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Elongated stars in one corner only - help please


DrRobin

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Hi,

I have elongated stars in one corner of my shots, but the rest of the frame, including the three other corners are all round. The stars look to be elongated in RA and only slightly.

I am trying to work out why, but need some advice. Here is my set up

RC250TT, AP 0.67 FR, SX-H18

Could it be

Collimation (checked with a HG laser);

Focuser or camera off centre;

FR not correctly aligned;

Tracking/stacking error?

I can try and post a single sub and a stacked image later if it helps?

Thanks

Robin

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Thanks Les,

It was clear last night, so had another go.  I think it is collimation.  If I look at the stars to the left of the frame they are all elongated, but at different angles, sort of like in a circle.  Projecting back from these stars perpendicular to the elongation, gives a centre point some way to the right of the centre of the frame.  Zooming in to a bright star in the centre seems to show a circular star, except it has a slight collimation error on it.

So it looks like the collimation is off.  I will need to test again with the laser, but might need to take the scope out of the obs to actually adjust the collimation.

Robin

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Hopefully you have solved your problem. If not, I had a similar problem with my Newtonian reflector. My problem was that the image plane was not perpendicular to the light path. My Skywatcher has screws to adjust the angle of the focuser (basically lift or low each corner) - this worked for me. Yours is an RC? I think it is possible to buy a collimation ring for some models of RCs that allows a similar adjustment. Anyway, best of luck, I hope you fix it.

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Hi,

Thanks for the replies.  I checked the collimation last night, the secondary was a fair way out.  I adjusted it and think it has fixed the problem, I had an imaging run afterwards, but haven't processed the results yet.  It was spot on before I put the scope in the obs, I am amazed it was so far out, lets hope it doesn't drift too much from now on.

Mike, my RC does have adjustment for the focuser tube, you set that first and it is spot on.  I think my camera might not be square, stars in the middle and one side are in good focus but the stars to one side are slightly out.  The camera has separate adjustment for tilt, so I will need to look at that as well.

Robin

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  • 2 weeks later...

Hi guys,

Okay got to the bottom of this, my scope was out of collimation, caused in no small part by the focuser. I never realised but the stock focuser uses a linear bearing and is only supported in one place (rather than the usual 3) so there is a fair bit of flex. Plus it is a 3" focuser with a 2" adaptor, but the length of the 2" adaptor is quite small, so you get flex there as well.

If you were using a webcam, small camera or light Canon DSLR then it would be okay, but my mono camera and filter wheel is far too much weight and you need something better. Fortunately I have a Moonlite and have just ordered a flange from the US to fit so that should sort it out.

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Hi,

My Moonlite flange raced across the Atlantic in a couple of days and then left Heathrow on Monday.  Since then it has gone in to a black hole or possibly RM, certainly it has disappeared somewhere.

Not to be put off, I had another go at collimating at the weekend, the scope is still off, probably due to setting it up with a laser in the focuser which is off centre.  Note to self you really can't align a scope if the focuser isn't square to start with.

I have read about Tak Collimating scopes, so I had an idea, take an ASI120MM and attach the fish eye lens it comes with.  The outer body of the ASI120MM is 2", so it clamps in the focuser, was that a coincidence or by design?  I couldn't guarantee it is perfectly square, but using the image, the focuser tube, mirrors should all appear to be in-line/concentric and I end up with an image of the mirrors and spider which I can then measure.

The two attached images have Mire de collimation over-laid and are centered on the spider, clearly my scope is still out of alignment.

Robin

post-10602-0-96918700-1449698587.jpgpost-10602-0-99624800-1449698591.jpg

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  • 2 weeks later...

Hi,

If I collimating so the laser is on the secondary, centred on the primary then I get this pattern through the front of the scope.

post-10602-0-38115200-1450703987_thumb.j

However, if I blank off the primary, I see this ring of light caused by light from the laser coming past the primary and secondary baffles.

post-10602-0-71888100-1450704153_thumb.j

Notice how it is not even, I think this means that the primary is not pointing towards the secondary correctly.

The only way I can explain this is the focused, secondary and primary are not all concentric, or the spot on the secondary is in the wrong place.

Sadly the weather won't allow me to star test.

Any thoughts?

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