SCT goes out of collimation after panning

[John]

I'm enjoying my C8 SCT but is does have an annoying habit of going out of collimation when I pan the scope to another part of the sky. Tonight I was getting lovely sharp images of Saturn, then panned the scope over to Gemini to have a look at the Eskimo Nebula only to find that it had slipped out of collimation during the "pan". So I re-collimated (relatively easy with Bob's Knob's fitted) viewed that part of the sky for a while then panned back to Leo - and it the collimation was lost again !.

I'm trying to figure out if this is because the secondary mirror is moving (most likely IMHO) or that the primary mirror is shifting (mirror slop ?).

Theres nothing wrong with the optics of the scope as the images are spot on when it's in collimation.

Has anyone experienced this sort of thing with an SCT and if so, what's the cure ?. I thought SCT's were quite good at holding collimation

Thanks,

John

[GazOC]

I'd guess its the primary, SCTs are known to have this type of problem.

[Astronut]

I've no experience of owning an SCT but I was planning on buying a C9.25 (plan on hold for now) and the various reviews I read (on C9.25 and SCT's in general) said that primary mirror flop is the cause. I can't remember what the solution is though but I have a vague recollection of reading about some sort of mirror lock system being put in place or stronger collimation springs. I think some SCT's are more prone to primary mirror flop than others.

[Kaptain Klevtsov]

I've never had an SCT but have read loads about imaging. I suspect that this is the dreaded "mirror flop" caused by the clearances necessary to allow the primary mirror to move for focusing being a little too generous. The problem is caused by the primary mirror moving slightly when changing the angle of the tube.

The imagers recommend using an off-axis guider when imaging using an SCT for this very reason. I've not heard it mentioned that the movement is severe enough to affect collimation but I'm sure that he severity is subjective.

BTW what exactly moves when you collimate an SCT? With the Mak. that I had, I'm almost sure that it was the primary mirror cell that was adjusted. The adjusters were through the back of the 'scope (the end that you look through).

Hope you can get it sorted easily John. I expect Rog. will have something to say about this as he has had a lot of time playing with SCTs.

Captain Chaos

[John]

BTW what exactly moves when you collimate an SCT? With the Mak. that I had, I'm almost sure that it was the primary mirror cell that was adjusted. The adjusters were through the back of the 'scope (the end that you look through).

Thanks CC,

With the Celestron SCT's (and Meade as well I think) it's only the secondary that is adjustable. The system works well and I've no problems in getting good collimation (I'm having quite a lot of practice at the moment ) but I'd prefer not to have to do it each time I move the scope to a different part of the sky.

It is quite an old scope (new in 1994) so I guess the mechanisms that hold the primary in place (and move it when you focus) could have become worn.

As you say, perhaps Rog will have some ideas.

John

[GazOC]

John,

In 99% of cases with SCTs its the primary. Can you tighten the focussing knob down to the mirror?

[John]

John,

In 99% of cases with SCTs its the primary. Can you tighten the focussing knob down to the mirror?

Gaz,

Not quite sure what you mean - is that tightening the 3 screws that hold the focussing knob assembly to the back of the OTA or do you mean focussing fully in or out, or something else

Probably me being dense

John

[russ]

John, you can buy a kit that minimises the mirror flop. Telescope House used to do it and there was another called Peterson. My LX200 had it but the previous owner fitted it. It was a sprung loaded thing on the back of the scope that could be screwed in or out to increase tension on the primary mirror. Sorry that's a bit vague, I just can't remember the details. But Gaz and CC are right, it's flop in the primary and is dead annoying. I had it my first C8 (orange tube), Meade LX6 and I presume the LX200, hence why the kit was fitted. But my last C8 didn't suffer.

Regards

Russ

[Kai]

It must be a age thing as mines okay.Either that or my eyes go out of collimation to the same degree as it pans

[John]

Thanks for all the responses.

From further web-based research it seems that my problem is causeed the infamous "mirror flop" which many SCT's are afflicted by to a greater or lesser degree.

