SCT focusers are attrocious!

[koputai]

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?

Cheers,
Jason.

[frolinmod]

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.

[koputai]

Thanks Frolinmod.

When you say "lock my mirror down" what do you mean? How is this achieved? My OTA just has a knob at the back.

Also, when I change focus direction, through the eyepiece I can see the mirror moving in a number of planes. Is this normal?

Thanks,
Jason.

[jenchris]

my 8" meade has no shift at all. Even in live view at x10.
Maybe it's just your OTA out of adjustment

[5ash]

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

[bmitchell82]

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 :D

[kustard]

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.

[frolinmod]

The Celestron EdgeHD series OTAs have mirror locks.

Image shift during focusing using the focus knob on the back of an SCT is unfortunately a fully expected "feature."

[rainwatcher]

Have a look at the thread Millionaire Observers in the Equipment forum.

[John0z]

Jason,

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.

[casstony]

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.

[GrampianStars]

J :D
Here's all you need to do...........
a mirror lock..
http://www.isomedia.com/homes/cvedeler/scope/mirrorlock.htm

All your troubles will begone ;)

[John0z]

Rob

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.

-John

[Merlin66]

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!

[dcalleja]

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

Dan

[Merlin66]

I added a manual index (Vegemite lid!) and then the skywatcher focus motor to my C11.......works 100%

[koputai]

Wow, so many replies! Thanks guys!

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.

Cheers,
Jason.

[John0z]

Jason,

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

[g__day]

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!

[raymo]

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

[Poita]

Whether SCT, Newt, RC or refractor, it seems in the consumer space that the standard focusing option included almost always could use replacing.
I added a cheap GSO crayford to the back of the Meade 10" and never looked back. I lock the mirror and use the crayford for fine focus and it is rock solid.

[toc]

I agree they are pretty bad - my 12inch Meade SCT has not only mirror flop, but runaway focus - when you move the focus anti-clockwise, it will slowly drift out of focus all by itself. I suspect I need to open it and lube up the shaft. (ew, that sounded wrong)

For any sort of planetary imaging, the stock focusers on the larger Meade SCT's appear pretty useless. My old C8 focusing was great - light as a feather, and almost no mirror flop.

[rainwatcher]

Some interesting replies here, it really does seem like luck of the draw.
As for locking the mirror there is no facility on the C8, and if you lock it how can you microfocus it, implies there is enough slop in the lock for some micro movement.
Anyway i have found that my C8 original (and of couse horribly expensive) DC dec adjuster fits and i am first going to try that just to see if it it will hold good focus. Of course its a single speed so i would have to take it off and on after rough focus.
Then i am gong to strip the back end and check the adjuster plate, connection and lube.
I am also still keen on the GSO 10:1 crayford - at $99 its a bargain and i will feel more comfortable adjusting the eyepiece than the whole mirror.

[koputai]

So it looks like the options for my LX50 vintage Meade 10" is either the Feathertouch microfocuser upgrade to the standard system, or lock off the mirror somehow and add a proper focuser like a Moonlite/Feathertouch/etc to the back. I think I will head in that direction.

Thanks everyone.

Cheers,
Jason.

[Poita]

I can send you my GSO unit to borrow for a week if you want to try it out.

[Poita]

On my 10" meade the standard focusing had none of those issues, but I went to a crayford for the fine control, and because it is what I am use to on the refractor anyway. It makes a nice upgrade whether the SCT has issues or not really.

[koputai]

Does anyone have a link to a site with teardown instructions for SCT's? Or how-to's on servicing the focuser?

Cheers,
Jason.

[Karls48]

An other possible solution is to replace plastic washer in the focuser with trust bearing. On my LX90 it improved mirror shift considerably. I cant say if it was the bearing or re-greasing. I did use Peterson EZ focus kit but you can use just right size trust bearing.

[koputai]

Thanks Peter, that's very kind of you. I think though, that with a GSO being worth under $100, the postage here and back would be a waste.

Regards,
Jason.

[rainwatcher]

http://ngc1514.com/Celestron/disassem.htm

has a dissasembly page.

[marki]

On the LX200R/ACF tubes you can remove the original focuser knob (grubscrews in the end) and there is a nut underneath. If you do this right up then back it off about 1/8th of a turn it will remove a lot of slop from the mechanism only really leaving free play between the baffle tube and mirror to worry about but this is minimal. Only problem is that the focusing action can become very heavy and you need to find a balance between slop and focusing action which means mucking around with the adjustment for a bit. I did this when I first got my Meade with good results but have now switched to a feathertouch microfocuser and a moonlight stepper crayford. For visual the FT is ideal, for AP the moonlight takes over.

Mark

[rainwatcher]

I am sure you must all be getting bored with this subject by now, but i have had a minor win. I stripped my focuser on the C8 and did as suggested in most of the posts, moved the object through all its travel to redistribute the grease. One thing I did notice is that the bearing on the shaft is at least 6 mm diameter smaller than the machined bore it goes in. It is located in place by the three screwed plate that locks it in. I am not sure if this is good design, in fact I know its not. However I centred it the best I could and locked up the plate. The wobble was noticeably less, still worse than my standard Newtonian focuser, but if I were only doing visual I would accept it. My main interest is photography so I will be buying the GSO Crayford.
By the way I have given up on my JMI/Celestron Dec motor option for focusing as the electronics for it are interconnected into the DC drive of the scope and I have been spectacularly unsuccessful in finding a circuit diagram that I can use to redesign. It may be surplus to requirements anyway as I don’t like it, and will be replacing it with a stepper motor system of my own design and build.
On a final note I am looking for something to de-dust the front of my C8 corrector plate and also something to clean my eyepieces, which my photography is showing up as quite dirty, one lens seems to have what looks like little water marks on it. Any advice .

[Damian Couzoff]

I use a Bintel Crayford (2 inch) 10:1 SCT focuser on my CPC 1100-11 inch.It works very well on high power/visual planetary.The standard focuser worked well enough,but the crayford is easier to fine focus at high power.

[multiweb]

This is quite clever actually. Locking the focusing arm rather than the mirror. On my C11 I have mounted 3 bolts spaced at 120 degrees that hold the mirror once in focus. It does a good job but you can tilt the mirror if you apply the slightest pressure on one side or even defocus the whole field. With practice you learn to get the mirror a bit lower out of focus then push it back up to focus with the three lock. It seems to hold better with a little pressure. It's a big chunk of glass anyway and it would take quite a lot of pressure to degrade the image quality.

In the end best focus is of course achieved at the visual back with a good quality focuser. But you can't use one with an hyperstar so having a feather touch micro-focuser attached to the primary is also a must.

[koputai]

I've made and fitted the spring loaded gizmo, will see how it goes if the skies clear. Have also bought the GSO focuser, which I expect to be a lot better.

Cheers,
Jason.