C11, colimation issue

[bojan]

Last night I was imaging Jupiter, Saturn, Mars...
Weather conditions here in Melbourne were not great so results were also way below my expectations...

Anyway, while focusing, I noticed the image and colimation was wandering across the cropped FV of Canon 60D (in 640x480 video mode), depending on focuser knob movement and postion (it was best when I was approaching focus point by rotating focus knob in CCV... obviously previous owner had the same issue as I didn't attempt to re-colimate the scope since I've got it. The issue is also widely discussed on the web (CN for example).

I am aware that this is the consequence of (poor) design of C11 focusing mechanism (one arm, off-axis push-pull), and many people are using additional external focuser to minimize the problem.. which will potentially always be there ("mirror flop") unless the primary mirror is properly fixed and supported in optimal position - and that requires redesign of the whole thing that is inside.


If anyone in our community did something about it, I would like to know about any ideas and solutions.

[Merlin66]

Bojan,
I've used many (!!) SCT over the years, mainly Meade - 8", 10" and 12" and it's only in the past eight years or so that I actual moved to Celestron and the EQ6 mount (for spectroscopy)
The mirror slop/ flop is much worse in the Meade design and I had to fit a mirror locating screw (MAPUG design) this worked "Ok"

With the C9.25 and later the C11 I found the slop/flop problem to be very minor and didn't affect my focus/ guider (I use a Lodestar on a "on-axis OAG") what can and does make a difference is to run the focuser through the whole travel a few times to re-spread the grease on the baffle tube.
Needs to be a regular part of the observing technique......

[floyd_2]

Interestingly I purchased and installed an SCT crayford focuser on my new C9.25 in the first instance. In retrospect, I don't think it was really necessary for visual anyway as the stock focusing mechanism turned out to be excellent. Still, the fine focus on the crayford is nice. Perhaps adding a crayford to your C11 may be of assistance Bojan.

[bojan]

I have one DIY motorized Crayford, intended to be used with my 10" Newt, whenmounted under the dome and on permanent pier.
Now I think I will have to design and build adapter for C11.

Still, mirror needs to be fixed properly to avoid issues with flip-flop.

[Merlin66]

Hmmm,
Fixing the mirror in an older SCT is not an easy task....

On the latest Celestron HD design they build-in location screws....

[Merlin66]

Dean,
I found the FeatherTouch 10:1 Micro focuser excellent when I was using the C9.25.
Highly recommended.

[bojan]

This (https://www.cloudynights.com/topic/463047-is-there-a-way-to-fix-sct-mirror-shift/page-10) is interesting discussion about how to fix SCT mirror.

I am thinking about something similar.

[multiweb]

You could use a ticker grease on your baffle tube to minimise image shift. There is a depression in the middle so you can add more. If the shift is still there then you could insert fine shims in between.

[Kal]

I always bring final focus in by turning counter clockwise, as suggested by celestron themselves as this pushes the mirror inwards (up against gravity), and that is what I collimate for. I also added an electric focuser to the back as well.

[bojan]

THIS (https://www.cloudynights.com/topic/299511-m809-complete-makeover-a-photo-journal/)is how Celestron should have designed the focusing mechanism.. like Intes (http://www.photoinfos.com/astronomie/Intes-Micro-Alter-M603-03.htm) did.

I am thinking about doing something similar, or adding arm/lever which would be moved with existing focus knob, but which will push the mirror at its centre.
It all depends on how much space is available between base and mirror.
For that, I need C11 drawings.. or dismantle my scope and measure and reverse-engineer it myself.

[bojan]

The arm / lever should look something like this below.
The actual design must incorporate spherical joints and possibly linear bearings, to enable the required movement of the arm.

[Merlin66]

Bojan,
Why move the mirror??
Could it be "locked" - three edge/ rear supports to the tube??

Set at infinity focus should give enough backfocus......then a rear mounted focuser??

[bojan]

Fixing mirror is definitely an option, if others prove to be too complicated (apparently, they are..).
It all depends on current situation - if my C11 has 3-vane support like this one (https://www.morningstarobs.com/uploads/4/5/9/0/4590917/5307413_orig.jpg) (very likely), then I will simply add two screws with knobs and counter-nuts to fix it.
If not, then I will have to make suitable flange (http://www.tamanti.it/Tips/C11MOD.htm), so screws can't touch the glass directly.

[Wavytone]

Bojan the Intes link is broken.

FYI their mechanism was usually quite stiff, to the point of irritating.

[bojan]

It works for me...

Try this again:
http://www.photoinfos.com/astronomie/Intes-Micro-Alter-M603-03.htm

[Marke]

You can buy a secondary focuser and not move the primary .

[Merlin66]

Mark,
Where can you buy one???
Understand the concept, it was used on a Meade SCT then dumped....

[Marke]

Optec make them
www.optecinc.com/astronomy/catalog/fastfocus/index.htm

[Merlin66]

Hmmmm
Us$1200 is a hellava price to pay for a focuser!!!!

[Marke]

When your the only one in the market I guess but it gets rave reviews

[bojan]

I have DIY focuser (http://www.iceinspace.com.au/forum/showthread.php?t=88216&highlight=focuser) (I made it for my 10" Newt), only adapter for C11 is still missing

[Marke]

I'll be getting a C11 edge very soon but will be using a motorised crayford and just leaving the mirror locked .

[bojan]

Working on adapter to fit my Crayford on C11...
Anybody could confirm the optimum focal plane distance from 2" collar is 5"475?

I contacted Don from Bintel with question about threads & pitches...
Below is what he came up with, it may be usefiul for someone:

2" Filters 48mm x 0.6mm
1 1/4" Filter 28.4mm x 0.6mm

Meade Large SCT
3.25" 16TPI without adapter
2" 24TPI with adapter fitted

Celestron SCT
3.3" 16TPI without adapter
2" 24 TPI with adapter fitted

T Thread 42mm x 0.75mm

Takahashi 2.7" Focuser 72mm x 1.0mm
Takahashi Sky 90 55.9mm x 0.75mm

Atik 11002 Camera front 54mm x 0.75mm

QHY Filter wheel in and out 54mm x 0.75mm

Takahashi Finder scope light M8 x 0.75mm

Vixen VC200L Focuser rear thread 60mm x 0.75mm

GSO 8" RC rear cell 90mm x 1.0mm

Web Cam adapter M12 x 0.5mm

EQ6 Pro altitude adjusting screws M12

[billdan]

Thanks for that list of thread sizes Bojan, however I am confused with the 2 inch filter thread pitch.
I thought the 2 inch filters are 48mm x 0.75mm thread pitch.

[Merlin66]

Bojan,
I agree with Bill re the 2” thread size.
I’ve see that 5.475” measurement mentioned before for Celestron....

[bojan]

This is what Don from Bintel send me... maybe his typo?

I will ask...