Mirror shift - Crayford Focuser

[lalande21185]

Hi

I do not even have the scope I want yet but one option is the CPC 9.25.

I have been reading up on it and I worry now about the mirror shift of these scopes.

Does anyone use a SCT with a 2" Crayford Focuser?

Especially the CPC 9.25 as this is one scope I am serious about at the moment but could change as so many about.

Sounds like a great solutuion as just do not like the idea of moving the main Mirror to focus, just does not sit right with me.

I mean its going to cost the best part of £200 for a 2" Crayford Focuser, but I think worth it to save moving that main mirror.

Cheers

Karl

[Merlin66]

Before spending the funds I'd wait until you've used the scope.

I've just changed over from a 10" and 8" Meade SCT to the Celestron 9.25" and find it much, much better.

The focusing mechanism in the C9.25 is a solid leadscrew type and far more rigid than the Meade....

Yes, a 2" crayford works well on SCT's ( I have the Meade #1209 zero-shift crayford on the 12" Lx200)

[barkis]

Not relative to your question, but I have a Meade SCT, and I use a Crayford, because of mirror shift. It was so bad using the mirror focuser, the Image would disappear from view in the eyepiece when reversing direction, it was so bad. I locked the mirror in an optimum position, as you can do that with the LX90, Then focus using the Crayford. Much Much better.

Ron.

[Merlin66]

Ron,

May be we should explain to Karl that the issue of mirror flop is far more noticable during the precise focusing only and that always doing the final focusing from one direction usually does the job.

For comparison: With the 10" Meade the target star would move well off the spectroscope slit ( 30 micron wide) in one direction but back onto the slit from the other - the C9.25 just drops the star off the slit then back again....

I can't say I've experience any significant mirror flop during normal operations - so it's pretty stable when left alone...

Just my 2c

[johninderby]

9.25 Edge HD with mirror locks..............now that would be interesting. If only it wasn't so expensive.

John

[barkis]

Ken, I agree that approaching the focus point in one direction gave me no trouble, but shooting past it, and reversing direction was quite severe on my LX90, so much so, that I intended doing the modification to the mechanism that is available, to eliminate that problem. As it is, I have not got around to it yet.

Ron.

[ollypenrice]

I use a Crayford with an SCT (Meade) and consider it essential.

The 9.25 Edge looks great but what about focal reducer? Essential for imaging but not yet available? This is a pain.

Karl, be aware of this; when using a focal reducer for imaging there is an optimium distance from reducer lens to chip which must be respected. Only the Moonlite focuser, I think, carries the reducer in the drawtube so as to avoid changing this distance as you focus. With other systems you should set the Crayford to give the right distance, focus with the moving mirror, then make tiny final adjustments with the Crayford. In theory this should be okay, I think.

Olly

[Psychobilly]

hmmmm Edge HD ... nice....

As well as focusing in one direction doesn't the gloop that the mirror runs on make a difference?

http://www.astronomiainumbria.org/advanced_internet_files/meccanica/easyweb.easynet.co.uk/_chrish/lx_flop.htm

I know I did the full travel thing on mine a few months ago...

Just reading some of Ken's thoughts on Iceinspace

Peter...

[Merlin66]

Yeah...

You need to run the mirror through the whole of the focus travel every so often to "spread" it out.....

[barkis]

Well I haven't done that, so I am my own worst enemy, and it makes obvious sense. If there is no gloop as Peter calls it, then taking the weight off the cell by changing focusing direction, will allow the flop to take place. "Simples" So why didn't I think of that?

Anon.

[ollypenrice]

For some Meades Astro Engineering do a device that screwsthrough the back of the scope into the transit locking screw hole in the primary and then has a spring externally which pulls back all the time. I find this thing pretty good.

Olly

[barkis]

Thats what I did Olly, but I just locked it with a 1/4"Whitworth bolt and nut, very carefully, and used the Crayford henceforth.

However I will action the focusing back and forth to it's full extent to distribute the "Gloop", it should eliminate most of the flop. After removing the locking bolt first though.

Ron

[Psychobilly]

I used "gloop" as a generic term... It also worth mentioning that some gloop can get quite runny in hot weather so care needs to be taken when parking or storing the scope if its in a warm environment...

I Know I helped Helen track down some Dow Corning High Vacuumn Grease when she was working on her SCT...

Peter...

[barkis]

"Gloop" fits the bill just fine Peter, And even though I read most of the posts way back on this subject, It never once entered my head to run the focus through to max and back. I have made a mental note to do so next time I'm in the Obs. though.

Ron.

[Psychobilly]

I used the motorised focuser to do it on mine as there are quite a few turns end to end on the travel...

[lalande21185]

Hi all.

Thanks for all your replies, that has been great.

I reckon, to keep things simple for my first serious scope is to just get the scope and then get the gadgets for it as and when needed.

Otherwise my shopping list is adding up to a hell of an amount and I just do not think I should spend so much first time out.

I have always been "Got to get all the gizmo's" all at once.

So after reading your replies I think I should get the scope and enjoy that first.

All replies have been great.

Kind Regards

Karl