C8 Collimating and Corrector Plate Question
My name is Chris. I live in Bellingham, WA. I'm fairly new to all this. I was given a 114mm/1000mm Meade reflector from an acquaintance a while back. About 3 months ago, I got that cleaned up and collimated as best I could. The issue was with the tripod. An abomination, I'm sure. Teeth of the RA stripped out around about a third of the round. I really enjoyed the small successes, but the issues with the tripod were to much to bear. I digress, and long story short, I discovered that a decent tripod was going to cost a bit of money, so...
I've recently purchased this C8 from a man in Enumclaw, WA. Everything seems to be in good working order, but I'm needing some help with collimating the scope, especially regarding the corrector plate and secondary mirror housing.
I've followed stringent procedures for cleaning the plate, primary and secondary. All that looks great compared to the state it was in when I purchased. I've attempted to collimate the secondary with good results, but I'm sure something's not 100%.
Besides the 3 adjustment screws on the secondary mirror housing, does anything else need to be done to the corrector plate? I marked the plate and replaced it in the same position as before the cleaning. The picture I'm showing is from the "before" cleaning pictures (there was debris on the inside of the corrector). The recess for the plate is in fairly tight tolerance - not much room for the plate to shift in any direction - perhaps 1/16" at most. I attempted to put the plate back in that position, although there was no shimming that did that originally... again, can barely move the plate around in the recess.
The secondary mirror housing is snugly screwed onto the corrector plate.
I've noticed that when I secure the plate in place with the corrector ring, the secondary housing can still spin counter-clockwise and clockwise (this is NOT turning the entire secondary mirror-already verified that). Using a collimating cap, I've noticed that this rotation of the secondary housing effects the centering of the primary looking through the collimation cap. Is that supposed to be the case on this scope? If I turn the secondary housing enough, either clockwise or counter, it eventually gets tight and feels really solid, but the centering of the primary is usually not at its best.
I was able to get things fairly well collimated at some point, but I may not have done it correctly. I realized later that I needed to go all the way through all my eyepieces (32/25/15mm + 2x Barlow). This may have affected my use of the 15 and Barlow since I may not have gone all the way up to that point. Is that true?
The other question I have is about the perpendicular aspect of the corrector plate. Is there any recommended adjustment to that aspect of things? I ask because I noticed that I could see some of the side of the secondary mirror housing when looking through the collimation cap. With the plate securing ring off, I carefully checked if that changed if I tilted the plate out of its recess. It affected and could correct some of that discrepancy. Is this type of adjustment necessary? Does it indicate a major issue with the scope (hope not)?
There is a small dent in the tube. Seems minor to me. Not a major dent at all. Also noticed that there are two empty screw holes in the gray main corrector plate housing which attaches to the orange tube. Are there supposed to be screws there? I notice that the housing is very secure to the tube and cannot be moved. The screw holes of the gray ring and the tube are perfectly lines up as well.
Ever since I was a kid, I wanted a telescope. I'm hoping I can get this working to the best degree possible. Any advice? Videos? Forum threads? I've done a lot of research, even before I bought this scope. I've watched countless YouTube videos. Sometimes Google just doesn't get you where you need to go. Any help is greatly appreciated.
All the best,
RSXM11+ : Do you have any information that you could share with me? Anything already known about this scope that I might find out about that isn't in the PDF? I'm interested to know its history
I'm sorry but your scope is not known to me personally.
Serial number [taken from your photo gallery] is 370206, previouslly registered by CN member whizbang in August 2018 and subsequently discussed by Masvingo in October of that year:
Here's my Orange tube C8 purchased in April of... 2018.
sn: 3 7020 6
Following on from my comments on the C8 Registry update in a previous post (#554), whizbang’s serial number is a bit of an oddity.
