C11 SCT mirror flop potential fix? Advice please?

[oymd]

Good morning everyone

I am hoping for some advice regarding my devious plan...

I have acquired a C11 XLT OTA a couple of months ago, and only stated using it regularly over the past few days. Focusing on it is really hard, but achievable. I have noticed that whenever I slew to a target at a HIGHER altitude, the stars get slightly out of focus, and I need to refocus?

Is this what is called "Mirror Flop", which is inherent to the design of the SCTs, fixed with the lockable mirror in the newer Edge HD models?

Would getting a Dual-Speed 2" Crayford Focuser for SCT Telescopes, like this one over come this problem, and also help with imaging when I attach a ZWO camera?

 

 

My thinking is that I would drop the mirror using the focus knob all the way to the back of the SCT, so it cannot sag anymore, and achieve focus with the Crayford focuser?

Is that achievable, or is it being done already routinely, and I am just late to the party?

Many thanks

Ossi

[Davey-T]

Not tried it but I don't think you will be able to achieve focus with the mirror wound right back and it may still flop.

You could jig it up to see if focus is achievable before buying focuser although it is a nice mod in it's own right.

Dave

[oymd]

5 minutes ago, Davey-T said:

Not tried it but I don't think you will be able to achieve focus with the mirror wound right back and it may still flop.

You could jig it up to see if focus is achievable before buying focuser although it is a nice mod in it's own right.

Dave

Thanks Dave

What do you mean by "You could jig it up to see if focus is achievable before buying focuser"

Sorry, did not get that?

[Davey-T]

Just now, oymd said:

Thanks Dave

What do you mean by "You could jig it up to see if focus is achievable before buying focuser"

Sorry, did not get that?

Wind the mirror right back, insert an eyepiece, find a bright star and slide the eyepiece in and out to see which way it needs to go to achieve focus.

Dave

[RT65CB-SWL]

Hi, I am not au-fait with a C11, but a few years ago I was having a similar issue with my 're-modded' ETX105. It turned out that the locking ring that holds the primary mirror in place and slides up & down the baffle tube was slightly loose.

Maybe worth sending an email/phoning Steve Collingwood at https://sctelescopes.com/ and arrange a visit to get him to check over and overhauled, when the  COVID-19 virus restrictions have been lifted.

[oymd]

24 minutes ago, Davey-T said:

Wind the mirror right back, insert an eyepiece, find a bright star and slide the eyepiece in and out to see which way it needs to go to achieve focus.

Dave

oh...nice trick

will give it a try

Do you know which way should I turn the knob to wind the mirror back?

Should I go clockwise or anticlockwise? I do not think there is a way I can know by myself?

[Davey-T]

Clockwise I think, maybe if you look down the front you can see which way it's moving.

Dave

[oymd]

3 minutes ago, Davey-T said:

Clockwise I think, maybe if you look down the front you can see which way it's moving.

Dave

Many thanks, will give it a try tonight.

Do you know if adding the Crayford Focuser might be detrimental to imaging? I remember I read somewhere that it will significantly lengthen the FL of the scope beyong F10?

I plan to use the F6.3 reducer as well, so hope this will work?

Even if I do not fix the mirror flop issue, I would rather bypass the focus knob, as it is not fine enough for subtle adjustments to focus.

[Captain Scarlet]

if you're planning on that focuser upgrade, I'd go for the Revelation one, which is identical in all respects except for the Revelation Astro branding on it, and the price. I got one myself for my 6" Intes, and it's sitting in front of me right now. I'm very happy with it.

https://www.harrisontelescopes.co.uk/acatalog/revelation-sct-crayford.html 

 

[oymd]

6 minutes ago, Captain Magenta said:

if you're planning on that focuser upgrade, I'd go for the Revelation one, which is identical in all respects except for the Revelation Astro branding on it, and the price. I got one myself for my 6" Intes, and it's sitting in front of me right now. I'm very happy with it.

https://www.harrisontelescopes.co.uk/acatalog/revelation-sct-crayford.html 

 

That looks very smart!

Are they identical!? wow...one goes for £140 & the other for £90??!!

So, do you move your primary mirror all the way back, and rely on the crayford to achieve focus?

Did you try imaging etc?

Edited March 31, 2020 by oymd

[johninderby]

When you add a crayford to an SCT you focus using the regular knob and then use the crayford. The crayford only has a very limited travel so just for final focusing.

[oymd]

Just now, johninderby said:

When you add a crayford to an SCT you focus using the regular knob and then use the crayford. The crayford only has a very limited travel so just for final focusing.

So in your opinion will not really help with bypassing the 1ry mirror focusing?

[Captain Scarlet]

Just now, oymd said:

That looks very smart!

So, do you move your primary mirror all the way back, and rely on the crayford to achieve focus?

Did you try imaging etc?

The external focuser has quite a limited range, only 2.5cm, so i use the coarse knob on the back of the scope itself (which has much bigger range) to get me into the ballpark, and then fine-focus on the external thereafter. This scope though has a more-or-less flop-free integrated focus mechanism different from the SW-type ones I think, so flop isn't a problem for me.

[johninderby]

Or you could replace the stock focuser with a FeatherTouch microfocuser. Works really well but rather expensive. ££££££ 🙀🙀🙀🙀🙀

https://www.firstlightoptics.com/starlight-instruments-feather-touch-sct-microfocusers/microfocuser-for-celestron-c11-sct-edge-hd.html

Edited March 31, 2020 by johninderby

[oymd]

5 minutes ago, johninderby said:

Or you could replace the stock focuser with a FeatherTouch microfocuser. Works really well but rather expensive. ££££

http://starlightinstruments.com/store/index.php?route=product/product&product_id=112

Never liked that option.

It sticks out of the OTA so far back, and makes packing the OTA in the carry bag a nightmare. Possibly on a permanent setup, but not when I need to pack the C11 after every session.

[johninderby]

But an add on crayford sticks out a lot more. 

[Davey-T]

As mentioned the focuser adjusts final focus after using the mirror knob, I have an electronic focuser for fine adjustment for imaging after first rough focusing.

 The focal length of an SCT alters continuously when focusing so probably never exactly f/10, you can plate solve an image to get and idea of the focal length .

Dave

[johninderby]

Interesting thread on CN on the subject of SCT focal length during focusing.

https://www.cloudynights.com/topic/466794-sct-focal-ratio-variance/

[oymd]

29 minutes ago, johninderby said:

But an add on crayford sticks out a lot more. 

Agreed, but its much more stout and hefty.

What I meant with the moonlite is that it's quite slim and sticks out, and always felt its a breakage hazard when carrying around

[newbie alert]

I have the revelation one on my c8 and I'm more than happy with it..

I've read that with the stock focuser you should perform the final focus anti clockwise

[oymd]

5 minutes ago, newbie alert said:

I have the revelation one on my c8 and I'm more than happy with it..

I've read that with the stock focuser you should perform the final focus anti clockwise

Does that imply that an anti clockwise turn moves the mirror BACKWARDS towards the rear cell?

[newbie alert]

7 minutes ago, oymd said:

Does that imply that an anti clockwise turn moves the mirror BACKWARDS towards the rear cell?

No idea

If you watch the astro imaging channel the speaker from metaguide mentioned it

Edited March 31, 2020 by newbie alert
Added info

[oymd]

One last question please

How does the Croyford Focuser play with the Celestron f/6.3 reducer?

Can they work together?

Many thanks

[johninderby]

Some useful info in this thread.

 

[oymd]

Just now, johninderby said:

Some useful info in this thread.

 

Excellent

Thanks John