Initally I'm going to try a suggestion I picked up from another web site - go through the whole range of focus 2 or 3 times (all the way in to all the way out) to evenly re-distribute the lubricant along the screw that moves the primary mirror in and out (this may have been what Gaz was suggesting).

I'll see if that helps the situation. At lease I'm not alone with this issue

John.

[GazOC]

Yes, sorry John, I should have been more specific.

[russ]

John, been searching around the web and found the Peterson Mirror lock but it's for Meade only. However, Hutech do one for the Celestron SCT:

http://sciencecenter.net/hutech/misc/focuslk/index.htm

Locks the mirror in place, which means you'll have to add a Crayford for focusing. Hope your tip pays off Much cheaper.

Russ

[CELESCOPE]

yup i would say 100% , its the age thingy , having owned the SCT for around 4 years i have never found that the P/ mirror moved in any way ,i have had 3 of them and had no problem , so maybe a good idea to get the mirror fix and go for the crayford, at least that way you gain an improvement , for one , no mirror slop ,and less collimation , and no image shift with the crayford, having said that , i have heard stories that bobs knobs are,nt always a good thing , cant comment really as i have had nothing really to do with them, but have heard of Scopes wanting collimation more often ,

John did u have this problem before bobs Knobs , if not may be well worth removing them and go back to the screws and trying to see if it stays collimated better ,

Rog

[Stardust]

I don't a problem with my Meade LX10, I was going to suggest racking the focusser in and out a few times but I see you've found that one. I use bobs knobs and haven't come accross a problem, do they all have some tension? if one was loose it could interfer.

good luck

DAve

[John]

Thanks for the further suggestions folks. I fitted the Bob's Knobs almost as soon as I got the scope so, if the focusser trick does not do some good, I'll try putting the original collimation hex screws back in and try that.

Some of the web pages I looked at suggested disassembly of the whole OTA - I really don't want to go that far.

Cheers,

John

[MartinB]

The only way Bobs knobs could be an issue is if the secondary is too loose. Presumably the knobs all have a little pressure on them. The other possibility to consider is a secondary housing thats become loose in the corrector plate. I had that problem with my Nexstar 8" because a gasket had perished - although the design is different to yours since the secondary is designed to be removed to accomodate a fastar lens. Just worth checking.

[tom.yates]

I have just bought a 925 And am having no trouble with the coli and have been panning all over the sky.

[John]

I have just bought a 925 And am having no trouble with the coli and have been panning all over the sky.

That's great Tom - I think this is something that affects older SCT's. Thanks to SGL I've got a few ideas to try and minimise it though.

John

[John]

A quick update on this,

After trying the trick of turning the focus know through the full range of focus a few times I have not noticed the problem re-occurring despite pointing the scope in all sorts of directions.

I reckon that the scope had been in storage for sometime (maybe as much as 12 months) before I took ownership of it. The focuser has been left static for all that time so it's possible that the grease within the mechanism had become unevenly distributed along the focus screw. At least that's my theory !.

Anyway the problem seems to have gone away which is great

John

[Stardust]

John

good news it's righted it's self, must have been a bit worring for a while there.

DAve

[John]

John

good news it's righted it's self, must have been a bit worring for a while there.

DAve

Another thing I have done is to remove and re-fit the Bob's Knobs - this time without the little white spacers which, according to the instructions, are optional. The tighness of all 3 knobs seems more even now and the added benefit is the knobs don't hit the underside of the dust cover which they did with the spacers in place.

One of the good things to come out of this is that I'm a dab hand at collimation now - I've developed a tendancy to fine tune it each time I use the scope as it's quick and easy now.

With the collimation right the scope has great optics - I was looking at Epsilon Bootes the other night at over 500x and it showed nice tight airey disks, a single diffraction ring around each star and the colours and separation very clear indeed - almost a "refractor like" image. Saturn looks like a picture from a book at 270x with the Cassini division etched sharply all the way around the rings and the crepe ring on show during the moments of clearest seeing.

Can't wait to get a shot at M13

John