AM

I bought a Baader T-2 90º prism diagonal for use with my C8.  It has a helical focuser that has turned out to be a great feature to have with an SCT.  Now, I just get it close with the SCT focuser, and fine tune things with the helical focuser. It's a much bigger help than I imagined when I bought it.

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#7 Andrew Brown

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Posted 27 June 2020 - 07:12 AM

Same as earlyriser I too use a baader helicoil focuser  on a 1.25 TE diagonal.


(For straight through however I also have a motorbike handlebar rubber grip the end of which has 3 different sized concentric round flats so that gives me 4 different focusing ratios, or 5 if I use the clothes peg, all for UK £1.85)

#8 SeymoreStars



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Posted 01 July 2020 - 06:17 PM

I hope this doesn't violate CN rules. This adapter from AP is perfect for Celestron SCT's C11 & C14.

Works like a charm and allows the use of "normal" 2 inch diagonals and stuff.



#9 SeymoreStars



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Posted 01 July 2020 - 06:18 PM

Oh well the link was removed, but astro-physics makes an adapters that are great for various SCT's

#10 Farzad_K


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Posted 02 July 2020 - 09:12 AM

I bought my C8 Edge HD in 2017 and I recall I wanted it t image the moon. Then of course I started to use it for imaging deep sky and  can agree the scope is a handful. I also changed the stock focus knob with the feather touch and it is a major improvement. Then I updated that one to the focus motor by Celestron and I am on my second one now, the first one went bad in less than a year. I think it is because of gear backlash that sofware like SGP and NINA have a hard time auto focusing the scope.

#11 coopman


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Posted 06 July 2020 - 11:00 AM

I use a screw-on SCT diagonal with my Evolution C8HD. It's functional on this alt-az mount because I never have to change the angle of the diagonal. I agree that it would be a PITA on a GEM though.

#12 jrbarnett


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Posted 06 July 2020 - 08:53 PM

Nice write-up.


I pretty much agree with everything you've written, though I am 100% visual so my motivations are a little different.


My scope is a C9.25 Edge HD.  There are a couple of oddities regarding the C9.25 EdgeHD model not affecting the other Edge HD models.  Like the standard C9.25, the Edge version uses a slower primary mirror (f/2.3 vs. f/2 in the C8).  In the Edge version this moves the two-element corrective optical group way back in the baffle, almost to the rear aperture.  This means that you have to be EXTREMELY careful in making changes to visual back and diagonal.  More on this in a second.


Another quirk of the C9.25 Edge is the aperture of the rear opening.  It's big.  Much larger in diameter than the opening on the C8, C8 Edge and standard C9.25.  In fact, it's the same diameter as the rear opening on the C11 and C14.  That alters your aftermarket visual back options a bit.


Given these two C9.25 Edge HD oddities, I went with the Astro-Physics 2.7" visual back.  This unit uses 3 equidistant set screws and a compression band.  On a C11 or C14 it works GREAT as there's plenty of room on the large rear casting.  On the C9.25 Edge there's less room so you need to be a little creative when attaching the VB and tensioning the set screws due to the close proximity of the focuser knob (a Feathertouch microfocuser in my case).


The other accommodation you need to make with a C9.25 Edge with a very low profile aftermarket visual back is on the diagonal front.  The nosepiece MUST be short or else you will shatter your corrective optical group inserting the diagonal.  I use the Baader Maxbright with clicklock.  It has a sub-1" nosepiece and just avoids hitting the corrective optical group when fully inserted.


After years of messing with dew strips, batteries and all the cables and clutter than go with them when using a catadioptric I now use a passive insulated dew shield instead - a Dew Master by Anacortes.  It's all I need in all but the dewiest of conditions and it gets rid of the fuss and clutter of active systems.





#13 groupoid


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Posted 09 July 2020 - 10:22 PM

I've been thinking about FT, but each time I feel that it's too expensive and doesn't really help with image shift or shake. However, I haven't used it in person.


Instead, I'm using a JMI motofocus with a controller, very simple setup which allows to focus without touching the telescope. It shows on CN sometimes, well under $100, but does not seem to be available now.


For the diagonal, I have negative experience with SCT diagonals, especially in winter. I'm currently using Baader clicklock visual back and Celestron twistlock diagonal (in the "refractor" mode). Very robust setup that doesn't involve any thumb screws, easy to rotate, etc.


You can also get a Celestron motor, but then it will not fit in the C8 soft case that I'm using, and you will be tied to Celestron mounts (or a laptop). I prefer simlipicity and independence, even though I'm using a CGEM.

#14 SeymoreStars



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Posted 09 July 2020 - 10:35 PM

My imaging is done remotely I love my feathertouch focuser.

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