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Old 15-03-2013, 06:51 PM
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Identifying Flexure..Faulty Focuser???

Hi guys.

If my PHD guiding graphs appear quite good but I am still getting slightly elongated stars and the images I am getting are gradually shifting in one direction in the field of view is this likely flexure?

Can a helical focuser with "slop" in a short focus refractor used as a guidescope contribute greatly to flexure or is it most likely the attachment rings?

Thanks in advance for any advice anyone can offer!


John K.

Last edited by John K; 23-03-2013 at 01:23 PM.
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Old 15-03-2013, 07:49 PM
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Shiraz (Ray)
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Hi John. if this is with your JMI, could also be the mirror shifting around slightly with attitude (eg settling on the supports or sliding sideways a tiny bit). If that is the case, OAG should fix it.

Last edited by Shiraz; 15-03-2013 at 08:31 PM.
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Old 15-03-2013, 08:12 PM
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naskies (Dave)
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What direction is the elongation in? If it's along the RA axis, then it could be related to guide settings or problems with the mount tracking (spiky periodic error curve).
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Old 15-03-2013, 08:22 PM
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nobbygon (Angus)
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Perhaps you could post a photo of whats going on with your stars.
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Old 15-03-2013, 08:49 PM
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Thanks guys,

i'll check the mirror cell lock screws - I think this could be an issue perhaps.

Some images attached - the image movement is consistent in one direction mostly.

These are 2 min exposures at 1600mm focal length.
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Old 16-03-2013, 03:15 AM
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Tandum (Robin)
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John, it will be the helical. Unless you can lock it off it's no good.

I'll bet if you shoot east and see which direction the subs march, they'll go the other direction when shooting west.

I'm guessing this is a gso rc8 (1600mm). An OAG solved all my issues with that scope.
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Old 16-03-2013, 09:09 AM
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Quote:
Originally Posted by Tandum View Post
I'm guessing this is a gso rc8 (1600mm). An OAG solved all my issues with that scope.
The scope is a 12.5" f/5 reflector JMI NGT 12.

The guidescope is a Lumicon 800mm superfinder with a helical focuser:
http://www.lumicon.com/store/p/230-D...er-System.aspx

Any ideas how to remove slop from a helical focuser on an 80mm short focus refractor if this is the cause?

Otherwise will have to give OAG a go.

John K.
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Old 16-03-2013, 09:51 AM
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Craig Stark recommends glue - guess that would turn the superfinder into a dedicated guider - if araldite solves the problem though, its probably a cost effective solution.

http://www.stark-labs.com/craig/reso...iding_RTMC.pdf

Last edited by Shiraz; 16-03-2013 at 11:19 AM.
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Old 16-03-2013, 03:27 PM
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Quote:
Originally Posted by John K View Post
Any ideas how to remove slop from a helical focuser on an 80mm short focus refractor if this is the cause?
I seem to remember Ponders drilled/tapped in a grub screw into one once but you'd have to be able to pull it apart I guess.
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Old 23-03-2013, 01:06 PM
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Videos of the potentially offending focuser here:

http://www.flickr.com/photos/johnkaz...n/photostream/

http://www.flickr.com/photos/johnkaz...n/photostream/


I think that the main issue is the screw that sits inside the helical focuser is smaller in diameter than the worm channel thread in which it rotates. As well, there is a small gap in between the focuser and the sliding 1/ 1/4 barrel tube.

Some images here:


http://www.flickr.com/photos/johnkaz...n/photostream/


Have just emailed Lumicon for a response.

Is this meant to be good enough for a $400 80mm short focus refractor/finder? Can it be repaired or an alternative focuser sourced for an 80mm ID barrel?

Look forward to comments.


John K.





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Old 23-03-2013, 06:04 PM
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Does look a bit sloppy John.
Is it more tight/less sloppy when fully turned in, even if the image is out of focus? If so, you could then focus by sliding the guide camera in and out manually and securing with the set screw?
Gary
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Old 23-03-2013, 08:42 PM
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Saw that vid, I would not guide with that unit as is. That sucker will move more than a schmidt-cassegrain mirror at different parts of the sky! Shame that you would even need to consider changing the focuser on a guide scope! I really hope that Lumicon can recommend a fix here... keep us posted.
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Old 26-03-2014, 03:02 AM
Jupe (John Briol)
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This is late but I wound dental floss around the screw in the helical focuser to make it fit snug in the groove.
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Old 26-03-2014, 10:08 AM
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Quote:
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This is late but I wound dental floss around the screw in the helical focuser to make it fit snug in the groove.
If dental floss works then plumber's tape would probably work even better.
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Old 26-03-2014, 01:36 PM
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1 Focus it then drill three holes x120 degrees through the thread and the focuser tube. Tap in some screws to fit and it won't move.or
2 Cling wrap over the thread then insert and wind to position - that will take up a lot of slack.or
3 Oil or wax the female thread and dry out the male thread - apply a thin layer of resin to male then screw in til set -... then focus as normal.
4 Screw dried male and female threads in to focus and then drizzle in some glue. allow to set
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Old 27-03-2014, 06:38 PM
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Thanks for all the suggestions - some great suggestions there.

Areldite did the trick and Lumicon also sent me a new focuser.

Also now guiding with an OAG which for 1600mm FL is the way to go.

John K.
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Old 27-03-2014, 08:56 PM
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Wait til you want to photo sculptor - oag won't help you there!!(well it may be ok, but away from the milky way, I had a dickens of a job to find a star to track on)
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Old 27-03-2014, 09:27 PM
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Wait til you want to photo sculptor - oag won't help you there!!(well it may be ok, but away from the milky way, I had a dickens of a job to find a star to track on)
No probs so far - with a Lodestar and a 12.5" in the OAG have had no issues so far sol lets see.
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