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ASI1600mm cool


Andyb90

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7 minutes ago, Andyb90 said:

Do you know the approx distance from filter to sensor with your Star 71?

Hi Andy.

Thanks for the link. Interesting that.

I asked zwo about the distance but they don't know. I spoke to someone who also uses a star 71 and 1600mm. They don't have the issues i experienced with the ed80, and my images so far seem to reflect that.

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35 minutes ago, Allinthehead said:

Hi Andy.

Thanks for the link. Interesting that.

I asked zwo about the distance but they don't know. I spoke to someone who also uses a star 71 and 1600mm. They don't have the issues i experienced with the ed80, and my images so far seem to reflect that.

I checked my spacing and it's approx 26mm from filter to sensor. I have an m48 to t2 adapter between my flattener and filter wheel. I think I can potentially drop that and connect flattener directly to filter wheel. This will give an extra 10mm that I can add between filter and sensor. Taking me to approx 36mm.

I'm waiting for a new mount but when I get it I'll check for the artifact at 26mm spacing and then increase it if I see the pattern on a bright star.

Andy.

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Just now, Allinthehead said:

I got 2 hrs on the horsehead last night. No sign of artifact. This is with 60 second subs gain 399 offset 50. I also tried 120 second subs and they were artifact free.

 

Really impressive results and superb processing!

David

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5 hours ago, Allinthehead said:

I got 2 hrs on the horsehead last night. No sign of artifact. This is with 60 second subs gain 399 offset 50. I also tried 120 second subs and they were artifact free.

 

Very interesting.  So you are saying that changing from an ED80 to a Star71 cured the problem? Are you using the same filterwheel with the same spacing from the camera?  Or did that change as well?

Mark

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10 hours ago, sharkmelley said:

Very interesting.  So you are saying that changing from an ED80 to a Star71 cured the problem? Are you using the same filterwheel with the same spacing from the camera?  Or did that change as well?

Hi Mark. Yes the only change i made was the scope. Bear in mind the ed80 also had a reducer/flat and a spacer between reducer and filter wheel for correct distance. The spacing between the camera and filter wheel has remained the same. I literally unscrewed the filter wheel with the camera still attached and then screwed it directly to the Star 71.  

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54 minutes ago, MattJenko said:

 

have you tried taking an image without the reducer/flattener on the ED80?

 

Hi Matt. No i never got around to trying this as the star 71 arrived and i was keen to get some images with it. In looking for others with the same issue i have noticed that the problem is ever present when a reducer is involved. I'm suspecting however that it's a spacing issue between the sensor and reducer as someone told me they have the issue when the filter wheel is attached and when the camera is attached directly with filter the problem goes away. I don't know if a reducer is involved in that scenario. I can find out if you wish.

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2 minutes ago, Allinthehead said:

Matt have you noticed it with your Ts 60?

Just had a look and can't see any although maybe a hint on the M45? I have the first gen ASI1600 if that makes a difference. M37 image is with with an FLT110 and x0.8 WO reducer and nothing there I don't think

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Mark after reading through that thread on CN and locating the image of the jellyfish taken with star 71 i went to photoshop and had a really good look at Alnitak. With some quite severe curves i think i was able to see the artifact. Very feint and with a curve that would never be used in processing an image.

Star diffract.jpg

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8 hours ago, Allinthehead said:

Mark after reading through that thread on CN and locating the image of the jellyfish taken with star 71 i went to photoshop and had a really good look at Alnitak. With some quite severe curves i think i was able to see the artifact. Very feint and with a curve that would never be used in processing an image.

 

Interesting that he has the problem and you don't. What I really want to see is the artifact imaged with Oiii and Ha filters with the same equipment.  The similarities or differences would confirm if this is a diffraction pattern or not.

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3 hours ago, sharkmelley said:

Interesting that he has the problem and you don't. What I really want to see is the artifact imaged with Oiii and Ha filters with the same equipment.  The similarities or differences would confirm if this is a diffraction pattern or not.

I'm not sure what you can tell from this? but here are Menkib in Ha and S2 (don't have O3).

Setup is identical for both, subs are 5min @Unity with offset 21.

The stacked images is 4.5 hour Ha and ~1.5 hour S2

Ha

Ha_Menkib_stack.pngHa_Menkib_sub.png

S2

S2_Menkib_stack.pngS2_Menkib_sub.png

The reflection is not showing a lot in Ha ,but in S2 it's very clear!

Scope is Equinox 80, with SkyWatcher non reducing Field Flattener and Baader NB filters.

Please share what this means.

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9 hours ago, Magnus_e said:

I'm not sure what you can tell from this? but here are Menkib in Ha and S2 (don't have O3).

Setup is identical for both, subs are 5min @Unity with offset 21.

The stacked images is 4.5 hour Ha and ~1.5 hour S2

The reflection is not showing a lot in Ha ,but in S2 it's very clear!

Scope is Equinox 80, with SkyWatcher non reducing Field Flattener and Baader NB filters.

Please share what this means.

To be honest, the artefact is not clear enough in either image.  I need to see the distinct discs that are grouped around the star both orthogonally and diagonally.  Measurements on those discs then gives useful information about their probable cause.  If this is a diffraction pattern, which is my guess, then the Ha and Oiii wavelengths are different enough to visibly affect the size and spacing of those discs when the same equipment is used for both.  Unfortunately, Ha and Sii wavelengths are too close to give patterns that are sufficiently different.

My working hypothesis is that this is a diffraction pattern involving reflections off the individual photosites (forming a 2D diffraction grating) and a reflection off the integral cover glass which sits a fraction of a millimetre above the sensor surface.  It is possible that an entirely different process is at work.  An alternative theory is that it is a parasitic image formed by a curved surface in the focal reducer.  However, I don't find this convincing because focal reducers vary too much in their construction to be able to form almost identical artefacts.  Another possibility is reflection off the filters but I fail to see how this particular artefact can be formed in that manner.  Filter reflection certainly can produce a diffraction pattern and there's a lovely example here:  https://stargazerslounge.com/topic/255758-first-light-with-g2-8300-and-ext-fw-disaster/  However it will always be a much more widely spaced pattern because of the distances involved. 

If it can be convincingly proved that cover glass reflections are causing the problem then it should be possible to put pressure on the camera makers to pressurise the sensor manufacturers to use anti-reflective coatings on their cover glasses.  Note that the popular KAF8300 sensor is available both with and without anti-reflective coatings.  I suspect that some cameras out there use one type and other cameras use the other.

Mark

 

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