Collimation failed

[jonathan]

Recently I took the corrector plate off my Celestron 8SE and cleaned it (it was quite badly fogged with dust or something on the inside), this part went well, however it seems that the central part didn't go back quite as it was so now it's rather badly out of collimation (the out of focus bright star shows a black dot far off to one side).

I had the scope out tonight attempting to collimate it by twiddling the three screws on the central part however they were extremely stiff, to the point where the heads started to strip, also when I was attempting to turn the screws I noticed the whole central part moving.  This worries me greatly - how can I possibly collimate this thing by adjusting the screws if they won't budge, but somehow the whole central part moves about?

Anyone who knows what's going on here, please help!  I was not able to achieve focus with the scope tonight, a very clear night by the looks of it. 

I will need to remove the corrector plate and examine the central part, I'm not sure why the screws are so very tight nor how it is able to shift up/down/left/right over the correctly plate.

[x6gas]

Hi Jonathan,

The secondary carrier on my Edge 11 also became loose (I can't honestly recall whether the scope arrived like this or if it just became loose).  I think you'll find that the carrier can be tightened against the corrector plate by tightening a screw collar on the backside of the corrector.

I also found that the collimation screws on the secondary were made of something akin to cream cheese and strip really easily.  It's very important (as ever) to use the correct size cross-blade screwdriver to remove them and I would then strongly advise to replace them with something better (many people use stainless steel thumb screws - aka "Bob's Knobs").

I have seen it said that the orientation of the secondary is not important now that computerised manufacturing is so accurate.  I am a bit sceptical of this in honesty as I can't see why Celestron would bother with an orientation pin in the secondary if it made no difference.  I think the only option to be sure is gradually rotate the secondary to "tune it in" and then marking the position before tightening the secondary carrier.

Aside from that there are two adjustments: the tilt of the secondary (which is adjusted by the collimation screws) and the position of the secondary in the aperture which is adjusted by moving the corrector plate.  This used to be achieved with shims but on newer scopes there are plastic adjusters holding the corrector.

To be honest I would try again with the tilt adjustment and see how you get on.

Otherwise I think the full process you'll need to go through is:

1. Adjust the secondary tilt as best you can - obviously ensure that the secondary is seated properly in the carrier
2. Check that the secondary is centred in the optical train
3. Adjust the secondary again to get collimation as good as possible
4. Focus the scope and then gradually rotate the secondary / secondary carrier until you get the best focus
5. Mark the orientation of the secondary and then tighten it against the corrector
6. Check the collimation again.

No way to dress it up: it's a painful process to check every avenue as there are quite a few possible adjustments (orientation of the corrector plate to the primary is another so be sure to mark that position too!)  If you can do the leg work with an artificial star that obviously helps.

Good luck.

[kirkster501]

No need to worry.  These scopes are not precious snowflakes - they are mass produced, commodity items and are quite robust.

Get those stripped collimation screws out first and foremost and get some Bob's Knobs (or equivalent) installed so you can collimate easily.  Then, even if it did twist the secondary assembly ever so slightly, it won't matter that much.  In terms of the rotating secondary, you will see if you remove the corrector again that this can be tightened from the back of the corrector (i.e. inside) of the scope.

Edited September 10, 2020 by kirkster501

[jonathan]

Yes, I've heard of Bob's Knobs, I think it must be time for those.  I had a thought that perhaps the screws have become stiff over time (I haven't adjusted them for many years), perhaps they've corroded with dew or something like that.

I feel that I should remove the secondary module from the corrector plate purely so that I can see what's going on in there.  There is a central disc on the outside of the secondary, does that just flip off to reveal a screw?

I do recall vaguely unscrewing the inside half of the secondary but I don't rememeber what it looked like behind the mirror.  Time for another adventure in telescope maintenance!

I see on FLO there are many different Bob's Knobs - does anyone know which would be appropriate for my Celestron 8SE?  It has an orange tube but I bought it new only about ten years ago.  I'm thinking these will be the ones, I need to remove a screw to confirm that it is metric.

Edited September 10, 2020 by jonathan

[kirkster501]

I would research "Bob's Knobs" and you will see there are thumb screws that you can get off of ebay for £1 or so.

[x6gas]

2 hours ago, jonathan said:

Yes, I've heard of Bob's Knobs, I think it must be time for those.  I had a thought that perhaps the screws have become stiff over time (I haven't adjusted them for many years), perhaps they've corroded with dew or something like that.

I feel that I should remove the secondary module from the corrector plate purely so that I can see what's going on in there.  There is a central disc on the outside of the secondary, does that just flip off to reveal a screw?

I do recall vaguely unscrewing the inside half of the secondary but I don't rememeber what it looked like behind the mirror.  Time for another adventure in telescope maintenance!

I see on FLO there are many different Bob's Knobs - does anyone know which would be appropriate for my Celestron 8SE?  It has an orange tube but I bought it new only about ten years ago.  I'm thinking these will be the ones, I need to remove a screw to confirm that it is metric.

