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Some Useability Improvements for Celestron 8” EdgeHD and Similar Telescopes

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Some Useability Improvements for Celestron 8” EdgeHD and Similar Telescopes


Tom Fowler


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.  Perhaps the best part about the improvement is that they do not utilize any parts made in China!


First, let’s start with the focuser.  Over several years I’ve experimented with electronic focusers, one by Orion and the new one by Celestron.  They work, and the Celestron focuser has the advantage that it allows you to record the position of the focuser.  However, I always preferred the feel of directly controlling the focus with my fingers, and I found that having to rely on a separate control pad or several commands on the hand control to even get to the focus panel made the whole process very cumbersome.  So I decided to try the FeatherTouch focuser, model FTM-CPC8, which fits the Celestron CPC800, NexStar 8SE, 8” Edge HD, and some other scopes.  See Figure 1.  This focuser is rather expensive at $315, but in my experience with it, definitely worth the money.  Plus, made in USA!


Figure 1.  FeatherTouch Focuser for SCT


The focuser is extremely smooth, with the fine focus knob making very small adjustments possible.  The general impression is that it makes focusing the scope very refractor-like, as opposed to the somewhat clumsy focus knob that comes with the scope.  I found it very easy to get extremely good focus on all objects, better than ever it seems.  Everything appears sharper than it did before—whether that is an illusion on my part or not, I cannot say; but the ability to do the kind of fine focus that this device enables is a huge improvement and makes using the scope a real pleasure. 


Installing the focuser is very easy. It comes with a CD-ROM with complete instructions.  You just need to remove a screw and retaining washer, unscrew the old focuser shaft, screw on the new one, and screw down a plate.  Total time is about 10 minutes. 


I did one other thing which may be of interest.  I found the most eyepieces focus in a fairly narrow range of shaft rotation.  I copied a 360 degree circle from the Internet, printed it, laminated it with clear package sealing tape, and attached it to the back of the SCT around the focuser (see Figure 2).  Then I picked one eyepiece as the standard (a 40mm Pentax XW, but any one would do), focused it, and marked the 0 degree point on the black focus knob with a piece of tape.  Then I took other eyepieces and focused them, noting how many turns (usually less than one) were needed relative to the standard, and the final degree position on my circle.  This I recorded so next time I can just dial in the focus.  Same can be done for cameras or other accessories. 


Figure 2.  360 target for recording focus position


This works well, as well as the numerical value on the Celestron electronic focuser, which always varied a bit from one session to the next. Note that FeatherTouch makes similar focusers for other SCT models, so you can look up the one for your scope. 


Next I worked on the diagonal problem.  With the SCT design, unlike refractors, there is an optimum position for the eyepiece (or any device), one which gives the flattest field and least distortion.  Celestron gives this for the EdgeHD as 133.25 mm from the back of the projecting threaded shaft.  (On this scope, Celestron is emphatic that the large black nut on the shaft should not be removed.  On the C9.25 and C11, the design is different and the nut can be removed).  The trick therefore is finding a configuration to replace the screw-on diagonal that is close in overall light path distance to the optimum 133.25 mm.  There are several screw-on visual backs available that allow you to use standard 2” accessories such as diagonals, but they tend to be rather long.  Celestron even sells one, about 2” long.  These I found were all too long, i.e., they made the optical path much longer than 133.25 mm.  Then I hit upon the Lumicon Short adapter, # LA1066, by FarpointAstro.com.  See Figure 3.  This allows for a shorter opticaI path length. 


Figure 3.  Lumicon Short SCT Visual Back Adapter, With Larger Thumbscrews


I purchased one from B&H photo, about $20, also made in USA!.  This works right out of the box, though I did replace the included 8-32 thumbscrews with larger ones, available for about $1.50 from Home Depot (must be ordered, 8-32 x 3/8). This works well, attaches easily, and holds a diagonal firmly.  Just one problem: it is so short that the nose on many diagonals is too long to fit completely into it!  However, that problem is fixable as well.  I use a Baader 2” BBHS click lock mirror diagonal (made in Germany), which has a short nosepiece.  This fits perfectly and goes all the way into the visual back, and is a superb piece of equipment in its own right, especially the click lock feature.  See Figures 4 and 5.  This gives an overall optical path length from the end of the projecting shaft to the top of the diagonal of about 136 mm, 2.75 mm greater than the optimum.  Interestingly, the Celestron screw-on diagonal gives an optical path length of about 128 mm, or -5.25 mm from optimum, so the Baader configuration is actually closer to the optimum.  Whether this is reason for the improved performance, or the better optics of the Baader, I cannot say.


