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Oooooh! What is wrong???


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This is from the Megrez 90 that I happily displayed last night here. I wonder what the heck is wrong in the optical path.

Any ideas?

Canon EOS1000D an a WO Flat 4 on a Megrez 90. Nothing else.

I will try tonight without the flattener!

m82_m90-1.jpg

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If this happens with every pic you take then take some darks which will hopefully counteract this effect.

You mean flats?? :hello2:

I think this is more likley a problem with the flattener. I am no expert and have never used one yet. But I think I remember reading a similar problem and it being down to the flattener not being quite a snug fit or the camera etc.

So yes try without and if it solves it then when you do put it back on just make sure there is no slop between things.

BTW Nice image all the same! :hello2:

Michael

Edited by msinclairinork
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You mean flats?? :D

I think he meant darks. It may be amp glow. I don't use Canon, so I don't know what it looks like on a 1000D. It also depends on temperature and exposure time. On my D50 amp glow becomes a problem for exposure more than a few minutes and appears as pink light bleeding in from the edges of the frame. Darks will remove it along with hot pixels.

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The star shapes could be caused by the objective spacer tabs sitting into the light path.

How many subs did you use? Total exposure time?

Flats and darks will clean things up a bit and allow you to better determine what other issues you may have.

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The way the light fills in from one side is strange. Did you have dew perhaps? I've seen a thin film of dew scatter light and give something of this effect.

Do you have any off-axis light problems? Street lights, or otherwise? Some open tube designs can have that problem - a cheap shroud of black cloth held on with velcro will help if that is the case.

This seems very odd for a Canon DSLR, and I'm not sure a dark exposure would help - that is usually used to eliminate electronic noise within the camera circuits and CCD chip. This coming from one side like that doesn't look like any noise I've seen - not on a functional camera, anyway. Mostly, the noise I have is little pixels turning on when they shouldn't - kind of like a random scattering of snow on the image. Your effect doesn't seem random at all. :D

Does it take good pics in daylight?

Let us know what you find.

Dan

Edited by Ad Astra
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OK, I have borrowed a Flat 3 from a friend and skies should be clear tomorrow.

The picture was taken with a Canon 1000D that has performed flawlessly and has taken great shots with the Explorer 250 OTA.

There are 20 subs of 5 minutes each. 30 flats were taken this morning with the telescope left out during the night.

Flats and lights were pre-processed with Nebulosity 2 and left that world as de-bayered FITS files destined for Pixinsight.

In Pixinsight I registered and stacked them, then did a dynamic background extraction followed by some simple histogram stretch.

Throughout the whole process, and in every single image, the stars have a three-spoke pattern simular to the nuclear ("nucelar" in George Bush lingo) symbol or a three bladed rotor on, for instance, a Eurocopter AS350.

The only new parts in this setup, from alignment and capture to finished image, is the OTA and the field flattener.

If the results are the same with the Flat 3 that I borrowed, then the tube is bad, otherwise, the flattener may be faulty.

Actually, I just need to shoot one frame of a single star in order to properly assess the situation.

Meanwhile, thanks for all input so far. This forum is great!

/per

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Hello Per,

Sounds like you are on your way to solving it. :D

The 3-bladed spikes sound like diffraction spikes off of a secondary mirror spider - but that seems very wrong for a refractor.

The only thing I can think of that would give you a 3-bladed pattern is pinched optics. With multi-element objectives (air-spaced), it is possible to tighten down the optical cell (or collimation screws) enough to actually flex the glass. This is more common on large mirrors than on lenses, but the effect can be significant. The thing that brings this to mind is that most collimation setups are essentially triangular - and a pinch could result in an essentially 3-sided distortion in the lens.

This is a guess (I'd have to model the darn thing on a computer to be sure) - but it seems worth investigating.

On a reflector - you just have to live with the spikes (like the nice people at Mt. Palomar!) or go to an exotic spider design that will change or minimize these problems for you.

I hope that helps, let us know how you get on.

Dan

Edited by Ad Astra
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The 'nuclear' stars sound like pinched optics or the ends of three clamping or adjustments screws are inpinging in the light path.

The overall glow is not amp glow on a Canon 1000D. Is the house or a strong light behind the mount, it looks like light bleeding in through a gap somewhere in the optical train?

Flats would help but it's best to find and cure the source if you can, Darks do something else.

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Looks like three little somethings in the light path, perhaps partial dewing on one of the surfaces? and I think a little amp glow. You need to take some darks urgently - this will show if you have a amp glow problem. There are some weird lighter circles on the image that your flats should have taken out (maybe dust on the sensor). Do you do much processing in Nebulosity? Could you have forgotten or processed with the wrong bias/flat frames? My imagination is piqued with this - please let us know what you find! Incidentally I use Neb 2 and am happy to help.

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I do not beleive it can be the camera. Just a few nights ago I shot this M51 with the same camera on my Explorer 250.

The flats could have been off and I had no darks. I think it is the optical path... We'll see tonight when I test with a Flattener 3 and completely without flattener.

M51_2011-03-19_Proc4.jpg

Edited by perfrej
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I changed the 3 bolts that hold the focuser in my OTA and didn't realise they were a few millimeters longer than the old ones. That was just enough sticking into the light path to give the effect you say you are seeing.

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OK, something is really wrong here. I have tried the following:

1) EOS1000D on Flat 4

2) EOS1000D on Flat 3

3) EOS1000D on photo extension tube (no flattener)

4) Meade DSI on extender tube

The results are the same. All the frames were focused with a Bahtinov mask.

I'll have to contact the factory...

The following is a 2s exposure of Capella at ISO200 with the Canon. Go figure...

nuclear.jpg

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Look into the front of the telescope and see if the three small tabs (which hold the lenses apart) are visible... it may be that they are intruding too far into the lens and causing the triangular distortions.

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Ah! I have looked through the tube, into it from both ends, all illuminated by a nice flashlight. There is nothing intruding into the light path and the lenses are separated not by three thingies but by a cylinder...

I am starting to think warped lens or something. I have contacted WO.

This is sad.

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Allright! I feel informed.

The problem I have indicates perfect collimation - because the artifacts are totally symmetrical around the center. The problem itself is the rear lens being fastened a little too tight, which causes "pinching" (mentioned above in he thread).

I found an article on the web where a guy had exactly the same problem with a Megrez 80, and he solved it, documented it and posted it.

I'll wait for WO's reply, but my guess is that I'll simply get the old tool box out of the closet and release the fastener ring for the rear lens element. Waiting several weeks for WO to process the return will take me out of season. I'm at about 60°N, so in a few weeks I will lose the dark nights.

Thanks again for all replies - the energy on this forum is just incredible!

I'll post my findings.

All the best,

Per

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Years ago I had one of the first Megrez 80's that showed a similar problem - though not as bad as yours. If you look at my page here you can see an example about half way down (click on it for a bigger version).

Mark

Yes, that was the page that I found when I totally googled the internet to pieces. Thank you for posting that; it was the key that unlocked the whole thing. I do not have proper tools for disassembly of the front assembly, but WO actually asked me to take it off and send it back express. Their instructions regarding removal were not quite right, but I got it off anyway. It should leave here today. I did see something that looked like an oil smudge on the enclosed lens surfaces...

Thanks again!

/per

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