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Bloated Red Stars Issue

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#1 Griglia

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Posted 17 October 2019 - 07:37 PM

Hello everyone, long time lurker - posting for the first time! I am looking for some help on the issue below.

 

I have been using a ZWO ASI1600MM-pro with Astronomik filters for a while now and have had an issue with my stars ever since I switched to mono. My raw subs show bloated/out of focus stars taken through my red and luminance filters and tighter stars in my blue subs. Green stars are somewhere in between. NINA seems to confirm this, as even when autofocused the HFR readout shows a higher value for red than green or blue.

 

I've attached a google drive link with some fits files of individual subs that demonstrate the issue, calibrated and uncalibrated.

 

https://drive.google...WWoyX7z3ppFD-yx

 

I thought the issue would be resolved when linear-fitting the data in post, but that didn't seem to help as my final images always have bloated red stars across the field.

 

I've also tried manually focusing with and without my EAF to no avail.

 

All data is acquired using NINA

 

Equipment list:

 

Skywatcher Esprit 80 with field flattener, Skywatcher Eq6-r, ZWO ASI1600mm-pro, Astronomik Deep sky RGB filters with L-2 luminance, Astronomik 6nm OIII HA SII, ZWO EAF autofocuser, ZWO ASI120mm guide camera, Meade 80mm adventure scope as a guide scope

 

Sorry if I am posting in the wrong forum, I wasnt sure if this was a general or equipment related question. Also please let me know if there is any other information I can provide/have left out. Any help would be appreciated!

 

Thanks!
Uday Kapoor


Edited by Griglia, 17 October 2019 - 07:41 PM.

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#2 PirateMike

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Posted 17 October 2019 - 07:43 PM

Are you sure that your filters are perfectly clean? That's the first thing I would check and probably the easiest issue to correct.

 

Miguel   8-)

 

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#3 View2

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Posted 17 October 2019 - 07:50 PM

What is the bandwidth of your filters? Are they all the same?
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#4 View2

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Posted 17 October 2019 - 07:52 PM

I think the red and luminance filters would have different exposures than green and blue?
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#5 Griglia

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Posted 17 October 2019 - 08:13 PM

Are you sure that your filters are perfectly clean? That's the first thing I would check and probably the easiest issue to correct.

 

Miguel   8-)

 

.

Good point. I hadn't checked since installing them, but I just looked and there doesnt seem to be any visible layer of dust or smudges on any of the filters. And I forgot to mention that these are all 1.25in threaded filters. Sorry!

 

I think the red and luminance filters would have different exposures than green and blue?

I've been doing 60 second exposures for RGB and 30 seconds for luminance. But even when I do the same exposure length (10s for example) for all four I get varying star sizes

 

What is the bandwidth of your filters? Are they all the same?

I believe that the Astronomik deepsky RGB are all the same bandwidth, but I'm not 100% sure



#6 PirateMike

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Posted 17 October 2019 - 08:19 PM

You can't always see that the filters are dirty. I would give the red filter a real good cleaning and test it, that's the only way to know for sure.

 

I looked at your subs and they don't look to terribly bad.

 

 

Miguel   8-)

 

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#7 Der_Pit

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Posted 18 October 2019 - 07:46 AM

It can be the quality of the telescope.  Refractors always have some color error, even an apochromat will have a focus error in UV and IR.  So it could be yours is bad towards IR (i.e., the image gets unsharper for longer wavelengths).  That would create a more fuzzy blob around the star.

Another option could be filter orientation.  Filters usually are AR coated only on one side, and that side should face towards the sensor/camera.  Placing it wrong can result in halos around stars, i.e., rather some 'disks' around stars.


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#8 Griglia

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Posted 18 October 2019 - 12:48 PM

You can't always see that the filters are dirty. I would give the red filter a real good cleaning and test it, that's the only way to know for sure.

 

I looked at your subs and they don't look to terribly bad.

 

 

Miguel   8-)

 

.

I'll definetely do so and report back! Thanks



#9 Griglia

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Posted 18 October 2019 - 12:48 PM

It can be the quality of the telescope.  Refractors always have some color error, even an apochromat will have a focus error in UV and IR.  So it could be yours is bad towards IR (i.e., the image gets unsharper for longer wavelengths).  That would create a more fuzzy blob around the star.

Another option could be filter orientation.  Filters usually are AR coated only on one side, and that side should face towards the sensor/camera.  Placing it wrong can result in halos around stars, i.e., rather some 'disks' around stars.

If it is indeed my telescope, am I out of luck? And the filter orientation being wrong makes sense, but I just remembered that the filters are threaded so there was only one way for me to put them into my filter wheel! I'll double check when I clean them to make sure that they go in correctly



#10 Der_Pit

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Posted 19 October 2019 - 08:29 AM

Indeed mounted filters are usually safe against wrong orientation, unless you reverse the whole filter wheel. wink.gif

 

A hint whether it is an issue of your scope could be the focus values for the Hα and SII filters.  If they are clearly different that points to the focus curve.  If also those show the bloat its some reflection issue of the filters or the camera...



#11 sg6

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Posted 19 October 2019 - 12:54 PM

Does the camera have a UV/IR cut filter integrated into it, or do you need one?

IR will show up as Red and is not accounted for in the focus so you would end up with bloated red "bits".

 

Have looked at the camera spec but can see nothing

 

Edit:

Maybe not, just found the filters and they cut at 700nm, so the red should block IR.

 

One thought but cannot find anything on it is that the red filter is 600-700nm pass.

Scope could be designed for say 420nm, 530nm and say 640nm and the 640-700nm bit starts to be out of best focus. Would need sprectrum focal plane curves for the Espirit to offer a suggestion and they are not seemingly around.

 

It is the Espirit triplet ?

Just Skywatcher seem to swap names around.


Edited by sg6, 19 October 2019 - 01:16 PM.


#12 Griglia

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Posted 19 October 2019 - 03:06 PM

Indeed mounted filters are usually safe against wrong orientation, unless you reverse the whole filter wheel. wink.gif

 

A hint whether it is an issue of your scope could be the focus values for the Hα and SII filters.  If they are clearly different that points to the focus curve.  If also those show the bloat its some reflection issue of the filters or the camera...

My filter wheel seems to be mounted correctly, at least according to ZWO's manual. And the filters are mounted with the AR coated side facing the sensor.

 

Sorry I dont quite understand what you are asking here and what I should look for. From my memory the focus values for Hα and SII are different, but they arent bloated relative to eachother, while they are bloated relative to OIII. I will have to wait for a clear night to verify.

 

Thanks so much for your help!



#13 Griglia

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Posted 19 October 2019 - 03:07 PM

Does the camera have a UV/IR cut filter integrated into it, or do you need one?

IR will show up as Red and is not accounted for in the focus so you would end up with bloated red "bits".

 

Have looked at the camera spec but can see nothing

 

Edit:

Maybe not, just found the filters and they cut at 700nm, so the red should block IR.

 

One thought but cannot find anything on it is that the red filter is 600-700nm pass.

Scope could be designed for say 420nm, 530nm and say 640nm and the 640-700nm bit starts to be out of best focus. Would need sprectrum focal plane curves for the Espirit to offer a suggestion and they are not seemingly around.

 

It is the Espirit triplet ?

Just Skywatcher seem to swap names around.

Yes the Esprit is a triplet.




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