Figure 4.  Comparison of Diagonals Showing Baader on Left with Short Nosepiece, and Standard Diagonal on Right with Longer Nosepiece



Figure 5.  Baader Diagonal in Lumicon Short Visual Back


I tested the overall configuration by turning the diagonal 90 degrees so that it was parallel to the ground, and inserted a heavy eyepiece (TV Panoptic 41mm).  Everything held quite firm.  I was afraid that the visual back might unscrew, but it did not.  And the clamps on the visual back held the eyepiece well.  On slews I never had any problems, which I often did with the screw-on diagonal.  The only issue with the shorty visual back, as you can see from Figure 5, is that you cannot rotate the diagonal more than about 30 degrees to the right before it hits the focuser.  I never found this to be a problem—if I needed to rotate the diagonal, I just did so to the left. 


Of course, if you are going to use the Celestron 0.7x focal reducer, you will have to remove this visual back.  But that reducer is exclusively for imaging, so you won’t be using a diagonal either. 


One caveat: Celestron claims that there is some tolerance on the optical path length, about 20mm.  I did not attempt to test this. 

  • king1522, Jon_Doh, bcloud and 9 others like this


My own improvements:
Baader Clicklock visual back
Baader UFC filter slide
Baader 2" diagonals with clicklock
Celestron shield
Reflectix on both OTA and shield
    • beammeup, Lem Jettison, AJK 547 and 2 others like this

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!

    • edl, king1522, donstar and 3 others like this

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...

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. 

    • mac1969 and Old Speckled Hen like this

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.

    • REC and tikiwonder like this
Old Speckled Hen
Jun 27 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)

Jul 01 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.



Jul 01 2020 06:18 PM

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

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.

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.

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.





    • bushatz likes this

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.

    • donstar likes this
Jul 09 2020 10:35 PM

My imaging is done remotely I love my feathertouch focuser.

    • radix655 likes this

Well this raises a bunch of questions I was trying to get answered on the Cats and Casses about the upsides and downsides of these scopes. 

If I buy a new one of these are you saying they don't come with the ability to focus well or to hold their eyepieces?  They seem to be exactly what I want but not if they don't focus unless I buy other, more expensive, things 


Or am I missing something?

Well this raises a bunch of questions I was trying to get answered on the Cats and Casses about the upsides and downsides of these scopes. 

If I buy a new one of these are you saying they don't come with the ability to focus well or to hold their eyepieces?  They seem to be exactly what I want but not if they don't focus unless I buy other, more expensive, things 


Or am I missing something?

Hello. I've had a CPC 800 for about 8 years now, and it focuses fine and holds EPs fine. I, too, added a Feathertouch focuser, but I don't think you absolutely have to. Lots of us astro junkies just always like to fiddle with our scopes and equipment. And if you go with a CPC version of an 8", you certainly do not have to change the diagonal in order to hold eyepieces correctly. Yes, a screw-on diagonal is not a great design, but as one responder here noted, it's more of an issue with equatorial mounts than alt/az mounts like the CPC. Even if you go with an equatorial setup (8" OTA on a GEM), there are lots of options for changing the diagonal to avoid the loosening problem that screw-on diagonals have. (Click-lock visual backs, for example.) And relative to the cost of the scope, diagonals are not that expensive and can have the added benefit of improving observation f you select a good quality one.


Bottom line: get into the game! You'll find yourself tweaking your equipment as you get more familiar with it and with what you want to do with your scope. But be forewarned: this is a hobby that has a way of becoming quite consuming!


Clear skies,



For those that have used a focus motor (any brand) AND a Feather Touch manual, which is do you like best? 


I understand it comes down to personal preference but wanted to get some feedback because I am debating which one to purchase. 



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