Optimising a C8 for APS-C. - Drawtube v Celestron Focus Motor

the old c8 have mirror flop, most people  add a sct focusser  behind. then you dont focus with the mirror.
The new  edge sct have  no more  problems.  meade have also  mirror lock.

You can make it yourself, but its easyer to get a sct focusser like this. https://www.teleskop-express.de/shop/product_info.php/info/p3947_TS-Optics-2--MONORAIL-Dual-Speed-Focuser-for-SC-Telescopes---SC-Thread.html

John:
the old c8 have mirror flop, most people  add a sct focusser  behind. then you dont focus with the mirror.
The new  edge sct have  no more  problems.  meade have also  mirror lock.

You can make it yourself, but its easyer to get a sct focusser like this. https://www.teleskop-express.de/shop/product_info.php/info/p3947_TS-Optics-2--MONORAIL-Dual-Speed-Focuser-for-SC-Telescopes---SC-Thread.html

Hi John -- I'm also interested in adding a focuser to my old Meade SCT. 

Regarding the focuser you suggested (and I see other similar brands), on the eyepiece side that uses a compression ring to attach a camera (with a 2" nose piece), wouldn't the compression ring cause a slight tilt?

Versus attaching the camera using threads. I struggled with this issue when I was imaging with a newt...

Did you check that your focus position is consistent?
In Nina you can either have your focuser correct backlash with a left or a right turn, depending on where you put the number.
With my Edge HD I get consistent focusser position when I correct backlash from the left, but not when I correct from the right. Easy to understand, in one direction gravity works for you, in the other not.
Once I had the right direction the tilt adjustments in HokusFokus worked just fine.

Hope this helps,

Michael

Hmm.. correct me if I am wrong, but an anti clockwise turn moves the mirror down, so Gravity is not helping you, the mirror can sag down a little bit, only with a clockwise move the mirror's weight will work for you because gravity pushes the mirror against the thread of the screw.
You can verify this with a simple nut and a screw.
This is why I asked if your focus position (position of the stepper motor in steps when focussed) is consistent, in your case there is a chance that the value fluctuates.
Also it seems that either the thread of your focusser is a lot more coarse than the thread on the Edge HD or your autofocusser works with larger steps...
Here's an example of one of my focusser curves, I have 350 steps per point, you seem to have something like 25 steps per point. I am using a ZWO Autofocusser, actually one focusser step is 0.13um

Hi Michael,

Thanks for taking the time to get back to me. You have got me thinking here
All the research I have done has suggested the last turn should be anti-clockwise. I understand your model of a nut and bolt, however, if the nut is captive, then an anti-clockwise turn will indeed result in a movement against gravity?

I may be wrong and will look into that some more. It's also frustrating that the direction of the Celestron Focuser cannot be reversed, neither can the focus direction in NINA which means that, my focuser has to overshoot and change direction for every sub in the focus routine. That means the mirror is shifted at each and every point, rather than just at the beginning of the run. I cannot find a way out of that unless I do reverse the direction of backlash compensation, but was of the belief that I had that right as it is?

You are absolutely correct about my focuser, a step size of 25 is quite normal for a Celestron Focus Motor and yes my focus position is consistent across different runs. It is the results in Hocus Focus tilt measurements that are differing between runs. I'm guessing that although th emirror distancing is consitance, it's angle is changing slightly or shifting laterally and so changing the recieved tilt angle at the sensor.

Out of interest, do you lock your mirror once you have fixed focus and tilt?


Ben,

Thanks for such a detailed write up. That's very kind of you to take the time and is appreciated. With an external focuser I could fix my tilt perfectly, say at Zenith as you have suggested. However, you have confirmed my suspicions for me and it seems a mirror lock would be necessary. Any mirror flop from movement around the sky would result in innacurate tilt if the mirror then flops. I was hoping that 'locking' the mirror with the Celestron Focus Motor would help against that somewhat but as you say probably not without locking the mirror. To what extend I would suffer from this I think I could only find out by trying it and i am hoping to obtain a second hand PrimaluceLab Esatto which will hopefully fir along side the Celestron Fosuser.

It's curious that my Hpocus Focus runs are not consistent even when I an pointed to Zenith. This must eb down to mirror shift and may be exasperated by the change of direction I mentioned earlier in this post. I can see how an external focuser would eliminate this but it's interesting that Michael was able to dial tilt in I am asssuming without an external focuser?

Looks like I may be looking into investing in an Edge HD. As well as the mirror lock benefis, I suspect there is a wider flat field for APS-C and presumably reduced vignetting also. It's just out of budget at the moment and I've got great mileage out of this old scope, would love to keep it running

Lastly, do you see a difference in the final image if you focus with the Celestron Focus Motor or with the Crayford or are you just getting the benefits of being able to lock the mirror and still refocus without any changes?

Thanks again Michael. You know you're right, I should just try it and see if it helps. Definately before making a plan to spend more money!

I suspected that you would not lock the mirror, without an external focuser you would lose the advantages of refosuing. I shoot overnight and I tend to set a new run every hour, just to keep everything tightened up.

I'm managing to pull data tonight. I've been doing so much tinkering and calibrating that I'm in danger of forgetting what this is all about for me, taking images. Next clear night, no predicted here for at least a week, I'll give it a whirl

Tell you what, when you can afford a Planewave CDK, I'll buy your old EdgeHD 11 🤣

I too have a Hyperstar but still want to keep it for now, hence the reason to keep the  Celestron Focuser in situ.

That's interesting that the field curvature suffers with a focal reducer on the EdgeHD 11.
As you say, sometimes you have to live with the limitations and thank you for helping me to understand what they are with my set up.
I want to squeeze the best I can out of it without chasing shadows. I think I'll get an external focuser anyway if only to satisfy my curiosity