SCTs - tuning your focuser rigidity & minimising mirror flop
Well my big question is what can you do?
Trying to correct my tracking I removed my Canon 400D - which I haven't had to re-focus on my C9.25 CF for 6 months and in its place put a 3" extender and a 12mm illuminated reticle.
I re-set focus by hand and got to experience just how much flop was in the focuser. Once the star approached focus - if I minorly reversed focus on the SCTs main focus knob - well the star jumped about 15 percent of the way across the field of view! Is this normal - or indicative the focuser is too loose?
I had forgotten how much moving the main focuser cause the centre of the star to shift. Ideally you'd want no shift - here the shift is so great as to make focusing near impossible. I remembered that's why I bought a Meade motor focuser and did fine tuning off this - this made it easy.
1. For a well tuned focuser on a moderate SCT - how much should you expect to see a star jump acorss the field of view when you change direction of focus as gently as you can?
2. For the jump I see - is this indicating mirror flop or focuser travel?
3. What can I readily do to improve things (or is this only shop tuneable if at all)?
I am now staring to wonder if my tracking problems (star appearing to gain in RA 1 arc minute and hour is either play in the imaging training (that reduces with the sine of the elevation) or mirror flop!
I also remember one experience imager saying the wound the mirror all the way back to stop mirror play / flop and used an external focuser with I presume alot of play to bring stars into focus! Someone else drilled holes in the back of the OTA, threaded them and inserted plastic screws until they hit the primary to lock it into position permanently!
Agree - noting the Crayford style focuser solution isn't really any different to the Meade electronic focuser I am using externally - other than not being motorised or computerised. Its kinda saying the default focuser is no good - use something else isn't it?
On my old LX200GPS, I had to wind the focuser back and forth end to end about 30 or 40 times to redistribute the grease on the baffle tube. This made a huge difference in mirror flop, taking it from the full width of the eyepiece down to under 1/8th. I can't remember what eyepiece I was using to test it, though.
In other words, high viscosity well distributed grease on your baffle tube might make a huge difference in slop. I wonder how long till someone patents a Crayford/multiple roller bearing style primary mirror support for baffle tubes (if it has not been already)? It shouldn't be too tough to do.
See my thread on SCT mirror weights for details of the "Harry Mirror Mat" will remove ALL mirror flop.
The baffle crayford idea is toooo complex to work and the three nylon screws doesn't appear to be a robust solution.
The locking screw ( See MAPUG) works reasonably well, but I think my pneumatic mat will be the no- machining, easy, reliable answer!
Keep you posted on the prototype results.
I have almost eliminated mirror flop on LX90 by replacing plastic washers in the focuser with trust bearings. See Petersons Engineering – EZ Focus Kit.
Improving the thrust bearings on the foccusser may give some marginal improvement, but as the root of the flop problem lies in the fit between the primary mirror sled and the running face on the baffle, there needs to be a positive way of "locking" the moving parts together.
The join pin between the focusser rod and the mirror sled is probably another area of potentional "slop".
I asked Celestron themselves about a mirror lock on C8's and they said if you turn the knob counter-clockwise the mirror moves in the forward position and this is how the mirror should be moved when trying to final focus and avoid mirror movement during exposure.
With image shift it is usually a wear problem and can only be repaired, a crayford style focusser would help but you would have to make sure the crayford will lock also.
Hmm, it seems to lock the focus knob in place, but not the actual mirror. I'd be curious about how those play numbers measure up on the average scope. It would seem they are blaming focuser creep for the slop more than anything.
I have almost eliminated mirror flop on LX90 by replacing plastic washers in the focuser with trust bearings. See Petersons Engineering – EZ Focus Kit.
I achieved the same on my LX with a replacement feathertouch 10:1 focusser from Starizona. The 10:1 fine focus is a definite plus as well, the unit itself is easily fitted and is beautifully made.
I've been looking for one of these - is the Feather touch the best micro focuser for SCT's out of interest?
The installation instructions show the unit mounts to the thin metal backplate - with could bend - rather than the base of the OTA like the original focuser. Is this a problem in your view?
Lastly does is lock well and what can you note about its performance before and after fitting - how did you diagnose how much improvement you saw please!
Changes to the thrust bearings, and focussing knob system, will not make any real change to the mirror movement along the baffle, there's still potential for the mirror to flop sideways.
The focuss rod is usually at three o'clock looking at the rear cell, it therefore ( and the use of the "locking screw" method) can ONLY affect the flop in the 3 to 9 o'clock direction, in the 6 to 12 o'clock etc there's nothing preventing the mirror from tilting and hence giving image movement during long exposures. I'm not to worried about some movement during focussing, it's when you 're guiding and taking a long exposure photo that it can and does become a headache.
Note when mounted on a wedge the 3-9 o'clock is usually the RA direction ( good news!) but the 6-12 is the Dec and the off-set loading of the mirror will vary with altitude.
Members with the R series telescopes which also have the "improved" locking collar around the baffle, still say mirror flop is an issue.
I tried alignment testing with my 127mm MAK today - and tracking stars closer to the SCP.
Surprise, surprise - Tracking first Acrux then Rigel Kentarus - I did see DEC drift East in a series of 30 minute checks - so this is wierd. Tracking Sirius on my C9.25 for 3 hours - no DEC drift. Tracking Sirius on a Megrez 80 for 3 hours - minimal RA and no DEC drift. But 10 minute snaps of a star alot closer to the pole shows there is discernible DEC drift or this is some complex system of wierd mirror movement.
Anyone guess what I'm seeing? MaxPoint said I was within 40 arc second of the SCP on both axes - is that enough to show DEC drift near the pole but not further away from the pole?
PS
I probably will get a Microfeather focuser - because it will be useful for Visual, and it might help mirror shift - you never know, it certainly shouldn't hurt!
The installation instructions show the unit mounts to the thin metal backplate - with could bend
The unit bolts on in exactly the same place as the old focusser, not sure where this "thin metal backplate" idea comes from. The improvement on mine was enormous, when using the fine focus there isn't any movement at all after releasing the knob, mainly due to the fact that there is so little pressure needed to actually move the finr focus knob. I found that on the old focusser just touching the focus knob was enough to move the mirror and the focus often changed after letting the knob go due to flex in the focusser, the Feathertouch is so much more rigid and had decent bearings as well. It's never going to be as good as a crayford and mirror lock combination, but it's a huge improvement over the standard.
Last edited by acropolite; 17-02-2008 at 11:32 AM.
Because in the link you supplied there is a how to attach PDF that doesn't make it look like a secure structure on a Celestron. I believe from the Piccys Meade goes right onto the solid back whereas for Celestron http://starizona.com/acb/Feathertouc...-P984C654.aspx
The insutruction and pdf are quite poorly illustrated and infer the turning moment and weight is on a thin, sub 1mm backplate that is a dust cover for the focuser's internals
I've been looking for one of these - is the Feather touch the best micro focuser for SCT's out of interest?
The installation instructions show the unit mounts to the thin metal backplate - with could bend - rather than the base of the OTA like the original focuser. Is this a problem in your view?
Lastly does is lock well and what can you note about its performance before and after fitting - how did you diagnose how much improvement you saw please!
Matthew
Matthew,
I've got a FeatherTouch Micro on my C8. Man it works a treat.
Mirrir shift is ALMOST totally removed.
I get it as close as I can with the micro & then I use the motor focuser to get that final little bit.
Get onto the guys at FeatherTouch directly. They are great to deal with & very helpful.
. The lock doesnt fit all C11's.
