Product Description
We received a small vintage "treasure": a Celestron C5 white,

which wonderfully illustrates the product development at Celestron. More on that below...

Problem:
The customer of this Celestron C5 asked us to repair it because he was having difficulty focusing on objects he was observing. On the workbench at the artificial star, it became clear: The secondary mirror embedded in the Schmidt-C5 could no longer be adjusted using the three outer adjustment screws. This is necessary to adjust the mirror to the correct angle and distance from the primary mirror, thus ensuring focus and sharpness.

After unscrewing the Schmidt-C5 retaining ring, the secondary mirror was no longer at the correct angle to the primary mirror, which is still very important for older Celestron telescopes.

A brief excursion into Celestron product history:
As mentioned, this small Celestron C5 demonstrates the continued development of Celestron products. With this design, the Schmidt plate rests loosely on the opening with approximately 2-3 mm of play relative to the tube. Only the screw-on metal ring holds the plate in place. There are no adjustable spacers, cork pads, or other holding measures. However, this retaining ring must not be screwed on too tightly, as this will cause tension on the plate. A good feel is required here.

Optical grinding processes have also improved a lot, so today it's no longer necessary to pay attention to the angular position of the secondary mirror and/or Schmidt plate.

Solution:
Before you begin; we can't get tired of mentioning this: Wear gloves!

The small, almost dainty C5 in particular doesn't offer much surface area for safe handling around the Schmidt plate, unlike its larger "brothers."

When unscrewing the Schmidt plate retaining ring, please remember that it only rests loosely. Therefore, the plate must always be held firmly by the mirror adjustment plate in the center! Then, tilt the retaining ring past the two upper and lower metal pins and set it down – never let go of the plate here either!

Afterward, carefully remove the Schmidt plate and place it with the mirror sleeve facing down on a soft, lint-free surface.

Now loosen the three upper screws from the secondary mirror, and the secondary mirror will fall out (hence the soft, lint-free surface).

Tip: Try to prevent the now loose screws from falling out of the housing. Putting them back in place is quite fiddly.

A look inside revealed nothing suspicious, but on the holding side of the secondary mirror, the inner component that keeps the secondary mirror at a distance and at the same time allows play, so that the three screws can be tightened or loosened, allowing adjustment, was missing in the middle.

Alternatively, you can insert a suitably large/small sphere or similar object into the recess. Celestron Schmidt plate and secondary mirror. Important: It should be made of hard, rust-proof material and not too high.
Then, assemble and screw everything together properly. It is important to ensure the correct alignment of the markings on the secondary mirror with the Schmidt plate on the tube. On most smaller Celestrons, the 3 o'clock marking is correct. This can often be determined by the (factory-applied) marking and number on the secondary mirror with the engraved number and marking on the Schmidt plate – these must all be aligned.

If this is not the case, as in this case, you can loosen the Schmidt plate by gently turning the mirror sleeve at the bottom and the adjustment plate at the top in opposite directions, thus rotating it into the correct position.

Please only tighten it slightly, just so that the plate is secure, as otherwise unnecessary tension can occur here as well.

In the end, the serial number, the logo, and the Celestron logo on the outside of the tube must be legible horizontally, and all the markings must point past 3 o'clock.
 

Despite their age, the telescope and optics were in clean and good condition.

A word about cleaning a Schmidt plate:

A Schmidt plate rarely needs cleaning. Dust particles or small streaks do not affect the optics!
The glass surface of the plate is coated! This coating reacts sensitively to mechanical and chemical stress.
We only clean Schmidt plates in absolute exceptional cases!
If you absolutely want to clean the plate, please strictly observe the following points to avoid causing permanent damage to the plate:

Do not allow (skin) oils or acids to come into contact with the plate - wear gloves,

human skin is a carrier of oils and acids!!
Never use household cleaners - no glass cleaners, dish soap, or similar.
Use Optical Wonder and a lint-free microfiber cloth.
Do not apply pressure to the plate.
Never wipe away particles, always blow them away.
Do not blow with your mouth, only with a bellows.
Pay attention to the markings and retaining plates to ensure they are returned to their original position.

Conclusion:
And lo and behold, the telescope was properly aligned with the artificial star on the workbench.
We wish you many exciting nights of observation.