I’ve confirmed with him that it is 3 7020 6 which would indicate a scope manufactured in the third quarter of 1976 – however, the sequential production number (the middle 4 digits – 7020 in this case) is out of sequence for the third quarter production although it would fit into the fourth quarter looking at the serials on the registry from around that time:
A scope with production number 7020 would fit in the 4th quarter but would have serial 4 7020 6 or if it is a 3rd quarter scope then a production number of 6020 (serial 3 6020 6) would make more sense. It may be that the Celestron employee stamping the serial numbers was having an off day and either used a '3' instead of a '4' for the first digit or a '7' instead of a '6' for the second.
- 2 5694 6 – a 2nd quarter 1976 scope with production number 5694
- 3 6040 6 – a 3rd quarter 1976 scope with production number 6040
- 3 6121 6 – a 3rd quarter 1976 scope with production number 6121
- 3 6542 6 – a 3rd quarter 1976 scope with production number 6542
- 4 6620 6 – a 4th quarter 1976 scope with production number 6620
- 4 6623 6 – a 4th quarter 1976 scope with production number 6623
- 4 6793 6 – a 4th quarter 1976 scope with production number 6793
- 4 7189 6 – a 4th quarter 1976 scope with production number 7189
The motor date may help to narrow down whether the number should have been 3 6020 6 or 4 7020 6 as scope # 3 6121 6 is reported to have motors dated 12/75 (so the Dec 1975 motors stock lasted into the 3rd quarter of 1976) whereas scope 4 6793 6 is reported to have motors dated 9/76.
Gotta love these Celestron serials – very so often they throw you a curveball!
As Masvingo notes in those posts, there are some oddities about your scope's serial numbering. However, this is not entirely unheard of with these and we have accumulated other such production variances over time. If you fully disassemble your scope be sure to note and post all the numbers written on your optics. [they usually all match one another but not necessarily that of the scope]
You have a C8 produced in Q3 of 1976, in the first year of the "die-cast fork era". You can verify and refine this by examining production dates on your RA drive motors. [again, please post them if you do so]
That's as much history as I can divine at the moment.
I gather from your description of the "loose" secondary housing and the fact that there were no shims present around the corrector, that your scope had been disassembled previous to your obtaining it and that these shims were lost. Correct reassembly and subsequent alignments exceeds what the moderators like us to pursue in this thread, so I suggest to get full assistance on that you begin a new topic to seek help under "Classic Scopes" or some more specific area for that.
To summarize points you will need to understand:
1) the corrector plate has an "optics" number scratched into the outer face near the edge. This is covered by the retaining ring, and should be positioned at 3:00 when facing the objective end. (see many descriptions)
2) the secondary mirror also has this same number on it's back, along with a mark - also to be oriented at 3:00 when assembled.
3) the secondary holder and corrector plate require gaskets to protect the corrector from being abraded or damaged by the metal of the holder and scope. (I have purchased replacement gaskets for my secondary from Starizona - call Dean). These and corrector gaskets (four in total) should also be available from Celestron. (I hope) This is also the point at which you should consider upgrading the secondary holder to a "Hyperstar adapter". I did so with my 1975 C8 [see my previous post for helpful images] and found it handy to be able to remove the secondary to re-check the corrector plate position at will.
4) although there is no need to, you can also verify the same optics number and directional marking on the back side of the primary mirror should you decide to remove it.
5) optical centering of the corrector plate is most easily done with the secondary holder removed. You'll need to remove it to place new gaskets on it anyway, so the corrector can be centered with the primary at that time. The 1/16" space around the corrector should allow enough adjustment for you to "null" it's position by eye and then create new spacers to keep it in that position thereafter. The secondary holder is then installed with it's gaskets and centered in it's opening in the corrector.
Generally these measures should return the scope close to factory positions, but more proper optical tests would involve additional equipment and attain finer outcomes. See publications by Robert Piekiel "Collimating Schimdt-Cassegrain telescopes" and "Testing and Evaluating the Optics of Schimdt-Cassegrain telescopes". Robert is a member of these forums.
FYI - I am not affiliated in any way with Starizona or Mr. Piekiel except as a satisfied customer.