C8 - Issues and Modifications

Motivation and Overview of the Modifications | Finder, Finder Shoe, Second Finder Shoe | 2" Zenith Mirror and its Delights | Preliminary Conclusions | Links

Motivation

Why did I make or had modifications made to my Celestron C8 tube and which ones? The best way to explain this is to look at the modifications themselves:

Replacing the 3" Losmandy rail with a Vixen rail (dealer)
> My Star Discovery mount and also the newly purchased AZ4 mount only accept the Vixen rail.
Replacing the fixed finder with a standard finder shoe plus a red-dot finder (dealer)
> This allows me to (1) remove the finder when storing the C8 in a tight bag and (2) use a finder of my choice, for example a simple red-dot finder.
Installing a second finder shoe
> This allows me to mount the finder in the upper left corner and thus look through the finder in a reasonable position; the original finder (and also the finder shoe mounted instead) is unfortunately located in the lower left corner when mounted vertically in my "one-armed" mounts (Star Discovery, AZ4).
Shifting the SC zenith mirror backwards (failed)
> This should allow the use of the SC zenith mirror that I had bought from a starfriend without any restrictions; without any displacement, the 2" eyepiece sleeve of the zenith mirror hits the focuser (this modification is only necessary if 2" accessories are used!).
Converting the C8 eyepiece receptacle from a SC thread to a 2" receptacle
> This 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 an bag or was taken out of it.
Conversion of the SC zenith mirror into a "pure" 2" zenith mirror
> This was necessary after converting the eyepiece receptacle from an SC thread to a 2" receptacle (alternatively, I would have had to buy a 2" zenith mirror).

I will not write anything further about exchanging the rail here. The other items will be discussed in the following.

Here, I present the contents of a small package, which my astronomy dealer Mr. Kloß sent me to find a solution for two of the issues listed above:

Photos: Package from Mr. Kloß, which was meant to solve some of my C8 problems (second finder shoe, moving the zenith mirror). At the bottom right the contents of the package: spacer ring to be inserted into the union nut with SC thread, Baader finder shoe.

Finder, Finder Shoe, Second Finder Shoe

The Celestron C8 is delivered with a fixed finder and, depending on the model variant, either a magnifying or a red-dot finder. This finder is not removable and does not fit in small bags. I therefore I asked Mr. Kloß, from whom I bought the C8 tube, to replace the existing finder with a standardfinder shoe and a red-dot finder. Mr. Kloß used a Baader finder shoe, the same type as I bought myself for my refractor some time ago. Regrettably, it did not fit the refractor, and so I gave the shoe together with some other stuff to Mr. Kloß. Now he could use it (or a similar sample) on my telescope!

When inserting the tube into the Star Discovery mount, I noticed immediately that the finder shoe is located at the bottom left (as with the Skymax models). By the way, it is the same with the manual AZ4 mount I bought later. This is, of course, not ideal! When using the Celestron StarSense module, I might even need a dual finder shoe like the one a starfriend bought for his Meade LX200 tube (where the finder shoe is also at the bottom). With the Explorer 150PDS, however, I managed to calibrate the StarSense module without a finder...

Left and center: the finder in its original position is located at the bottom left of the Star Discovery mount

The same applies to the AZ4 mount

At first, I had considered to glue the second finder shoe of my Rigel finder on the C8. But the plastic ring at the back of the tube is too narrow for the Rigel finder shoe, and I do not want to glue the shoe on the tube, because when inserting and removing the Rigel finder from the shoe, strong shearing forces can occur and bend the tube.

Finally, I asked Mr. Kloß whether certain screws on the tube were suitable for holding another viewfinder shoe. He confirmed this and sent me another Baader finder shoe (together with a distance ring for the zenith mirror - see above). I had no problems to screw this second finder shoe to the tube, and in this shoe the finder is in a more favorable position. However, if I want to use the StarSense module, either the finder or the module must be mounted in the lower finder shoe. Alternatively, I might use a dual finder mount.

The finder shoe next to the two screws for fixing the shoe

The two screws removed

Finder shoe in place and screws fixed*; to the left, the other finder shoe

Ditto, closer view*

Finder inserted into the new finder shoe for a test*

Ditto

Finder in the lower ("original") finder show

Finder in the upper, new finder show

Ditto with eyepiece

Photos: Mounting of the second finder shoe, finder in the new finder shoe

*) Note: Later, I turned the finder shoe around so that the finder foot fits the recess in the finder shoe (I had initially mounted it in the same way as my astro dealer had mounted it).

2" Zenith Mirror and its Delights...

As reported above and page Celestron C8 OTA Information (8" Schmidt-Cassegrain), I bought an SC zenith mirror from a star friend after a short time. This mirror collided, however, with the focuser on my "one-armed" mounts where the rail sits on the side. In addition, I found the screwing annoying when fixing or removing the SC zenith mirror. But it was necessary in order to store the C8 without a zenith mirror in its small bag. Thus, on the one hand, I wanted to move the zenith mirror a bit further back so that it does not collide with the focuser, and on the other hand, I wanted to find a solution that allows a faster mounting and dismounting of the zenith mirror. In retrospect, I have to admit that I did not choose the most direct way to arrive at satisfying solutions...

2" SC zenith mirror (bought later)

Ditto, you can see that the zenith mirror is not mounted horizontally, but a little oblique...

...because otherwise the tube for the eyepiece would touch the focusing knob; with 32 mm eyepiece and 2"-to-1.25" adapter

Ditto, larger

You can see from above that the focuser might hits the eyepiece tube of the zenith mirror if you mount the mirror horizontally.

Here, the focuser more or less hits the eyepiece sleeve of the zenith mirror - the SC adapter is only a little less than 4 cm long.

