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Star Shape Mystery - Help please!


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Hi all

Am playing around with a new refractor and seem to be getting odd star shapes. So I've done some tests without the 0.79 reducer/flattener, just in case that was the culprit, and seem to have a notch on one side of my stars.

If I rotate the camera, the position of the notch rotates too so this seems to suggest the problem is with the scope, not the sensor. I wondered if the front clamp was too tight, but I don't think so - loosening it didn't seem to make a difference. One problem with this is its very difficult to avoid tilt, because without the reducer I need a long extension and this is prone to tilt. However, I've done the test several times and the notch always seems to be in the same place. In fact having done tests without the reducer, I repeated them with  the reducer and the result is the same.

Also I've tried various filters, and it's the same with all of them.

That said, I've done some further tests yesterday and the notch seems reduced. Does anyone have any ideas please? I've also done some star tests and have posted the patterns - they look about right though TBH I've never done this before. So how is the star showing a notch, but the circles show no distortion? BTW I've taken care that these aren't binary stars. I used mostly Phad and Merak. I've kept the exposure short, so there is no trailing type effect.

I'd be very grateful for any ideas! 

First test, obvious notch 

475730790_meraknored_0001221_53_12_Notchcrop.png.e8277894a1305bb6261c8b80e79a77fc.png

 

Second test, this time with the reducer because I can be sure that its square - notch less obvious. Note it was also less obvious even without the reducer on this occasion - I cant think what might have been different.

1876673227_PhadRedL-15_0000222_02_44_smallnotchcrop.png.1b5de663c88f573d534ba5406eff9148.png

 

Star tests

658838070_MerakRedL-15_0000222_29_00_startestcrop.png.5b0ea353f9c0618f0322ea1178493bd8.png

1778872731_MerakRedL-15_0000222_23_13_startestcrop.png.2af49a659813d561b754dd13faf226ce.png

 

 

Edited by Tommohawk
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Thanks for comment. Newish, bought End Jan, but only used few times due to weather. I guess I could return. Odd thing is how it seems variable in extent, but always same position in image. I guess I don't want to return it only to find its something im doing wrong like tilt.

If consensus is pinched optics I will return. 

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It says to me it's the camera not the scope if the notch rotates as the camera rotates.

When I first saw it I thought you were using a reflector and it was the focusser sticking into the star (as happened to mine until I chopped a bit off the end of the focusser. 

Not sure what to suggest with a refractor except my first line.

Carole 

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Hi Carole!

Not sure if this is my poor explanation or poor understanding! What I mean is if I rotate the camera in the scope, the position of the notch rotates on the finished image. So instead of being at 10 oclock its at 2 oclock. If the camera were the problem, the notch would be always in the same place on the image, no?

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13 minutes ago, Davey-T said:

Does it alter with temperature change ? it may have been colder for the first picture.

Dave

Hi Dave. Funny you say that, somewhere I read about tube current causing probs so I also wondered about temp - both ambient and sensor temp. I recently changed my default cooler to -15, previously -10, and wondered if this could affect it. But the latest test was done with and without cooler, and no difference I could see. 

Possibly to do with ambient I guess, but nothing extreme there, and not much change over Test period. 

anyhow, wouldn't any optical issue like pinching or current also affect Airy disc shapes? 

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1 minute ago, Tommohawk said:

anyhow, wouldn't any optical issue like pinching or current also affect Airy disc shapes? 

I'm not an optical expert and the only thing I can say regarding Airy discs and collimating as they never seem to look quite like the textbook examples :grin:

Dave

BTW. which scope is it ?

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17 minutes ago, Davey-T said:

I'm not an optical expert and the only thing I can say regarding Airy discs and collimating as they never seem to look quite like the textbook examples :grin:

Dave

BTW. which scope is it ?

LOL. yes agreed - stuff never looks quite the same as textbook examples in any field.

TBH I was trying to avoid naming names. Only because I've raised the issue with the supplier and Id rather post the whole story rather than cloud folks judgement about a scope/supplier that's probably perfectly good. Hope that makes sense!

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1 hour ago, Davey-T said:

Doublet, triplet ?

Dave

Air spaced Fpl53 doublet.

I don't do visual, but it seems near as makes no difference parfocal with my ZWO filters. So presumably minimal chromatic aberration. 

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Tom

It maybe worth throwing an email over to TS with the images to see what they say also?. To be fair they may just ask for it to be returned even if they cannot work out the issue. Nothing lost in opening the communication lines!

Rob

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3 hours ago, Rob said:

Tom

It maybe worth throwing an email over to TS with the images to see what they say also?. To be fair they may just ask for it to be returned even if they cannot work out the issue. Nothing lost in opening the communication lines!

Rob

Hi Rob - yes I've done that. I have had a reply, so they are listening. What I'm trying to avoid is returning it only to find that there's something weird going on with my sensor or whatever. So I'm hoping somebody will recognise the problem and concur that it is a scope defect. 

Also I'm not sure whether I should be discussing this with the manufacturer or with the UK supplier.

Ideally I would do further tests - I still have my Canon DSLR, although I haven't used it for a while, so if time and sky allowed, I would replicate the star tests with this. Good time at the moment in that the skies are clear, but too much going on at home!

 

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Have you looked at the glass edges, ie looked through the scope without eyepiece, and from both sides? The defocused star seems to have a small diffracting anomaly at the very edge. Just a thought.

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5 hours ago, wimvb said:

Have you looked at the glass edges, ie looked through the scope without eyepiece, and from both sides? The defocused star seems to have a small diffracting anomaly at the very edge. Just a thought.

Hi Wim - thanks for that suggestion. Yes I noticed the curious defects at the edge of the defocused star too. I've not done that test before so not sure what to make of it - initially I thought it was just a dust bunny, but the position would be about right for the defect, assuming the defect would be on the opposite side of the image.

I also did further tests over the Easter period with my DSLR, to eliminate any oddity with the sensor or the EFW - same result, notch at the top left, though less obvious due to noise.

I've had a really good look at the optics and cant see any obvious defects in the lens, the edges, the surfacing or the coating. It's not possible to inspect the lens edges properly without taking the lens form the cell - and I'm not going there!

I've contacted the manufacturer and just waiting for a response. I hope it can be sorted because mechanically its a great scope - I'll post back when I get a response.

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