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Strange effect around bright stars....


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I got a new scope and after a week of fiddling it's up and running. I took a few test shots this evening and noticed this strange effect around bright stars, does anyone have any idea what would cause it?

 

 

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Your colleague is very smart! Olly

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Is your secondary mirror set correctly? Take the eye piece out and using a thin strip of card, hold it inside the draw tube at the 12 o'clock position (opposite the focus rail) and slide it down until it "gently" touches the secondary mirror edge (at 12 o'clock). mark the card at the edge of the draw tube. Now do the same at the 6 o'clock position and see if there is a difference in the height. Adjust as necessary.

 

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1 minute ago, Alien_Photons said:

In fact just return it.

Really? I just thought it was something quirky about my setup....now you've got me a little worried.

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Just now, Daz69 said:

Is your secondary mirror set correctly? Take the eye piece out and using a thin strip of card, hold it inside the draw tube at the 12 o'clock position (opposite the focus rail) and slide it down until it "gently" touches the secondary mirror edge (at 12 o'clock). mark the card at the edge of the draw tube. Now do the same at the 6 o'clock position and see if there is a difference in the height. Adjust as necessary.

 

secondary looks lovely and round underneath the focuser, i spent a couple of nights making sure collimation was as good as I could get (meaning I could see nothing at all wrong with it).

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Just now, Peje said:

secondary looks lovely and round underneath the focuser, i spent a couple of nights making sure collimation was as good as I could get (meaning I could see nothing at all wrong with it).

But is it? From your photo, you are getting an angled shadow. The only thing that works at an angle is the secondary mirror. I don't get it and mine is bang on. 

Are your vanes absolutely straight? Does your draw tube lower too far into the OTA? 

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Just now, Daz69 said:

But is it? From your photo, you are getting an angled shadow. The only thing that works at an angle is the secondary mirror. I don't get it and mine is bang on. 

Are your vanes absolutely straight? Does your draw tube lower too far into the OTA? 

Some images to show the end of the OTA, the focuser is in the same position as when it was taking images

20170323_230504.jpg

20170323_230521.jpg

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Could it be additional diffraction being caused by the wire running across the top of one of the secondary supports ?. It might be having the effect of creating a thicker and not straight support vane profile ?

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Am I seeing it again in the lowest stars in the centre? Looks like it. 

Are you getting any light coming in through the draw tube or between the camera and the draw tube? Straighten the dew strap wire on the vane as that is a little wavy, but I doubt that it will do what is showing. 

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Just now, Daz69 said:

Am I seeing it again in the lowest stars in the centre? Looks like it. 

Are you getting any light coming in through the draw tube or between the camera and the draw tube? Straighten the dew strap wire on the vane as that is a little wavy, but I doubt that it will do what is showing. 

Yes the lower stars are also doing it.

It is very possible some light is getting in via the back of the focuser but I don't think this is soemthing I can fix

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The back of the focuser? Do you mean the base of the focuser, the part that is bolted to the OTA? If that part isn't sealing completely on the OTA, I'd be inclined to take it off and put some sealant underneath before putting it back. You will only need the merest smear of sealant. 

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1 hour ago, John said:

Could it be additional diffraction being caused by the wire running across the top of one of the secondary supports ?. It might be having the effect of creating a thicker and not straight support vane profile ?

That's exactly what I'm seeing in this, too. I suggest you remove that wire from the "equation" (set-up) and re-check the collimation. Then repeat your testing without the wire in place.

Please keep us informed?

Thanks -

Dave

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I remember when I had mine I used to place a black head shower cover over the rear of the scope, so there was no light leakage. The wire issue might be an issue as well if it intrudes into the light path.

May I suggest if there is a problem with the focuser base, why not try with the original focuser placed back on the OTA to see if it mimics the same effect.

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I'd certainly do a test without the wire. That would, if nothing else, eliminate or confirm it easily.

I don't like the look of the unblackened focuser retaining bolts either. Just in principle I'd blacken them. Use stove or barbecue matt black because those paints use pigments rather than dyes which can be reflective in some wavelengths.

If the wire is the culprit it might be worth not running down the vane in a straight line but, counter intuitiviely, in a wide curve to smear out the diffraction effects.

Olly

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10 hours ago, Daz69 said:

The back of the focuser? Do you mean the base of the focuser, the part that is bolted to the OTA? If that part isn't sealing completely on the OTA, I'd be inclined to take it off and put some sealant underneath before putting it back. You will only need the merest smear of sealant. 

I've attached a photo showing the open area I mean, I have black tape around the rest of the focuser body to reduce any stray light getting in but as this bit is against the moving part of the focuser I can't do much about it.

46 minutes ago, Skyline said:

I remember when I had mine I used to place a black head shower cover over the rear of the scope, so there was no light leakage. The wire issue might be an issue as well if it intrudes into the light path.

May I suggest if there is a problem with the focuser base, why not try with the original focuser placed back on the OTA to see if it mimics the same effect.

That's very interesting, I did notice when I had my main floodlight on I could see light through / around the primary. I wonder if this could be a contributing factor? I really don't want to swap the focuser back at this stage. I wonder why it would cause problems with this scope and not with my previous f/6 Newt?

2 minutes ago, ollypenrice said:

I'd certainly do a test without the wire. That would, if nothing else, eliminate or confirm it easily.

I don't like the look of the unblackened focuser retaining bolts either. Just in principle I'd blacken them. Use stove or barbecue matt black because those paints use pigments rather than dyes which can be reflective in some wavelengths.

If the wire is the culprit it might be worth not running down the vane in a straight line but, counter intuitiviely, in a wide curve to smear out the diffraction effects.

Olly

The heater shall be removed tonight and I'll see what difference it makes, I'm fairly certain it won't make any difference but it'll take 2 mins to take off and 5 to put back on so it's an easily tested theory. The silver focuser bolts are a job for Saturday :) Going to head to B&Q and get some matt black spray paint and just blast it down the tube....LOL

20170324_074538.jpg

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That definitely looks like a diffraction effect rather than light leakage into the tube. There's nothing untoward with the mirror clips or anything like that is there? Any of the baffle rings loose?

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Heater removed, scope fully recalibrated including secondary to focuser alignment to make sure it was a good as possible...effect is still there, allbeit slightly different now.

Any other ideas?

Procyon2.png

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Interestingly your dark areas are at 120 degrees now. Have you tried viewing without any security/back garden light being on. How long was the exposure for that photo above, as although it may not show in normally observing, taking a long exposure will show it. It could very well be light leeching in around the primary mirror. 

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Just now, Daz69 said:

Interestingly your dark areas are at 120 degrees now. Have you tried viewing without any security/back garden light being on. How long was the exposure for that photo above, as although it may not show in normally observing, taking a long exposure will show it. It could very well be light leeching in around the primary mirror. 

That was a 60 sec exposure, the same as last night. There are no bright lights on at the minute, just the usual house lights. I'm currently shooting east at m101 which points the rear of my scope towards a very sheltered area with no light sources, with a 6min exposure i am getting no effect...although there are no bright stars in the shot.

star.png

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Try covering the rear area of your OTA so no light can get in, turn the rear to the house lights, turn on a garden light (if you have one) and take a 60 second exposure of a bright star. If there are no dark areas in the shot, then there is your problem. Good luck.

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