Auto-Focusing a Celestron EdgeHD 8"

[PhotoGav]

I am struggling to successfully autofocus my Celestron EdgeHD 800. I am using a Lakeside autofocuser with the bracket connected to the main focus pin. It all works mechanically and I can alter focus with no problem. The issue comes when running the autofocus routine in SGP. I can easily achieve sharp autofocus with my two refractors, but the problem with the SCT is mirror movement when changing the direction of focusing. It goes through the V-Curve routine fairly efficiently, but when it racks out (with backlash compensation) to then rack in to the chosen point of focus, it is not quite in focus. It appears that the stepper motor value is not consistent when the change of direction happens, so what was focus point 13459 is not the same when it goes out and back in to that point. This is frustrating to say the least! I am achieving focus by doing it myself, i.e. using the motor focuser, but not the autofocus routine.

So:

Question one - does anybody have a crafty solution to get autofocus to work with my current set up?

Question two - I am considering the use of a Crayford focuser, which will make autofocus more consistent, but will have implications on the back spacing required from visual back to chip. Is anybody using a Crayford on an EdgeHD to autofocus for imaging? I would love to hear your experiences and whether this is a viable and sensible route to explore.

 

Thank you and clear skies,

Gav.

[Davey-T]

I dont think there's much hope trying to focus with the stock focuser Gav.

I've got an electronic micro focuser on my Meade SCT that works a treat once the mirror is locked to eliminate movement.

Can't be much help with the Edge but definitely more than [sensible] I'd say imperative to get some sort of precision focuser.

Dave

[PhotoGav]

Thanks Dave. What is your electronic micro focuser attached to?

[swag72]

I do remember an electronic focuser for the SCT being prohibitively expensive. To that end I stayed with manual focus then later changed scopes!!

[Stu]

I've never remote controlled these things, but have used a Baader SteelTrack SCT focuser with the SteelDrive kit which works very well. I understand this can be PC controlled quite well.

I have a Meade micro focuser on my C925 now, don't think that can be controlled remote though although I could be wrong!

[PhotoGav]

Sara - indeed, a long refractor is the best solution, I'm just struggling to justify the spend!!

Stu - thanks, I was just on the FLO site looking at the SteelTrack. My worry is how it affects the image? I am very tempted just to give it a go. A second hand focuser would be a good option - you haven't got yours floating about looking for a new home have you?!

[Davey-T]

41 minutes ago, PhotoGav said:

Thanks Dave. What is your electronic micro focuser attached to?

Just screws on the SCT thread, same as Stus, I focus it "manually" using FWHM /HFD in Maxim, haven't tried autofocusing it but don't see why it shouldn't be possible.

I can say it would be very time consuming, practically impossible to focus without it.

Dave

[Whirlwind]

I think the issue with the C8 Edge is whether you use the reducer or not.  With the reducer the in place you may not have enough back focus (105mm in total) depending on the imaging train - I see you are using a QSI wsg and they are about as compact as you can get (IIRC correctly about 50mm from chip to flange). 

I have only found one focuser that attempts to resolve this partially which is moonlite CHL (http://focuser.com/products.php) as the reducer fits into the focuser partially.

[Stu]

21 hours ago, PhotoGav said:

My worry is how it affects the image? I

Not sure what you mean by affecting the image? It doesn't change the image, just makes it possible to lock the mirror to avoid mirror flop and then it is easier to focus with the micro focuser.

I'm afraid I sold mine a while back. I do miss the C8 Edge. I have a C925 which is great, but it's just that bit bigger and heavier.

[Stu]

This is the Meade remote focuser I have on the C925. I just had a handset but presumably it could be PC controlled somehow? I just do visual and use this to help with fine focusing and to avoid mirror flop. It has a very short travel, probably 10mm but that's all you need as you can coarse focus with the standard focuser.

[Davey-T]

I think the Meade motor focuser can be controlled with the Hitec Astro thingy which connects to PC via USB but it's only a DC motor so not as accurate as a stepper motor version.

Dave

[PhotoGav]

Thank you for all your comments and advice. I like the Moonlite tip as I do use the reducer sometimes and would certainly want to be able to use it. I will do some maths to see what will work best.

[cfpendock]

Hello Gav. I've only just seen this post or I would have responded earlier.  I use a Lakeside focuser to focus my C1100 Edge.  The normal focuser is fixed at around half of it's total movement, and the mirror is locked.  I use a Moonlite focuser on the rear of the scope which is driven by a Lakeside stepper motor, and hence the Lakeside electronics.  

I find it works very well and I can easily move the focuser to my Tak 106 when needed - I just have to recalibrate the max and min position of the focuser each time.

I also find the V-curve pretty wide on the Celestron when compared to the Tak - a consequence of f10 instead of f5.  The stars suffer, though, never as sharp as I would like despite being in perfect focus.  The image shows the system attached to the Celestron - you can see the Monnlite focuser and the Lakeside stepper motor..

Chris

 

 

[PhotoGav]

Excellent, thanks Chris. Good to see hard positive evidence! I guess spacing is the only 'issue', ensuring that it is more or less as per the Edge spec recommendation. 

[PhotoGav]

UPDATE - I think I have fixed my autofocus problem... I had a screw loose. One of the grub screws that fixes the stepper motor bracket to the focus shaft was loose and obviously not holding the shaft consistently as it turned. I have tightened it up and autofocus appears to be working sweetly again. I need to test it thoroughly with a proper imaging run, but it certainly looks very positive. Phew!