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Old 26-01-2019, 10:25 AM
vaxvax (Peter)
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Help needed identifying a Celestron C8 Blacktube

Hi,

I am thinking of buying the Celestron Focuser – Focus Motor For EDGEHD and SCT. It says it fits all SCTs since 2006.

I have a Celestron C8 SCT Blacktube with Starbright coating but that is all i know about it. I do not know what year this was made or the model no.

It is currently mounted by a bottom rail but this may have been done by the previous owner. It works fine but i would like help identifying the year and model.

I have included some pics to help.

Regards

Peter
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Last edited by vaxvax; 26-01-2019 at 10:43 AM.
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  #2  
Old 26-01-2019, 10:58 AM
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Merlin66 (Ken)
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Hmmm,
Certainly an older tube maybe from the mid 90's..........
You'd need to check carefully which model micro focuser would fit.
(I can't find a link for the Celestron version, but the Feathertouch site gives some details:
http://starlightinstruments.com/stor...product_id=261)

JMI still seem to offer a electronic focuser motor which could fit.
http://www.jimsmobile.com/buy_motofocus.htm#Celestron
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  #3  
Old 27-01-2019, 08:17 AM
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Merlin66 (Ken)
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Ahhh
The Celestron has yet to be released....
Not suitable for older SCT's.
https://www.firstlightoptics.com/ast...nd-edgehd.html
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Old 27-01-2019, 11:27 AM
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Sunfish (Ray)
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I have the same scope. Mine is complete with original mount and is a Powerstar 4. I am sure there were other version which were very similar on other mount version of the scope, the Ultima , the Polaris etc.

I use a Celestron issued version of the JMI motofocus which is simply a DC motor and tube backer which slips over the focusser assembly and plugs into the mount handset. Works fine and 8 am not really worried about minor image shift.There is a version which has its own controller from JMI and it can also plug into a Shoestring USB focus controller which I have but don’t really use.

They also made a zero shift SCT model for the visual back but more expensive and complex.

You can buy the Celestron Motofocus second hand but it will cost $100US and then you need to get the right model for the eight inch size focus shaft.

You can PM me or post if you want photos of mine.
Cheers
Ray
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Old 13-02-2019, 07:13 AM
vaxvax (Peter)
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C8 Blacktube

Hi,

Thanks for the reply.

As i said in another thread, Celestron are releasing a new model of the Focuser mid this year which is supposed to fit many more SCT types.

I will wait till then and see if the new one fits.

Regards

Peter
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Old 13-02-2019, 09:58 PM
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Sunfish (Ray)
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As the link quoted by Merlin says :

[The focus motor is compatible with all SCTs from 6” to 14” in aperture that have been produced since the year 2006. It is not compatible with 5” SCTs or older models.

Yours is definitely an older 1990s model with the cast back and so has a different focus shaft assembly.

It really is good to focus without the shimmy and lock up the focus knob.

Your only option is to buy a second hand JMI / c8 celestron focusser unit on Cloudy Nights as I did ( or make one from another similar device) It is really only a geared DC motor with a friction fit rubber o ring coupler for the focus shaft. Very basic but also quite well thought out and somewhat foolproof.

Without a matching variable speed hand control however, it will not be satisfactory.


Good luck.
Cheers.



QUOTE=vaxvax;1416573]

Thanks for the reply.

As i said in another thread, Celestron are releasing a new model of the Focuser mid this year which is supposed to fit many more SCT types.

I will wait till then and see if the new one fits.

Regards

Peter[/QUOTE]
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  #7  
Old 14-02-2019, 05:57 AM
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The_bluester (Paul)
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I wondered about one of the Celestron units, but I replaced the focusser on the C925 with a Feathertouch two speed years ago and have not got the original any more.

I wondered about one of these but again I would need a standard focusser, or custom parts to fit it to the Feathertouch

https://www.astroshop.eu/motors-cont...-c9-25/p,57049

None of the options (Apart from something on the visual back, which I have not got space for in the imaging train) would eliminate image shift while focussing but without physically touching the focus knob it would probably be reduced a fair bit.
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Old 14-02-2019, 07:54 AM
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Sunfish (Ray)
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176 euros for the motor without the controller for a $20 stepper motor and a $10 gear and belt. That is an expensive bracket but you can see the design, although bulky , would work.

The two essential parts of the JMI / celestron focusser apart from the motor is an aluminium and rubber ring friction fit collet over the focusser shaft (or plastic knob adaptor ) and a plastic tube with a plastic grub screw on the focusser housing to mount the whole assembly.

I suspect you could have these made in aluminium tube to fit any focusser shaft or knob for older SCT or indeed a feather touch shaft.

The really useful part of this design is that they can be taken off and on easily and the focusser shaft is still available for quick manual use.

Recent SCT I think do not have a projecting tubular focusser housing to accept the plastic motor tube and thumb screw ( thus the new design) but I suppose adaptor plates are not too complicated either.


Quote:
Originally Posted by The_bluester View Post
I wondered about one of the Celestron units, but I replaced the focusser on the C925 with a Feathertouch two speed years ago and have not got the original any more.

