Does your EdgeHD mirror lock shift focus?
Posted 20 February 2013 - 12:40 PM
I own an C-8 EdgeHD and my primary use is imaging. I focus on a star near target DSO with Bahtinov mask. Once I am satisifed with the focus, I lock the mirror with mirror locks. I notice on my computer monitor that the middle spike from Bahtinov mask slightly shifted. If I unlock the mirror, the middle spike returns to center. I have to estimate how much focus shift so I purposely focus so the middle spike is slightly shifted so after locking the mirror, the middle spike is return to the center. It's kind of annoying.
I know I should get an auto-focusing software. One of these days I will, but for now I am sticking to Bahtinov mask for focusing.
Anyone else notice the focus shift after locking the mirror of EdgeHD scope? You probably will not notice the focus shift if you are a visual observer but the focus shift is enough to notice when using a camera.
Posted 20 February 2013 - 01:11 PM
Posted 20 February 2013 - 01:36 PM
I guess I will not use mirror locks anymore and check/refocus about every hour during the first two to three hours of imaging session and after that the focus is usually stable for the rest of the night. I am used to refocusing periodically that I can do it pretty quickly so it's not a big deal. In the Winter, refocusing is more critical because I live at 4500 feet elevation and temperature can initially drop pretty quickly and then after about 2 to 3 hours, temperature drop is less severe.
Posted 20 February 2013 - 01:40 PM
The rods are "Pinched" into position by small pistons that are cammed into place when you screw in the mirror lock knobs. There is no upward or downward motion at all. It is in fact a brake that just squeezes on the rods and nothing else.
So, what is happening?
I repeat this message about once every two weeks..
The primary reason for focus shift in the SCT design is a result of the mirror continuing to settle when the final focus is approached using clockwise motion of the focuser.
This is because the mirror can continue to settle due to the play between the threads of the focuser shaft and the carrier on the mirror.
If you approach final focus using a clockwise, you are "pushing" the mirror down. When you stop, gravity will allow the mirror to now settle away from the treads that are pushing it down to the opposite face of the treads so that they are now "Holding the mirror up."
The grease in the baffle also plays a role because the mirror carrier settles through that to.
The message I repeat is that for a variety of reasons, the best results with an SCT will be to always approach final focus using counterclockwise turns of the focuser.
For imagers, this "Lifts" the mirror into final position so that play in the focuser is never allowed to occur. The mirror is raised into position so that the mirror carrier on the baffle is always left at the same orientation, an there is no play left for it to settle back from.
For visual observers, the added benefit is that approaching focus using CCW turns will result in less chasing focus (inward direction for Refractors and Reflectors). If you overshoot final focus slightly, the eyes normal visual accommodation can easily adjust to refocus the image on their own.
I have had the Celeston EdgeHD locks apart and I can assure you that the design if the locks them selves cannot cause any mirror shift. This means that the only shift is the same that comes with regular SCTs and the solution is an aftermarket external focuser, or using CCW motions of the focuser to approach final focus (to may not eliminate, but will greatly reduce shift).
Hope this helps, but not the lock mechanism at fault. Take it apart and see for yourself. Just unscrew the knob and you can see how the design just pinches the rod.
Posted 20 February 2013 - 02:00 PM
I ALWAYS finish focusing in CCW direction. If I went too far, I reverse the focusing in CW direction by at least two turns so that focusing back in CCW works better like you described.
I am not the only one notice this issue.
Posted 20 February 2013 - 03:21 PM
Posted 20 February 2013 - 03:28 PM
Posted 20 February 2013 - 04:00 PM
I have seen shift an a lot of SCTs without them though, and since there is nothing in the design itself that presses or pulls in the plance of the plane of the mirror's movement, I would be at a total loss as to why the EdgeHD lock itself does would contribute.
I don't have the problem of mirror movement after focusing though my scope does have the usual amount of shift.
Sorry for not being able to contribute then. Good luck finding the answer.
