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Equipment Discussions >> Cats & Casses

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John Miele
Carpal Tunnel
*****

Reged: 05/29/05

Loc: North Alabama
Re: Does your EdgeHD mirror lock shift focus? [Re: Peter in Reno]
      #5691722 - 02/20/13 08:37 PM

Quote:

So it's consistent from three people so far that the focus actually shifts after locking the mirror. Sounds like a design flaw to me. I wonder if Celestron is aware of this (or reading this) and planning to improve it?

Peter




Peter, make it four! I see the exact same thing on my Edge 8. The shift is exceedingly small but is detectable when using my bhatinov mask. I handle it the same way you do...John


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Peter in Reno
Postmaster
*****

Reged: 07/15/08

Loc: Reno, NV
Re: Does your EdgeHD mirror lock shift focus? new [Re: John Miele]
      #5691745 - 02/20/13 08:53 PM

Thanks guys for responding. It looks like I'm not crazy or drunk when I saw the focus shift after locking the mirror. I am going to have to decide what to do to make my life a little easier.

I see advantages and disadvantages of using electronic focuser at SCT's stock focuser or external focuser behind the SCT scope.


Electronic focuser at SCT stock focuser:

Advantages:
1) No additional back focus to worry about.

Disadvantages:
1) Have to unlock mirror to focus.
2) May have to leave mirror unlocked all times for remote operation.
3) If mirror left unlocked all times, may shift collimation as the scope tracks across the sky due to mirror flop.
4) Backlash on stock focuser.



Electronic focuser behind SCT scope:

Advantages:
1) Leave mirror locked at all times. Collimation remains unchanged while tracking the sky.
2) No backlash in some focusers like FLI.

Disadvantages:
1) Adds back focus.

Can anyone else think of more advantages/disadvantages?

Dave, you are correct about the weakness in FLI DF-2 focuser. I quickly glanced at FLI Yahoo Groups and several mentioned about weak load and flexure for FLI DF-2 focuser. FLI PDF is expensive but pretty thin of 1" - 1.325".

Peter


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korborh
scholastic sledgehammer
*****

Reged: 01/29/11

Loc: Arizona
Re: Does your EdgeHD mirror lock shift focus? new [Re: wolfman_4_ever]
      #5691769 - 02/20/13 09:17 PM

Hi Peter, yes for focusing, I slew to a focus star nearest to the DSO and return back to the DSO. I try not to go too far as the mirror shift might cause focus to change. However, there is always a star close enough so this has worked everytime. Even with Ha filter where I pick a very bright star.

The main thing is to get back exactly to my guidestar after focusing, and for this I use plate-solve - sync- re-slew and guidestar back in the guiding window for Maxim.


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korborh
scholastic sledgehammer
*****

Reged: 01/29/11

Loc: Arizona
Re: Does your EdgeHD mirror lock shift focus? new [Re: korborh]
      #5691777 - 02/20/13 09:25 PM

Peter, the backlash is not a disadvantage of stock focuser. Its easily dealt with the software transparently that it does not come into play at all. Need to set the right amount of backlash in the software and always focus in one direction.

A big advantage of stock focuser is that it allows for a very rigid and stable imaging train for which otherwise one has to spend tons on a large/heavy focuser. The imaging train never moves and the load on the focuser is always the same and small. The adjustments are also fine due to the screw mechanism.
An external focuser makes sense for high focal lenght planetary, but for DSO imaging the stock focuser works very well.


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Peter in Reno
Postmaster
*****

Reged: 07/15/08

Loc: Reno, NV
Re: Does your EdgeHD mirror lock shift focus? new [Re: korborh]
      #5691821 - 02/20/13 09:57 PM

Good points about having rigid connection at imaging train when using stock focuser. My imaging train is extremely rigid with threads (SCT & T-Thread) plus very rigid rotatable dovetail connection at Hutech OAG. I guess I will stick with what I have for now. I will also stick with mirror locks. Once I have imaged for 7 straight hours without touching the stock focuser and the stars were well focused. I think that night the temperature drop was pretty minimal. Having mirror locks reduces the worry of mirror shift which can affect the focus if unlocked. I just wished Celestron mirror locks were a little more robust and convenient.

Thanks,
Peter


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edl
professor emeritus


Reged: 06/24/04

Re: Does your EdgeHD mirror lock shift focus? new [Re: Peter in Reno]
      #5693241 - 02/21/13 05:58 PM

I purchased a FT Crayford with 0.8" draw tube for my EdgeHD 9.25.
With the special adapter the amount of additional back focus is quite minimal, at least for visual.

Best,
Ed L.


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Peter in Reno
Postmaster
*****

Reged: 07/15/08

Loc: Reno, NV
Re: Does your EdgeHD mirror lock shift focus? new [Re: edl]
      #5693259 - 02/21/13 06:06 PM

I saw that a while ago but from the specifications for your FT Crayford plus adapter, the minimum back focus still appeared a little too long. Can you measure and tell me the minimum back focus of the focuser including the adapter (drawtube racked all the way in)?

Are you using these FT parts?

http://starlightinstruments.com/store/index.php?route=product/product&pat...

http://starlightinstruments.com/store/index.php?route=product/product&pat...

Thanks,
Peter


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edl
professor emeritus


Reged: 06/24/04

Re: Does your EdgeHD mirror lock shift focus? new [Re: Peter in Reno]
      #5693556 - 02/21/13 09:02 PM

Hi Peter,

The focuser is correct, but the adapter is-
A20-223---Adapter 2.0" for CELESTRON C11 /C14 SCT telescopes
$49.00

Unfortunately my EdgeHD 9.25 is back with Celestron for a repair, so I can't accurately make the proper measurements. I did remove the focuser before shipping, so could try to get a rough idea.

Best,
Ed L.


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Tori
sage


Reged: 01/10/12

Loc: Somerville, MA/Warren, NH
Re: Does your EdgeHD mirror lock shift focus? new [Re: wolfman_4_ever]
      #5703012 - 02/27/13 07:56 AM

For what it's worth, I notice no shift whatsoever when using the locks on my 14" edgehd, even when zoomed way in on a bahtinov masked star. Not even a little bit.

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wolfman_4_ever
Pooh-Bah


Reged: 07/15/11

Loc: El Segundo, Ca, So. Cal
Re: Does your EdgeHD mirror lock shift focus? new [Re: Tori]
      #5703585 - 02/27/13 02:10 PM

Hey Peter, I'm pretty sure you can get a smaller adapter made from precise parts...

Only issue would be if maybe the drawtube, when focusing in, hit the corrector lenses in the baffle..


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