I can't believe companies can sell telescopes with focusers as bad as the mirror shift system on SCT's. They are amazingly bad, totally inadequate for the job.
Having recently aquired my first SCT (Meade 10") I just find focusing a really bad experience, so I need a fix.
I know you can add a normal focuser to the back thread of the OTA, but what happens inside?
Do you take off the standard SCT focusing knob? Is there anything you add inside to hold the mirror firmly in one spot, or will it just flop about and require me to refocus all the time?
Does anyone have any recommendations as to new focusing on the SCT?
You end up using the SCT focus knob for rough focus (major focus changes) and the addon focuser (say TCF-Si or something) for fine focus. I only need to do rough focus when switching imaging trains (adding a reducer, different camera, going visual, etc). After I do rough focus I lock my mirror down.
i just do a rough focus , centre the image and do a fine focus.negligible image shift in fine focus. when i had an lx90 image shift was very small.
philip
Im not a expert on this but from experience i can suggest that once you lock the mirror down you should be able to "collimate" the scope to that exact combination of tilt with the camera in place. then you should be able to focus properly with the focuser on the rear cell.
With your system there should be some repeatability in the mirror flop and hence if you where to come into rough focus from the same side every time your collimation should be pretty much spot on.then you take it the last little step with the focuser.
Thats my thoughts either that or get a RC/ Newt/ Refractor
My C9.25 has a bit of mirror flop so I opted to get a crayford 10:1 focuser for it. As others have stated, I do a rough focus with the scopes original focus knob and then use the crayford for fine adjustments.
IIRC the Meade SCT's have the option to lock the mirror in place but the Celestron scopes do not.
One suggestion though is to occasionally wind the focus from one end to the other a few times to make sure there is grease all the way along the focuser gear, this certainly helps with smoothing out the primary focus.
I also have a Meade 10" SCT. This is one that I bought 13 years ago and has no mirror lock. The newer models come with a way to lock the mirror in place after focusing has been achieved.
For the older models, yes - there will be some mirror shift, and it varies between SCT's. I found the best way was to always focus in one direction, i.e. if I have gone past best focus, I turn the knob the opposite direction to go past, then come back. One direction which I now can't remember, was better - I think because the mirror is pushed against gravity.
I suggest getting an after market focuser - I ended up with a Motofocus from Jims Mobile 10 years ago.
-John
[Edit] I just checked my SCT - anti-clockwise on the focus knob moves the mirror away from the back. This is also how to focus if you have any significant backlash in the focuser.
Last edited by John0z; 05-12-2011 at 05:58 PM.
Reason: Update focus direction
Further to JohnOz's post above, turning the focuser anticlockwise pushes the mirror up towards the secondary minimizing subsequent movement. In my experience the smaller sct's have better focusing and benefit less from crayfords.
I can't believe companies can sell telescopes with focusers as bad as the mirror shift system on SCT's. They are amazingly bad, totally inadequate for the job.
Having recently aquired my first SCT (Meade 10") I just find focusing a really bad experience, so I need a fix.......
Thanks for this. I can't believe that I never noticed that little hole before. I suppose I must have seen it but not given any thought to it. I just shined a torch into it and certainly there is a threaded hole inside.
I've been trying to get documented evidence of the impact of mirror flop on image exposures - no success so far!
The attached write-up gives some background....
Comments welcome!
Agree on the SCT focuser issues. I did two things with my Meade LX200R out of sheer frustration:
1. Got a Feathertouch focuser from Starlight
2. When installing it I ran the mirror up and down the full play of travel to distribute the 'grease'
Very happy with this focuser and the results. The 10.1 fine focus is great for imaging and I've stopped using the meade motor focus unit completely. I get rough focus, lock the mirror and then just use the fine focus
I'll try the long bolt/spring thing and see how that goes. My scope is an
LX50 OTA, and doesn't appear to have a 'lock' as such, so no lock down.
I'd really like to be able to fix the mirror in the tube so it's FIXED. No movement,
no adjustment other than collimation, then just use a proper focuser on the back.
Hi. Actually I ended up buying a Jims Mobile NGF-S Focuser about 10 years ago - not the Motofocus which I had been considering. I had never used it because I stopped observing and began travelling. Now I have actually mounted it onto the SCT and will wait for the rain and clouds to stop and use it for the first time. Might be a while, the way the weather is looking.
I will also try the long bolt method to keep the mirror in position - but that really is only for photography, I never really had problems with visual observation - but then I hardly ever used this scope.
-John
Last edited by John0z; 06-12-2011 at 11:11 AM.
Reason: Fix spelling
The original focuser on my C9.25 had woeful slop and play - stars would jump wildly when I changed focus directions. The backplate the focus knob was mounted on was too thing and would flex easily too.
In the end I added:
1. Feathertouch microfocuser - like the one before - it is great and has a really solid - flex freee mounting plate
2. Meade Motor focuser
3. JMI USB controller for the Meade motor focuser
Add your Bhatinov mask and all is well. In fact with a Carbon fibre tube I only have to tweak focus one every few weeks if the temperature is pretty constant.
Since adding the above mentioned gear stars don't jump at all when I adjust focus - and that's great!
Sorry to hear you have excessive image shift. Due to both the basic design of the focusers in SCT's and Maks, and the pressures of
high volume production, it is pot luck how much image shift you get.
It can range from almost nil to incredibly bad. I had a new 6"Mak which was so bad it was almost unusable, even for observing, let alone
imaging. I returned it for repair, and it came back enormously improved.
If yours is under warranty, I suggest that you try that.
I have a Celestron C-8, which is 30 yrs. old, and the image shift is almost undetectable below 80x, and is approx. 4 to 5 per cent of the field of view
at 275x. Hope this helps.
raymo
