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Problem with narrow band filter reflections

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#1 cyber

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Posted 26 June 2017 - 05:09 AM

Hi all,

I purchased a ASI1600MM-C kit from ZWO, including EFW and 31 mm unmounted LRGB + NB filters.

 

Sadly, my narrow band subs show a bright ring around the edge and I cannot figure out what's going wrong frown.gif

Does anyone know what's causing this?

 

Here's an example stack (30 x 90 sec) through the OIII filter (gain 200, offset 50, -15C), calibrated with a master dark.

 

O3_DarkOnly.jpg

 

Master Dark:

 

MasterDark_30x90s_g200o50.jpg

 

The OIII flat has a similar bright ring.

But it does not remove the glow from the light frames completely.

 

O3_flat.JPG

 

The image below is calibrated with a masterdark and masterflat

 

oiii.jpg

 

It looks like a reflection in the optical train, but flats from my broad band filters look fine! confused1.gif

(I'll post these below)

 



#2 cyber

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Posted 26 June 2017 - 05:10 AM

Broad band flats

 

R_flat.JPG

G_flat.JPG

B_flat.JPG

 

NB flats

 

Ha_flat.JPG

S2_flat.JPG



#3 terry59

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Posted 26 June 2017 - 06:19 AM

Your flats do not seem to be well matched at all my friend. Keep trying...flats are hard to get right


Edited by terry59, 26 June 2017 - 06:20 AM.

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#4 diegocolonnello

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Posted 26 June 2017 - 06:45 AM

reinstall the filters upside down. ine side has an anti reflexion coating. the ar coating must face the ccd.

Sent from my SM-N920I using Tapatalk
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#5 cyber

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Posted 26 June 2017 - 07:07 AM

reinstall the filters upside down. ine side has an anti reflexion coating. the ar coating must face the ccd.

Sent from my SM-N920I using Tapatalk

Thanks for the suggestion, I've installed the filters according to the instructions on ZWO's website (it's near the bottom of the page)
https://astronomy-im...nd-31mm-filter/

 

But I'll definitely try reinstalling these filters tonight!


Edited by cyber, 26 June 2017 - 07:08 AM.


#6 buckeyestargazer

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Posted 26 June 2017 - 08:11 AM

Yeah, this situation can be quite maddening.  I believe the issues is caused by the AR coating not quite extending to the very edge of the filter.  Also, the filter edges are not blackened.  If you can't figure anything else out first and as a last resort you could try blackening the edges of the filters with a flat black model paint (and a tiny, precise brush!).  This pretty dramatically helped in a similar situation I had, but the best solution is to get Astrodon filters grin.gif .


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#7 cyber

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Posted 26 June 2017 - 09:31 AM

Your flats do not seem to be well matched at all my friend. Keep trying...flats are hard to get right

The light frames are wrong, that's the main problem... 

But indeed, taking good flats is hard!

 

Yeah, this situation can be quite maddening.  I believe the issues is caused by the AR coating not quite extending to the very edge of the filter.  Also, the filter edges are not blackened.  If you can't figure anything else out first and as a last resort you could try blackening the edges of the filters with a flat black model paint (and a tiny, precise brush!).  This pretty dramatically helped in a similar situation I had, but the best solution is to get Astrodon filters grin.gif .

The broad band filters have AR coating on both sides, I believe.

ZWO doesn't mention anything about their orientation in the optical path, so that might explain why my broad band flats look fine.



#8 xb39

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Posted 26 June 2017 - 10:02 AM

Hi Victor,

I've had a similar problem with my Baader filters (only Ha and OIII): https://www.cloudyni...hy9-wo-star-71/

 

See also in the above link the pdf document Baader provides.

 

It's often not the filter itself.

 

Best regards

Stefan


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#9 terry59

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Posted 26 June 2017 - 02:51 PM

So the darks indicate amp glow (to me anyway). How does a single sub look? 



#10 cyber

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Posted 27 June 2017 - 01:10 AM

Hi Victor,

I've had a similar problem with my Baader filters (only Ha and OIII): https://www.cloudyni...hy9-wo-star-71/

 

See also in the above link the pdf document Baader provides.

 

It's often not the filter itself.

 

Best regards

Stefan

Thank you for the link.

 

I first tried turning the filters upside down.

The edges were already painted black, so that's good.

 

O3_reversed_flat.JPG

 

Then I tried a different spacing of the front optical elements.

So I removed the reducer and attached the camera+FW+VariLock directly to the focus tube at native FL (F/6).

 

O3_F6_flat.JPG

 

As you can see, both of these actions did not affect the glow, even at a different focal length.

The problem seems to be a reflection between the FW and camera...?

 

I really don't want to increase the distance between the camera and FW, because this will result in vignetting.

ZWO designed this as a kit, why isn't it working? frown.gif

 

So the darks indicate amp glow (to me anyway). How does a single sub look? 

