C5 - Issues and Modifications

Motivation and Overview of the Modifications | Finder, Finder Shoe | Conversion of the Eyepiece Receptacle from SC Thread to 2" Receptacle with Baader ClickLock | 2" SC Zenith Mirror at the C5 - with and without Reducer/Corrector | Preliminary Conclusions | Links

On this page, I discuss some issues that appeared initially with my Celestron C5 tube, as well as some modifications to the original C5 tube and the zenith mirror.

Motivation

Why did I make modifications to my Celestron C5 tube and which ones? The best way to explain this is to look at the modifications themselves:

Replacing the fixed finder with a standard finder shoe plus a red-dot finder
> This allows me to (1) remove the finder when storing the C5 in a tight bag and (2) use a finder of my choice, for example a simple red-dot finder.
Converting the C5 eyepiece receptacle from a SC thread to a 2" receptacle
> This allows to use not only 1.25" accessories but also 2" accessories and should speed up and simplify the mounting and dismounting of the 2" zenith mirror (compared to an SC connection...), for example when the tube is to be stored in a bag or was taken out of it.

Finder, Finder Shoe

The C5 OTA comes with an erecting 6 x 30 finder scope, but the 5SE version with GoTo comes with a red-dot finder. I prefer the red-dot finder and, as with the C8, replaced the finder shoe and finder. This time, however, I did it myself.

C5 tube with supplied finder holder

Finder holder remove, the TS-Optics finder shoe is ready for attaching

TS-Optics finder shoe temporarily attached; the re-dot finder is ready

TS-Optics finder shoe temporarily attached with plastic screws from the supplied finder holder

The red-dor finder cannot be mounted correctly, because the screws prevent this

First, I tried to mount an available TS-Optics finder shoe with "long holes". However, the screws of the supplied finder shoe were too short and did not grip. The reason was that the screw heads were too large and therefore the screws did not fit into the provided recesses. Even if the screws would have been long enough, I would not have been able to slide the finder foot over the screw heads that stood too high, though. As a test, I removed two plastic screws from the supplied finder holder and using them I was able to screw the finder shoe on the tube reasonably tight. Here, however, the screw heads protruded even higher, so that I could only push the finder halfway in. But for my first tests this was sufficient!

In the end, I was not satisfied with this "temporary" solution, and tried to fix the finder shoe with longer metal screws, which I took from the C8. However, this did not work properly, because the screw holes in the tube are located so far at the edges of the long holes in the finder shoe that I might have "choked" even these longer screws only with effort - at the expense of the threads - into their holes ... In addition, the screw heads were again too large and did not fit into the recesses. So I finally decided to "borrow" one of the two Baader finder shoes from the C8.

Baader finder shoe at the C5 - in my opinion mounted wrongly (my astro dealer had mounted it on my C8 tube this way...)

Baader finder shoe at the C5 - mounted the other way round

Red-dot finder at the C5 - now the finder foot fits the recess in the finder shoe

This shoe fit right away and even with the short screws (the long ones went back to the C8...). I attached the "long-hole" finder shoe provisionally to the C8, but also ordered another Baader finder shoe for replacement, which was installed in the meantime.

Conversion of the Eyepiece Receptacle from SC Thread to 2" Receptacle with Baader ClickLock

By default, the C5 comes with the 1.25" visual back and 1.25" accessories, and from what I understand, the light path is so narrow (to be precise, the rear baffle has a diameter of 25 mm; that of the C8 has one of 38 mm) that the C5 is indeed only suitable for 1.25" accessories (up to about 25 mm eyepiece focal length at a FOV of 70° - or a 32 mm Plössl eyepiece).

In astronomy forums, I came across discussions in which the question was raised, whether 2" accessories can be meaningfully used on the C5. In the end, I have not been able to find a clear answer - actually it is "no". Despite this answer, some posters decided to buy 2" accessories. So I thought I would give it a try myself, since I have 2" accessories available from my C8 (and from my refractors). However, I failed after only a few turns: the Baader ClickLock adapter cannot be screwed onto the SC thread at the tube, because the locking lever of the adapter bumps against the focusing knob of the C5.