You can indeed buy thumbscrews cheaply on the Bay but Bob has gone to the trouble of researching which ones fit which scopes and, for example, ensuring that you can still fit the dust cover.

This might help identify what you need, though:

http://bobsknobs.com/SCT/page26/C8.html

Ian 

[Starwatcher2001]

I put a set of "Bob's Knobs" from FLO on my C9.25 yesterday and they make a big difference to the ease of collimating - not that you need to do it very often on an SCT.

If the heads on the fitted bolts are starting to go, I wouldn't hesitate to change them. The instructions that come with them are comprehensive and easy to understand. Once they are all replaced (one bolt at a time), you'll need to recollimate. It took me around 45mins last night, but most of that was scooting around looking for a star that suddenly wasn't obscured by clouds.

[billhinge]

I also have Bob's knobs but a good idea I saw elsewhere was a good set of  screws that you use allen keys with - forgot their name!

By the way  @X6gas wrote "I have seen it said that the orientation of the secondary is not important now that computerised manufacturing is so accurate.  I am a bit sceptical of this in honesty as I can't see why Celestron would bother with an orientation pin in the secondary if it made no difference.  I think the only option to be sure is gradually rotate the secondary to "tune it in" and then marking the position before tightening the secondary carrier."

What orientation pin?

[jonathan]

OK..  I've disassembled the corrector plate and secondary as far as I think I should, done some science / maths (or drawing, but a ruler and poor mental arithmatic was involved), and come up with the following photos.

I believe I have the secondary carrier centered, according to my measurements and doodles, it's just a matter of working out what these strange markings mean and how to put the pieces back together in the OTA.  Astonishingly there seems to be about 1.5 or 2mm wiggle room for the secondary carrier in the glass!  I wonder if this is on purpose to allow for metal expansion / contraction.  I didn't like to star prying at the carrier, there is no obvious way to remove it from the glass other than leverage with a screwdriver (the outside part of the carrier is metal, the inner cone where the secondary sits appears to be plastic, they are probably clipped together).

 

I used some precision measuring and a compass to draw myself a template to work from, then cut out the central obstruction hole so that I could fit it over the corrector plate and carefully draw around the edge of the whole plate.  This drawing marks the positions of the screw holes in the holding bracket and also the positions of each trangle that was scratched on the outside edge of the corrector plate glass.

The various markings didn't make much sense to my eyes, sometimes there were two triangles and what looked like a number (8, 0, 3... that's what they looked like to me), except for this one shown below which had no number.

 

And just in case anyone was curious as to what the secondary looks like in an SCT...   (the three collimation screws are underneath)

 

Anyone any ideas on how to proceed?  My only plan is to pick one of these symbol groups and align it to what I would guess at being the 'top' of the OTA, the point opposite the dovetail bar mounting.  There are no markings that I can see in the OTA though there is a screw hole so I'll just use that as a starting point.  As for orientation of the secondary, I can only go by photographs of other 8SE scopes, I believe it should be two screws towards the top with the 'FAST' logo upright. 

... wish me luck!

 

Update: I noticed that the secondary only fits in the carrier one way, there's a grub screw on one side of the secondary holder and a cutout slot in the carrier for it, that removes that potential problem from the equation at least.  Need to wait for the next clear night to test it (I don't have enough room indoors for an artificial star, couldn't place one far enough away).

The 'FAST' logo was not upright after I placed the secondary in the carrier, come to think of it I don't remember it being upright before anyway, probably changes from scope to scope.

Edited September 12, 2020 by jonathan

[PeterCPC]

I don't know whether any of this may be of help.

Peter

[jonathan]

Thank you Peter, though I think now I've discovered / learned everything I need or want to know about Celestron corrector plates.

Tonight I took the 8SE outside on a beautiful clear night, set up and set to fiddling with the thumb screws ('Bob's Knobs'), learned lesson #1: Don't use Jupiter for collimation!  It just didn't seem to work.  I pointed at Vega and got a much clearer bright star to defocus and collimate to, ten minutes or so and it was looking good, focussed and there it was, all looking great and no coma or other weirdness going on. 

I moved to Jupiter, the double vision was gone, focus was achievable, I could make out three bands easily.  This is the 8SE I remember from years ago!

I slewed over to Saturn, I fancied I could see some colour variation on the planet disc - a large band - and perhaps even a variation in the rings, at least the darker outer ring was discernable - possibly the Cassini Division at a pinch.

Then I noticed something bright and orange rising in the East, a bonus planet!  I shuffled over to Mars, it's been many years since I've been able to get some decent observing on the planet.  I fancied that I could easily make out the large darker area on the planet disc, and also a definite polar ice cap.

I'd call this job 'good enough' for now, there may be another 5% collimation to get out of it but maybe not really worth the effort.  I'd rather forgotten how things look through this scope, it doesn't have the pin sharp contrast of my refractor or even the 150P reflector, but those scopes don't seem to have the same raw magnification for planets (the refractor maybe comes close, I haven't compared them side-by-side).

The dew came in and said hello to my glass surfaces, I didn't have my heater tapes on so had to call it a night.

[PeterCPC]

Good news. Congratulations.

Peter