From what I've read, amp glow is normal with this camera and dark frame subtraction should remove it completely.

The reflection ring is also visible in my single (uncalibrated) subs.

 

O3_sub.JPG

 

S2_sub.JPG

 

It's different in Ha though, but the Ha flats are also different.

The glow goes diagonally, from top to bottom (see my second post).

 

Ha_sub.JPG


Edited by cyber, 27 June 2017 - 02:17 AM.


#11 FiremanDan

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Posted 27 June 2017 - 08:23 AM

I wonder if there is an issue with the ASI1600 and NB filters in general. It seems a lot of folks, myself included, are having a really hard time getting good NB flats. 

Someone mentioned there might be an issue with the sensor window. 

It's maddening. I was going to just buy a flip a flat in hopes it would solve my issues, but they don't make them anymore. 
 



#12 terry59

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Posted 27 June 2017 - 08:50 AM

I wonder if there is an issue with the ASI1600 and NB filters in general. It seems a lot of folks, myself included, are having a really hard time getting good NB flats. 

Someone mentioned there might be an issue with the sensor window. 

It's maddening. I was going to just buy a flip a flat in hopes it would solve my issues, but they don't make them anymore. 
 

 

Oh no...absolutely can't be Dan....people have been denigrating every camera except this one on here for some time now. They can't be wrong....right? smile.gif

 

In any event I think the OP has multiple issues going here....


Edited by terry59, 27 June 2017 - 08:52 AM.

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#13 mikefulb

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Posted 27 June 2017 - 10:29 AM

It would be good to systematically document any issues with the ASI1600MM to see if there is a problem.

 

I haven't seen any myself - I have used Baader narrow band and Astrodon with mine and it works fine.

 

One change I made that helped my flats in general was to use a thin layer of T-shirt like material between my light box and the OTA.  The translucent acrylic I used for my light box was reflective enough I think it was catching light off the filter/ASI window and reflecting it back down, etc.  Things seemed to have improved since using the material to break this "hall of mirrors" - so thats my theory and I'm sticking with it!


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#14 cyber

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Posted 27 June 2017 - 10:59 AM

It would be good to systematically document any issues with the ASI1600MM to see if there is a problem.

 

I haven't seen any myself - I have used Baader narrow band and Astrodon with mine and it works fine.

 

One change I made that helped my flats in general was to use a thin layer of T-shirt like material between my light box and the OTA.  The translucent acrylic I used for my light box was reflective enough I think it was catching light off the filter/ASI window and reflecting it back down, etc.  Things seemed to have improved since using the material to break this "hall of mirrors" - so thats my theory and I'm sticking with it!

By the way, the images in this post are daylight wall flats. The telescope was positioned about 60 cm (2 ft) from the wall.
The room has large windows with translucent curtains, basically giant softboxes, so the wall should be evenly lit.
Additionally, I covered the end of the dew cap with a white t-shirt to:
1) Diffuse the light even more.
2) Reduce the light intensity so I can use a slow shutter speed (> 0.2 sec)

 

The bright ring is likely not related to the telescope optics, I think.
It's IDENTICAL with and without my reducer, and that's a big difference in (potentially reflective) optical surface.

I'm going to take some new images with just the camera and FW (no telescope attached) pointing at the flat wall, to rule out this factor.

 

I wonder if there is an issue with the ASI1600 and NB filters in general. It seems a lot of folks, myself included, are having a really hard time getting good NB flats. 

Someone mentioned there might be an issue with the sensor window. 

It's maddening. I was going to just buy a flip a flat in hopes it would solve my issues, but they don't make them anymore. 
 

Dan, could you describe your flat problem a bit? (or point me to the correct topic here on CN)

How do you handle the NB subs to produce an acceptable image?


Edited by cyber, 27 June 2017 - 10:59 AM.


#15 FiremanDan

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Posted 27 June 2017 - 01:19 PM

Here is one of my posts... 

https://www.cloudyni...+flats +hardest

 

I have had a few posts about flat issues. I seem to get over/under corrected flats, weird patterns/gradients that are not always present in the light frames. Which leads me to think it's a reflection issue, maybe, probably not, who knows? 


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#16 Goofi

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Posted 27 June 2017 - 02:50 PM

I don't use your camera or filters (I use Astrodon's - worth every penny paid), but a thought to consider:

 

What light source are you using to make your flats?  Narrowband flats require good illumination in their bands, and longer subs. If you took too short of a sub, then you'll stretch harder and amplify minor variations more.  My flats are 15-20 seconds for unbinned (1x1) subs.  Also, normal lights tend to drop off near the IR region (where Ha and Sii are located) .. make sure you use a full spectrum light.


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#17 cyber

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Posted 28 June 2017 - 02:38 AM

Update: I found the source of the problem and a solution! 

 

Like Dan suggested, it's definitely a reflection issue.

Even without anything attached to the FW (no telescope or lenses), the bright ring was still present in my test images.