After an e-mail exchange with a Baader employee, I took another run (January 5, 2021), unscrewed the locking lever with quite some force, and screwed the ClickLock adapter onto the C5. Since the adapter was in a favorable position afterwards, I was also able to screw the locking lever on again (at least, this time...).

Locking lever screwed off

ClickLock open (locling lever screwed on again)

ClickLock "locked"

I tested my three 2" eyepieces (26 mm, 35 mm, and 40 mm) with the Omegon 2" zenith mirror on the C5 during the day and could not find any noticeable vignetting. I also observed that the FOV got larger with longer focal lengths. At night, I expect an even smaller effect. Only the 26 mm eyepiece showed orange reflections at the edges of the field of view, whose origin is unknown to me. Nevertheless, it showed less distortion and a more pleasing image than my TeleVue 24 mm eyepiece with 1.25" mount.

26 mm eyepiece (2")

35 mm eyepiece (2")

40 mm eyepiece (2")

After this result, I decided to purchase another ClickLock adapter for the C5. In the meantime, it arrived, and I installed it. I did an initial test at night with this one on January 10, 2021, and I did not notice any vignetting with the 26 mm and 40 mm eyepieces. I also took photos of the views that some of my long focal length eyepieces delivered on January 18, 2021. These confirmed that the FOV became larger with increasing focal length of my eyepieces. On the other, a test perfomed on the following day showed that the FOV is more or less then same, regardless of whether you use 2" accessories or 1.25" accessories together the with Celestron f/6.3 reducer/corrector. For details, see page C5 - Two Inch Accessories - Yes or No?.

2" SC Zenith Mirror at the C5 - with and without Reducer/Corrector

In order to try a 2" SC zenith mirror on the C5, I "rebuilt" the 2" zenith mirror from TS-Optics that I had originally purchased as an SC zenith mirror for the C8, but then converted to a 2" zenith mirror; rebuilding was very easy. The first thing I did was to try it out with the f/6.3 reducer/corrector, because I wanted to see whether there were any differences from the other zenith mirrors, since the light path is a bit shorter with the SC mount. And indeed, the vignetting seemed to be a bit more pronounced here to me with the 35 mm and 40 mm eyepieces. So this is not an option! Since the reducer/corrector moves the zenith mirror further back, there was also no collision between the zenith mirror and the C5's focuser.

Afterwards, I tested the SC zenith mirror with 2" eyepieces also without the reducer/corrector on the C5. Surprisingly, there was no collision between the focuser of the C5 and the SC zenith mirror, while there had been one with the C8.


2" SC zenith mirror mounted to the reducer/corrector, 26 mm eyepiece	2" SC zenith mirror directly mounted to the C5; the focuser is located next to the zenith mirror, but does not touch it.	Ditto

2" SC zenith mirror directly mounted to the C5; 40 mm eyepiece	For comparison purposes: 2" zenith mirror mounted via ClickLock	Ditto

2" SC zenith mirror directly mounted to the C5; the focuser is located next to the zenith mirror, but does not touch it.	Ditto (somewhat fuzzy)	Ditto (somewhat fuzzy)

To cut a long story short: From a purely mechanical point of view, the C5 can be operated with the 2" SC Zenit mirror from TS-Optics!

The results with the three 2" eyepieces were similar to those with the other zenith mirrors - perhaps the vignetting was slightly stronger than with these. In any case, much better than with reducer/corrector! For details see page C5 - Two Inch Accessories - Yes or No?.

Preliminary Conclusions

I was able to find a good solution for the finder. In addition, the Baader ClickLock with 2" receptacle allows using 2" accessories at the C5, even though it does not seem to be built for this (different sources report different opinons on this...). This is, of course, good news.

On the C5, a normal SC zenith mirror for 2" accessories could also be used instead of a 2" zenith mirror (via ClickLock clamp). Unlike the C8, the SC zenith mirror does not collide with the focuser at the C5, which I would have expected. But the ClickLock clamp is, of course, the more convenient solution...