Moving the SC Zenith Mirror Backwards (Failed)

First, I I tried to find a way of using the existing 2" zenith mirror with SC thread at the telescope side (SC zenith mirror, for short) without any restrictions. I had the idea to insert a distance ring t into the union nut to gain some distance to the focuser. After I had discussed different distance rings with a starfriend, and found none in the city of Braunschweig, there was only one option left, namely an offer on the Internet, where you have to order 10 rings (diameter outside 5 cm, inside 4 cm, available in thicknesses from 0.5 to several mm). I therefore contacted my astronomy dealer Mr. Kloß regarding this and the "finder shoe" topic. He sent me a 4 mm distance ring, which I had already noticed on the Telescope Express Website. Contrary to the assumption of my starfriend, this ring could be inserted into the union nut without any problems, but the thread of the union nut did not hold any more - the ring was too thick. The following photos are meant to demonstrate this:

Regrettably, the 4 mm ring (on the left side) was too thick

Ring inserted into the union nut

The caliper measures 4 mm...

Thread of the union nut

Ring inserted into the union nut - not much thread remains...

SC thread at the Celestron C8 tube

The thread of the union nut is 6-7 mm long

With inserted ring, the thread of the union nut is only about 3 mm long

The SC thread starts only after 2-3 mm; therefore, the union nut does not get a hold when the 4 mm ring is inserted in to the nut...

This was in my eyes the end of this approach! Even if a thinner ring would establish a connection, there would only be a small amount of thread left to screw on. That would never create a safe connection!

Conversion of the SC Zenith Mirror into a "pure" 2" Zenith Mirror

Of course, you can gain more distance with a "real" 2" zenith mirror, which also has the advantage that it can be mounted and dismounted faster. But I had just bought a SC zenith mirror from my starfriend! So this mirror had to be converted into a "real" 2" zenith mirror to avoid buying a new 2" zenith mirror.

For this purpose, you can either buy a 2" sleeve with a thread for the zenith mirror, for example from TS, or the sleeve has to be removed from an existing 2" zenith mirror. After discussing various solutions with my starfriend, he had to convert a "real" 2" zenith mirror into a SC zenith mirror himself due to the sale of his old C8. That isa, he had to go the other way round... He still had the part he needed to do this, and after the conversion, he had a spare 2" sleeve that he sent me. So I was able to use it to convert my SC zenith mirror into a "real" 2" zenith mirror. The following photos demonstrate the conversion of my SC zenith mirror into a "real" 2" zenith mirror:

SC zenith mirror fromTS and 2" sleeve with thread

Ditto, right: Omegon 2" zenith mirror

Ditto

Protection cap on the side of the SC thread (with union nut) removed

SC receptacle removed

2" sleeve screwed into the zenith mirror

Now I have a "real" 2" zenith mirror from TS - and the remaining adapter for the SC thread with protective cover

2" Zenith Mirror at My Refractor

Since I was at that time not able to try out the converted zenith mirror at the C8, I first tested it at my PS 72/432 refractor and compared it with my Omegon 2" zenith mirror:

Omegon zenith mirror

Ditto

Ditto with 40 mm eyepiece

Converted TS zenith mirror

Ditto

Ditto with 40 mm eyepiece

Conversion of the Eyepiece Receptacle from SC Thread to 2" Receptacle

But now I needed a receptacle for the 2" zenith mirror on the C8! Since the discussion on this topic had lasted a little longer, my starfriend and I had already ordered an adapter from China, which would take the 2" zenith mirror at one end and could be screwed on an SC thread at the other end - so we thought! But the adapter's thread was too small - so I can "forget" this adapter (about 7 EUR) as well...

Photos: The "China" adapter (2" to SC thread, 5 cm long) fits only at the 2" end (see photo at the bottom left)

In the meantime, I found, together with my starfriend, two solutions:

a TS adapter (35 EUR, 48 mm long), which indeed does the job, or
a Baader ClickLock clamp for more than twice the price (75 EUR, 47 mm long), but also offering more comfort.

Both adapters would probably fit into the C8 bag as well, so that only the zenith mirror has to be removed, which is done quickly. I discussed with Mr. Kloß, which solution I should choose - and decided for the Baader ClickLock clamp. It arrived at my home shortly before the turn of the year 2019/2020, and I was able to install it without problems. From now on, it is meant to remain permanently on the tube, even when I pack it in its bag. It looks as if all my eyepieces get into focus with the clamp installed. Below are some photos of the conversion and the result! You can also see that the tube, together with the ClickLock clamp, fits into the bag!

C8 with Visual Back 1.25"

Ditto, closer view

Visual Back removed, you can see the SC thread

Baader 2" ClickLock clamp (with cap) screwed on

Ditto, side view - the focuser is shorter than the clamp

SC zenith mirror and sleeve for 2" connection

Sleeve for 2" cvonnection screwed into the zenith mirror - now it is a pure 2" zenith mirror

Baader 2" ClickLock clamp attached, cap removed

Ditto, closer view

2" zenith mirror (with 2"-to-1.25" adapter) attached to the Baader ClickLock clamp

Ditto, from farther away

Ditto with inserted 2" eyepiece

With the attached Baader ClickLock clamp, the C8 tube still fits into my tight bag, so that for stowing the tube in the bag and transporting it in the bag, only the 2" zenith mirror needs to be removed.

Preliminary Conclusions

I was able to find a good solution for the finder. The conversion to a "pure" 2" zenith mirror and Baader ClickLock clamp also seem to solve all "zenith mirror problems"! It furthermore looks as if all of my eyepieces come into focus with this configuration...

Note: In the beginning of November 2022, I sold my Celestron C8 to a star friend, because I had used it only rarely. I therefore can no longer report any experiences with the C8 here.