Opinion seems to be divided on the accuracy of centering of the secondary required for imaging.
Some opinion is that spherical elements do not require perfect centering and that collimation corrects for this.
The secondary on my classic C8 varies in offset to the scope wall by 0.5mm. I have measured the centering of secondary in the cover plate while cleaning and this seems good so the external measurements hold.. The images are OK when very carefully collimated but not exceptional, particularly for planets.
Due to the rough cut corrector centering ( I presume), to centre the secondary I would need to push the corrector to one side and take out the already thin black paper spacer and use a slightly larger cork spacer opposite. Brrr.
Is there an easy way to establish the centre of the primary coincides with the secondary without removal and will it make a difference?
Ah I can see now when ask the question whether, logically , the corrector lense also needs centering that Multiweb has already posted the answer to this question. More reading required.
So if the hole in the corrector is not central but the corrector was figured to suit the final position then it is going to be very hard to know exactly if the corrector is centred on the primary without some sophisticated equipment. And if rotation is exactly critical also then perhaps marking should be exact to replace in exactly the same rotation.
On CN I got this reply which reinforces that everything should go back exactly the way it was originally set out by the scope maker for each scope. Tilt I can measure. I quote:
I've done a fair amount of analysis of the C14HD with regard to centering the corrector/secondary assembly. It turns out that the system is fairly insensitive to centering. On the C14HD systems getting the whole assembly centered to within +/-2 mm is plenty good enough. I don't have any data on the 8" systems but I suspect that they behave in a similar way and if you have a non-HD system, the centering tolerance is sure to be even less restrictive. That 0.5 mm centering offset will have absolutely no effect on your imaging performance. The key parameter to pay attention to is secondary tilt. The system is VERY sensitive to small alignment errors in the secondary and that's where you should spend all of your effort to make it as perfect as you can get it. Even a tiny error can have a significant impact on system Strehl and MTF performance.
Given the way that the secondary is mounted, it's a bit of a tricky calculation but I've been meaning to compute the sensitivity of the system to secondary tilt one of these days. I'll post the results if (and when) I ever get around to it.
I've been meaning to compute the sensitivity of the system to secondary tilt one of these days. I'll post the results if (and when) I ever get around to it.
Bottom line is that the classic Meade/Celestron SCT is relatively insensitive to decentering errors (ie lateral shift) of corrector + secondary. What it IS sensitive to is a tilt error in the secondary - as every SCT owner will know.
I am no expert but my scope looked original in orientation with the engraved number readable in the 3 o'clock position which also included a makers marker pen colour on the ground edge in the same position. Blackened cork packers and back folded paper packers where cork would be too thin at 12, 3, 6 and 9. My scope was also owned before me by a physicist and I think it would be correct.
So I suppose if the serial number is not reversed and is close to 3 it is original . I also understand there is a mark under the secondary cover pointing in the same horizontal direction as celestron parts are all matched but I did not look for mine. I was nervous enough cleaning the glass and puffing out the tiny bits of dust.
From diagrams it would seem the lense shape of the corrector is on the inside but it is not visible by eye to me and I would not touch it with any measuring device I own.
Quote:
Originally Posted by mldee
I have seen many comments on how to install the corrector plate with the engraved number at (I think) the 3 oclock position.
Should the plate be installed with the engraving on the inner or outer surface of the plate? ie, which is the correct " front side" of the corrector.
No doubt the plate on my 20-year old black C8 has been removed countless times, just want to ensure I put it back correctly next time
Bottom line is that the classic Meade/Celestron SCT is relatively insensitive to decentering errors (ie lateral shift) of corrector + secondary. What it IS sensitive to is a tilt error in the secondary - as every SCT owner will know.[/QUOTE]
Yep. But compact, everyone could afford one and the view was pretty good and detailed for the money. Incredibly easy to use also if you want to quickly view an object and don't load up the fork. I kind of like how those basic one man companies started who did all the work in an area no one else appreciated. All gone now and the stuff is made under contract somewhere else.
Once upon a time I enquired about the rate at which C8 and Meade 8's were sold. It is a surprisingly large number, just in Sydney.
Meade and Celestron went after the mass market with a design aiming to be jack-of-all-trades - but not particularly good at anything, and applying manufacturing management principles from the 1920s to keep costs down - ship a product just barely good enough to keep most beginners happy, occasionally defective, but never outstanding.
The strategy works - most customers probably use the scope a half dozen times then find it's all too hard, and it spends the next 20 years stored under the hose or in a garage until its sold after a cleanup.
But look more critically and you will find these scopes are not optimal as lunar & planetary visual scopes thanks to a gross secondary obstruction, focal ratio that is too short (f15 would have been a lot better) and sub-par optics that are not really tested or matched (as revealed by the russian test results), yet too slow to be much good for photography (they should have done an f/7 astrograph). It has been this way for the past 40 years BTW.
Post 1995 the reliability of the fork mounts and electronic from both have been so poor most owners have de-forked them and put the OTA on a mount made by someone else (mainly Skywatcher, Losmandy or iOptron).
But most customers don't know the difference anyway between what they have and "perfect" optics, nor have to tools to precisely measure the optical quality of their scope to know whether it is quarter-wave, or not (most SCTs aren't, by the way).
The other trick they have is this - keep a few good ones at the factory in case a smarter customer DOES know the difference and returns a scope as defective, so you can ship him a known good one. The wacky part about this is the customer will probably tell all on CloudyNights that the company saved the day, and more kudos to them.
Back in the 1960's I would guess Celestron made optics with little more than backyard grinding and polishing machines and hand-finished, with the correctors made the same way Schmidt did in the 1950s. Yet despite the widespread use of CNC machines and precision machining, Celestron never have attempted to:
a) modify the OTA focusser design to fix the primary mirror slop issue, despite other manufacturers showing it can be done;
b) build a decent mount better than beginner level;
c) build a mount that can survive long term frequent usage (observatories), or
d) improve the average production quality of the optics in as-shipped scopes.
Totally agree. These sorts of marketing games have been around since time immemorial. My first car, a brand new 1962 mini, what a POS. But gave me many opportunities to learn about car and engine design....
Same with my C8, cheap as chips to buy used, and still gives a newbie like me many opportunities to learn the ins and outs of AP.
As to the "goop" solution they used to minimise mirror slop...may they rot in hell.
Cars from the 1980s were usually scrap after 10 years but a Japanese car today can be 15 years old and ready for another 200000km. US companies like Celestron missed a great opportunity in the 90s. All the computing technology and automated manufacturing was theirs for the taking. However they did not move forward as the Japanese and Europeans did ,preferring cost over quality control and the long term . Too late now. Cloudy over here in NZ . Damn.
I have had a reply from Bill @ astronomyasylum.com with a link showing how Celestron corrector were made which is interesting. It shows that the individual figuring was all done on the secondary and shows the vacuum form mould method for the secondary which might explain slight variations in centering. It would be interesting to see the books about SCT by Bob Piekiel