I wondered about one of these but again I would need a standard focusser, or custom parts to fit it to the Feathertouch

https://www.astroshop.eu/motors-cont...-c9-25/p,57049

None of the options (Apart from something on the visual back, which I have not got space for in the imaging train) would eliminate image shift while focussing but without physically touching the focus knob it would probably be reduced a fair bit.
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  #9  
Old 14-02-2019, 08:34 AM
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The_bluester (Paul)
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To be honest I know I am pushing my OTA to do something it is really not designed for (AP) and it might be in line to be replaced. I should not spend a cent on it.

But focussing for AP is frustrating and it definitely changes focus over the night as the tube cools.
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Old 14-02-2019, 08:42 AM
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Merlin66 (Ken)
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Paul,
I used the C9.25 for a while...also fitted the 10:1 micro worked great.

I didn't find the focus slop too bad ( compared with various Meades I'd used)

You could consider mounting a SW motor and belt drive the existing micro knob. It would work OK.
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Old 14-02-2019, 09:35 AM
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The_bluester (Paul)
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I have considered doing something similar but if I do it I will probably look to use a stepper to make it possible to do repeatable focus. Taking a leaf out of automotive steppers they initialise them by driving them to the stop first by feeding a large number of steps in one direction (needing to be sure a stalled stepper against one end of the focusser travel won’t do any damage) and then backing off the other way a known number of steps.

If you know that 1000 steps is close to focus as an example then a setup like that should be close within ten seconds of switch on.
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Old 14-02-2019, 09:43 AM
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Merlin66 (Ken)
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Yeah, understood.
To ensure minimal impact of focus shift (mechanics, temp etc) I refocus every 30 minutes while collecting spectroscopic data so a stepper wouldn't help (unless it was linked to my Lodestar guider and could measure and react to FWHM changes)
The remote JMI Focus Pal seems to work for me.
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Old 14-02-2019, 10:02 AM
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that_guy (Tony)
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It being a DC motor, I just dont see it being worth the price
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Old 14-02-2019, 10:18 AM
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Sunfish (Ray)
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Here is what the motofocus looks like on an old c8 .

This must have a patent owned by JMI , now owned by Farpoint Astro. The key seems to be the fairly precise friction fit of shaft collet and close , but not too tight fit of plastic tube.

Also the motodec which is essentially the same thing on some models. Shown with a home made bracket made from plumbing fittings as I could not buy the correct stepper motor or bracket for my model handset. Plugs into the focus port and I can swap them over for final focus.

I think I will change this to a $20 stepper motor now I have sorted out what is required.
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Old 14-02-2019, 10:18 AM
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Merlin66 (Ken)
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Tony,
$100....
The DC pulse control under JMI Focus Pal works pretty well - check it out.
It really depends what you want to do - focus the scope or compensate the changes as you go....
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Old 14-02-2019, 07:12 PM
Ukastronomer (Jeremy)
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A few interesting links for the future

https://www.springer.com/cda/content...869-p179934459

https://skywatch.brainiac.com/used/used_sct.pdf
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Old 14-02-2019, 07:56 PM
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Sunfish (Ray)
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By JMI Focus Pal do you mean something similar to a FCUSB or is the JMI unit offering a more controlled focus?

The problem with DC motors for focus i have found is overrun unless the motor speed control can be set very slow or there is a braking mechanism. But overall ,with the celestron speed adjustable handcontrol , the function is quite good if not repeatable.

Using occasional refocus with a bahtinov mask, I have got much better at manual focus.So much that the motor focus gets less use. Or perhaps it is simply steadier with the scope bolted to a pier and tightening up the mount.

Quote:
Originally Posted by Merlin66 View Post
Tony,
$100....
The DC pulse control under JMI Focus Pal works pretty well - check it out.
It really depends what you want to do - focus the scope or compensate the changes as you go....
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Old 14-02-2019, 08:03 PM
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Sunfish (Ray)
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By JMI Focus Pal do you mean something similar to a FCUSB or is the JMI unit offering a more controlled focus?

The problem with DC motors for focus i have found is overrun unless the motor speed control can be set very slow or there is a braking mechanism. But overall ,with the celestron speed adjustable handcontrol , the function is quite good if not repeatable.

Using occasional refocus with a bahtinov mask, I have got much better at manual focus.So much that the motor focus gets less use. Or perhaps it is simply steadier with the scope bolted to a pier and tightening up the mount.

Quote:
Originally Posted by Merlin66 View Post
Tony,
$100....
The DC pulse control under JMI Focus Pal works pretty well - check it out.
It really depends what you want to do - focus the scope or compensate the changes as you go....
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Old 14-02-2019, 08:03 PM
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Merlin66 (Ken)
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Ray,
Yes the FCUSB controller.
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Old 15-02-2019, 02:21 PM
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Ken,

Perhaps I should use the FCUSB as I have one.

In a program such as sharpcap can that mean the program can correct focus..?

I am not keen on laptop use but if focus correction could work with parfocal off axis guiding camera and it may be worth the trouble. Particularly if it can work in IOS.

Quote:
Originally Posted by Merlin66 View Post
Ray,
Yes the FCUSB controller.
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