So, it had to be something in between the filter and the sensor.

 

I noticed that the filter edges are beveled, and I could see a colored reflection in this edge.

The color was also different in each NB filter.

 

Filter bevel color.jpg

Filter profile.png

 

This is likely the source of the problem.

I found a post on the ZWO forum, where a user reports the same problem:

http://zwoug.org/vie....php?f=8&t=6833

 

He made small rings to block the filter edges.

So, I cut out small paper rings and placed them on top of the NB filters.

 

Filter rings.jpg

 

That did it!

 

Paper is probably not the best material for these rings though, so I'm going to (try to) make new rings out of a thin plastic sheet.

 

 

I don't use your camera or filters (I use Astrodon's - worth every penny paid), but a thought to consider:

 

What light source are you using to make your flats?  Narrowband flats require good illumination in their bands, and longer subs. If you took too short of a sub, then you'll stretch harder and amplify minor variations more.  My flats are 15-20 seconds for unbinned (1x1) subs.  Also, normal lights tend to drop off near the IR region (where Ha and Sii are located) .. make sure you use a full spectrum light.

My old flats were only exposed for 2-3 seconds to reach 12k ADU, but now I know that this is due to the bright reflection ring.

The new flats needed 10-12 seconds with the same light source, so that's as expected. Thanks!


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#18 terry59

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Posted 28 June 2017 - 06:06 AM

Congratulations....the flat looks great smile.gif



#19 cyber

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Posted 28 June 2017 - 07:03 AM

Thanks Terry, I'm now looking forward to the next clear night smile.gif

 

Something's still odd though.

The broad band ZWO LRGB filters also have the beveled edges, but they don't show any reflections.

 

For example, here's a view through the green filter:

 

Through green filter.jpg

 

You can only see a small shadow from the filter screws (white arrows).

 

Must be a difference in the filter coatings?



#20 terry59

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Posted 28 June 2017 - 07:23 AM

Perhaps....as long as the flats work though laugh.gif



#21 FiremanDan

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Posted 28 June 2017 - 09:22 AM

Nice, glad it looks like you found the source to your issue. I have threaded filters, so I don't think this works for me. But I do plan to take apart everything, look for reflections. I'll pay close attention to the filters. 



#22 PirateMike

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Posted 28 June 2017 - 09:45 AM

I recommend washing all new filters before use.



#23 PhotonHunter1

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Posted 02 December 2017 - 10:48 PM

Update: I found the source of the problem and a solution! 

 

Like Dan suggested, it's definitely a reflection issue.

Even without anything attached to the FW (no telescope or lenses), the bright ring was still present in my test images.

So, it had to be something in between the filter and the sensor.

 

I noticed that the filter edges are beveled, and I could see a colored reflection in this edge.

The color was also different in each NB filter.

 

attachicon.gifFilter bevel color.jpg

attachicon.gifFilter profile.png

 

This is likely the source of the problem.

I found a post on the ZWO forum, where a user reports the same problem:

http://zwoug.org/vie....php?f=8&t=6833

 

He made small rings to block the filter edges.

So, I cut out small paper rings and placed them on top of the NB filters.

 

attachicon.gifFilter rings.jpg

 

That did it!

 

Paper is probably not the best material for these rings though, so I'm going to (try to) make new rings out of a thin plastic sheet.

 

 

I don't use your camera or filters (I use Astrodon's - worth every penny paid), but a thought to consider:

 

What light source are you using to make your flats?  Narrowband flats require good illumination in their bands, and longer subs. If you took too short of a sub, then you'll stretch harder and amplify minor variations more.  My flats are 15-20 seconds for unbinned (1x1) subs.  Also, normal lights tend to drop off near the IR region (where Ha and Sii are located) .. make sure you use a full spectrum light.

My old flats were only exposed for 2-3 seconds to reach 12k ADU, but now I know that this is due to the bright reflection ring.

The new flats needed 10-12 seconds with the same light source, so that's as expected. Thanks!

 

Very promising!! Any one know what the inner/outer diameters would be for the 36mm filters?



#24 Binsky3434

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Posted 22 July 2018 - 10:46 AM

on the astrodon website it has 31mm LRGB and narowband filters that r mounted. does anyone know if the mounted 31mm will work with the ASI1600 8 position filter wheel or not?

 

and are the E or I series LRGB astrodon filters best for the ASI1600?


Edited by Binsky3434, 22 July 2018 - 11:46 AM.


#25 nightfly85

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Posted 23 July 2018 - 07:59 PM

on the astrodon website it has 31mm LRGB and narowband filters that r mounted. does anyone know if the mounted 31mm will work with the ASI1600 8 position filter wheel or not?

 

and are the E or I series LRGB astrodon filters best for the ASI1600?

Both mounted and unmounted work fine in the 5 or 8 position ZWO FW. I run E-Series LRGB and narrowband Astrodon 31mm unmounted with no issue - and they are parfocal!




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