Celestron C11 XLT EdgeHD

The Celestron C11 XLT EdgeHD at InFINNity Deck.
Figure 1: The Celestron C11 XLT EdgeHD at InFINNity Deck.
For planetary work I wanted something with large aperture (higher resolution) and long focal length (more magnification). Initially InFINNity Deck was equipped with a Celestron C11 XLT Carbon (279mm aperture, f/10), of which an used one was available at a decent price. In order to keep the C11 parallel to the SkyWatcher Esprit 150ED I modified the ADM dovetail clamp and the C11 dovetail. The ADM clamp (see figure 4) now allows for adjustment in declination direction, while the dovetail (see figure 5) allows this for right ascension.
I soon found out that the XLT suffered from that much mirror-flop and mirror-shift that it became rather difficult to keep objects within view using both the Esprit and the C11. Being equipped with a mirror-lock system I was hoping that the C11 EdgeHD (also 279mm, f/10) would do a better job, so the XLT left and was replaced by a used EdgeHD.

Cleaning a SCT
Having been used in a damp observatory, most of the screws of the EdgeHD were rusting, while two spiders and some dust had found their way in. I cleaned the scope using the cleaning instructions from this video, but instead of cotton wool I used Kleenex wipes (as advised by Baader Planetarium) using the same method as shown in the video (please note that the Kleenex should be colourless and odourless and that a lot should be wasted).


The Celestron C11 XLT EdgeHD after being cleaned and provided with new nuts and bolts.
Figure 2: The Celestron C11 XLT EdgeHD after being cleaned and provided with new nuts and bolts.
Replacing EdgeHD bolts
For those running into the same issue, after a few days of (re)search I managed to find out what was used on my EdgeHD:
- finder scope: 5/32" BSW x 1/2";
- dovetails: 3/16" UNC x 1/2", 3/4" and 1";
- front and back ends: M4 x 12mm (screw with loose nut on the inside!);
- corrector plate: M3 x 17mm;
- air vents: M2.5 x 8mm.
The dovetail screws thread into a small aluminium plate that is fastened with a central countersunk M4 screw (also 12mm in length and secured with a nut on the inside). So when replacing them all without taking the corrector plate off one should mind that there should always be at least one screw in place and that, provided that it does come out, the central M4 screw will loose its nut.
The same accounts for the M4 screws that hold the front and rear ends to the tube. Those screws have a loose nut on the inside as well and replacing can only be done when the corrector plate is removed as they do not thread into the tube, only in the nuts.
Altogether it is a bit of a mishmash of bolts and nuts. Two of the finderscope holes had been widened to 5/32 UNC. Both BSW and UNC have the same pitch, but slightly different diameters and top angle (UNC being 0.2mm larger in diameter). Being made of aluminium, it is not too much of a problem to get the UNC bolts into the BSW thread. It takes a bit of force and several attempts assisted by some grease, but in the end the UNC will run smoothly in the former BSW thread (using a tap would be better of course). It appears that most imperial threads on the C11 have originally been produced as BSW, but replaced with UNC bolts.


Celestron C11 EdgeHD Mirror-flop

Mirror-flop of the C11 EdgeHD as seen on front of a collimator.
Figure 3: Mirror-flop of the C11 EdgeHD as seen on front of a collimator.
Soon after the EdgeHD was installed I found out that mirror-flop was not much better than with the XLT. As it was also causing collimation problems I made a collimator out of a 12" SkyWatcher Newton to further investigate. The collimator clearly showed that turning over the scope around its optical axis caused collimation to completely go astray. I contacted Celestron who told me "It does sound like the unit is suffering from mirror flop. Unfortunately there is nothing one can do at home to remedy this. This unit would need to come in for a repair." Having bought it second hand left room for a repair, so the unit went back to Celestron.
To cut a long story short: after several weeks and spending some 1700 euros on repair (US$1273.-) and shipping (€250 for a flight case, €250 shipment to the US) the C11 came back showing exact the same issue as before (see figure 3), and that while I was assured that "We will to make sure your telescope is returned to factory specifications." After commenting on this they replied "I am sorry about the issues you experienced." and refunded only US$365 as the rest (US$908.-) was spent on the shipment back to the observatory.
Needless to say that this was rather disappointing, but at least we now know what Celestron's factory specification for mirror-flop is. The plan is now to dismantle the C11 next autumn (2020) to see if I can reduce the mirror-flop to an acceptable level. As the EdgeHD is used in combination with a MoonLite focuser, I hope mirror-flop can be reduced by limiting the play of the baffle tube. The fear is, however, that the mirror sits loose on the mirror support, which is a more far-reaching repair. Until then I collimate the scope on a star close to the planet I wish to image, so that at least for that imaging session the collimation is fair.
To be continued...


If you have any questions and/or remarks please let me know.

The modified ADM dovetail clamp, the brass H-shaped notch for adjustment in declination direction.
Figure 4: The modified ADM dovetail clamp, the brass H-shaped notch for adjustment in declination direction.
 
Two pushing set-screws in the ADM dovetail allow for adjustment in right ascension direction.
Figure 5: Two pushing set-screws in the ADM dovetail allow for adjustment in right ascension direction.

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