Posted 20 February 2013 - 04:28 PM
We are talking about a tiny move here. Probably the locks, when engaged, lift the mirror a few microns. I am fighting this in the following manner:
- Of course, I reach focus by CCW approach, but after repeating this a few times I leave it at a point where I need to move the mirror a little bit more in the forward direction. Just a tiny bit, so that when I engage the locks, it will not cross to the other side.
- Then half engage the locks and continue turning the focus knob a tiny bit in CCW sense till I reach focus.
- Tighten the locks fully but not brutally.
Here I am talking about very little amounts. If you do this excessively, you destroy the collimation as the primary will be tilted since it is hold back by the two locks and pushed by the focuser screw.
When I am shooting with Hyperstar, I dont bother with the locks, but when shooting at 2800mm I prefer locking them.
I checked this during visual use, it is very hard to detect the move but when imaging you definitely see it on the Bahtinow pattern or on your FWHM values.
Posted 20 February 2013 - 05:44 PM
Posted 20 February 2013 - 05:51 PM
I know C9.25 or larger EdgeHD have longer 146mm back focus than C-8 EdgeHD (133mm) but are you able to meet the 146mm back focus with your Crayford focuser along with other imaging equipment like SX-AO, OAG, filter wheel and camera?
The only part I don't like about external Crayford focuser is that when you adjust the focus with external focuser, you are also changing the back focus as well. It may not be critical for smallish cameras but may be critical for large format cameras.
EDIT: Never mind. I see you are using FLI focuser which is probably pretty thin as compared to Crayford focuser so you should be able to fit the back focus requirement.
Posted 20 February 2013 - 06:01 PM
Once you get the FLI set, it's only going to move a few microns for filter changes or temp changes.
Posted 20 February 2013 - 06:18 PM
You got me interested in FLI focuser. I looked up FLI focuser and the cheapest is $995 for DF-2. Which one is yours?
DF-2 travels only 0.2" which is only 5mm. Do you have to preset it halfway (0.1"), focus with SCT focuser, lock it and tweak it with FLI focuser? Is 2.5mm travel either direction enough?
Also, can I attach FLI DF-2 directly to my C-8 SCT thread? Will the stock C-8 focuser be in the way? Do you have to rotate the focuser to screw onto the rear of the SCT scope? Does it come with an adapter to screw on the rear of scope, then snap the FLI focuser onto the adapter so that FLI focuser does not hit the stock C-8 focuser?
Posted 20 February 2013 - 06:27 PM
I set for half way.. it has plenty of travel. I use astrodon 3 and 5nm filters.
With my HD11 at f10, I'll need a longer adapter but would be way over backfocus. The locks and focuser are in the way.
I setup at f7 with the reducer. This clears the locks and focuser. The adapter is 0mm from reducer to PDF. The PDF to SX 72mm AO, needs at least a 9.5mm adapter to clear the PDF motor on the outside of the focuser housing.. (see pdf drawings on fli site)
The atlas motor is all internal and the external measurements are fixed.
Posted 20 February 2013 - 06:31 PM
Posted 20 February 2013 - 06:32 PM
Posted 20 February 2013 - 06:52 PM
Posted 20 February 2013 - 06:55 PM
Do you slew to a bright star near target DSO to check/refocus and then slew back to target DSO or can you refocus where the target DSO is at?
Posted 20 February 2013 - 06:57 PM
Posted 20 February 2013 - 06:57 PM
Posted 20 February 2013 - 08:37 PM
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, 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
Posted 20 February 2013 - 08:53 PM
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:
1) No additional back focus to worry about.
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:
1) Leave mirror locked at all times. Collimation remains unchanged while tracking the sky.
2) No backlash in some focusers like FLI.
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".
Posted 20 February 2013 - 09:17 PM
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.
Posted 20 February 2013 - 09:25 PM
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.
Posted 20 February 2013 - 09:57 PM