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Altair 115 and weird shaped stars


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I purchased an ex-display 115mm Triplet last year and have found that I get weird shaped stars. Originally I put this down to having no field flattener so I purchased the 1x FF and also the adapter to be able to attach my Atik 314L and Filter wheel to the scope.  This however has not sorted the problem and attached image should show what I mean.
 

I have checked spacing, that the seating of all of the items on the imaging train are 'square', guiding, you name it, I've tried it.

 
The scope is mounted on an HEQ5 Pro pier mounted and all is guided through a converted Finder Guider using PHD. 
 
Polar alignment is pretty solid - I use a Polemaster.
 
I should say that I use same camera/filter wheel on other scopes ( I have an RC 6" , ED80, ST102 and a WO ZS71)  with simple brass clamp type attachments, far less solid that on the 115, and I don't see anything like this strange star shape on them. In fact I get nice circular stars usually.
 
I'm really at a loss to understand why I am getting these star shapes.  Can anybody suggest anything.  Its got to the point that this, which should be my best performing scope, is now sat in its case gathering dust because I have despaired of what to do. 
 
 

M13 _115.jpg

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to me this looks like a combination of slightly out of focus and drift on the exposure.

The reason why I think its drift is because all of the stars seem to stretch in the same direction across the whole frame.

Try a shorter exposure or (if not using already) autoguiding and see if the stars are round.

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My suggestion is to align the scope so that it is pointing straight up in the air vertically.  Try a short - 5 second or so - exposure.  Examine the stars.

The short exposure will eliminate guiding/tracking errors as a possibiltiy.  The straight up in the air bit should eliminate any issue due to focuser sag (or other sagging bits).  If it is still skew whiff it points to an optical problem.  The fact that you get round stars with other scopes suggests to me that you know what you are doing and makes me lean towards it being an optical issue. 

It can be a real pain when you get problems like this.

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4 minutes ago, MarsG76 said:

to me this looks like a combination of slightly out of focus and drift on the exposure.

The reason why I think its drift is because all of the stars seem to stretch in the same direction across the whole frame.

Try a shorter exposure or (if not using already) autoguiding and see if the stars are round.

As I mention - it is a pier mounted , PHD guided, scope.  I can get perfectly round stars on 20 min frames with other scopes - the frames on this image were no more than 300s.  I get the same on 60s frames.

 

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How does the scope perform visually at high magnification?  Again, test on a decent  night with stars near the zenith. Defocus slowly both ways and note the star shapes. A star test at high mag will be more revealing than even a camera at native focal length. Look to see if the returned pattern both sides of focus at say 350-400X is not circular.

From what you have said it sounds like an optical issue. If I recall correctly, (and there is no saying I do :p ), I think some people have had some issues with the lens cells on that model telescope. If that does turn out to be the case, i'm sure that Nick or someone at AA can help you out with it.

If the star test reveals any issues, it is possible with care to take a snap through the eyepiece with a mobile to help us help you further.

Finally, and this may be too basic, how is the balance on the scope? Bigger triplets are famously front heavy and need careful balancing in every plane.

HTH

Tim

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Steve,

The stars bottom left are stretched more than the stars top right and share a common axis and this is indictive of tilt.

Whether that is tilt in the focuser or tilt in the lens cell you would need to investigate further.

Basic test for axial collimation with a refractor, make a pair of "V" blocks out of timber with the mouth of the V wide enough to support the OTA, line the V with plastic cut from a milk bottle or similar to prevent scratching the tube paintwork. Place the V blocks on a sturdy table or work bench and clamp or screw down so they can not move and far enough apart to support the full length of the tube excluding dew shield and focuser, take the tube rings off and lay the OTA in the V blocks, place a collimated laser in the focuser with the beam pointing inside the OTA and exiting the lens cell, pad out with aluminium baking foil or paper wrapped around the laser body to ensure it is a snug fit in the focuser, arrange the V blocks so that the telescope is pointing at a wall at least fifteen to twenty feet away, switch on the laser and mark the spot on the wall where the laser hits. Now rotate the OTA in the V blocks, if the focuser, tube and lens cell are in alignment the laser spot will not deviate from the mark, if it does then either the focuser draw tube or lens cell is not in axial alignment.

If your scope has a rotable focuser, keep the OTA stationary in the V blocks and just rotate the focuser, if the draw tube is correctly aligned the laser spot will remain fixed on the wall, if it wanders as you rotate the focuser then the draw tube support bearings are not adjusted correctly.

I don't know your scope but maybe it has a collimatable lens cell in which case you would need to unscrew the dew shield and remove it, or slide it back down the OTA to reveal the push-pull screws used to tilt the lens cell with respect to the tube, note the push-pull collimation screws, if used, attach the entire lens cell to the tube on the outside of the cell, if they are there it will be obvious when you slide the dew shield back, do not adjust any screws that point in towards the lens elements contained in the cell, you need an optical bench to adjust these and they should not be touched normally. If the lens cell is not collimatable then there will be no push-pull screws and the lens cell will simply be threaded and screwed into a matching thread in the tube.

If you do have a collimatable lens cell or tilt in the focuser draw tube then with somebodys help rotate the OTA and mark on the wall the circle traced out by the laser, use a pair of dividers or compass to find the centre of the circle and mark it on the wall, now keep the OTA stationary and adjust the lens cell push-pull collimation screws, and/or focuser draw tube bearings until the laser hits that centre spot. Final check after collimation, rotating the OTA in the V blocks the laser spot should remain stationary on the wall.

Rough sketch below explains....

HTH.

 

 

 

Edited by Oddsocks
Added sketch
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You need to test your OTA's  optical elements for Orthonogality.
You did say you had established 100% 90 degree angles were precise throughout.
If that is certain, then there has to be an issue with the objective. 
Your concerns should be taken up with whoever supplied the Telescope to you.

Some of the above recommendations should be  acted upon first though.
Eliminating possible drive/tracking errors by using short exposure images  at the  Zenith,
or High Declination areas, with your drive off. 


 

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20 hours ago, Wilsmaboy said:

 Its got to the point that this, which should be my best performing scope, is now sat in its case gathering dust because I have despaired of what to do. 

 
 

 

I bought the AA 60mm lightwave triplet. whilst not relegated to collecting dust, it is the most expensive guidescope I've ever owned :(. After back and forth with both them and neil at Astronomia lasting many months, and a second scope being issued, I gave up. lesson learnt.

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Previous comments aside, I did notice that the focus tube in the 2½" R&P focuser on my Altair Wave 102 had a bit of lateral slop and this was giving me some mis-alignment. A bit of judicious adjustment on the focuser screw(s) (can't remember which now) reduced that considerably. I suspect I'm not that critical when it comes to misshapen stars, but it might be worth checking.

Ian

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  • 2 months later...

Hi Guys - all exceptionally helpful advice and am working through the simpler routes for getting a solution.  I have tweaked a few things and seen slight improvement - mainly focuser related - star testing at zenith showed no discernable drift so ruled that out.  Onwards and upwards, pardon the pun.

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