1st Light in 2 years, New Equipment - Horsehead Nebula - What's going on?

[topgearuk]

Hi,

Please can I get some advise, I have started imaging again after 2 years of not getting a single result.. I have all new equipment after having an unsuccessful time last year in my limited windows I get, last year was written off due to technical problems (mainly me). And almost gave up.

I moved six months ago and have a good view of light polluted E,S,W sky, but due to size of garden and out buildings, I need to be either at one end then can see North or other end and can see South, so Polar Aligning has been a challenge, but I think i'm pretty close and I'm leaving the mount outside but covered.

Here is a screen shot of 1 x 10 min HA sub from last night and while I'm happy with the main image I have no idea what is going on around the edges, particular bottom right.. This is on all my subs including RGB subs. Can anyone point me in the right direction as to what is going on?

Camera ZWO ASI 1600MM, will update my equipment list below.

[vlaiv]

Did you calibrate your subs? This looks like amp glow on ASI1600 - you need calibration with dark frames to remove that.

[Bukko]

IFor a single sub, there is a lot of nice detail there.

I don't have an ASI 1600MM camera, but it is a dedicated imaging camera and if this was amp glow, to my mind, it would be a design flaw...

Although concentrated in the bottom RHS, there is signs of a problem all around the frame. I would look for light leakage... Is there any light source near the scope while imaging; such as a laptop, or something?

As Vlaiv says, it should be possible to calibrate a lot of this out, assuming it is amp glow...

 

Good luck and welcome back to imaging.

Gordon.

[topgearuk]

43 minutes ago, vlaiv said:

Did you calibrate your subs? This looks like amp glow on ASI1600 - you need calibration with dark frames to remove that.

Thanks for your reply, so this is just Amp glow? I haven't done my dark frames yet, will take them later. I wasn't expecting AMP glow to be as severe as this.

[Filroden]

The well depth on the 1600 is not very high so can be easily filled by light polluted skies. Given the time of year and your location, I guess this target was quite low? Maybe 25 degrees altitude? Was it near any buildings, e.g. just rising above a fence, etc that could be reflecting light?

I've only every pushed my 1600 to 4 mins for Ha and 2 mins for LRGB and I've never seen amp glow to be that bad. Typically my glow is in the upper and lower right corners. So my first guess would be a combination of light pollution/exposure time and possible light reflections/ingress.

[vlaiv]

45 minutes ago, topgearuk said:

Thanks for your reply, so this is just Amp glow? I haven't done my dark frames yet, will take them later. I wasn't expecting AMP glow to be as severe as this.

This is my master dark (x256 subs) 1 minute, unity gain, -20C for ASI1600, really stretched

So you see that there are couple of areas where amp glow is present, but it calibrates out nicely so it should worry you.

If you still have lighter areas after calibration, next thing to check out would be if there is light leak somewhere in optical train.

[IanL]

You've got a number of different things going on there.

The large bloom to the bottom right is definitely amp glow if you haven't done darks yet. My darks are similar to Vlaiv's above, but there are two patches  both on the lower right side with the one in the corner being larger.  Amp glow is a fact of life with many cameras, including the ASI1600-MM. Is this the cooled or non-cooled version (which would likely be worse)? It will calibrate out fine if you use matching dark frames - same gain / offset, same temperature (ideally), same exposure time - I stack about 100 to make a master for each of the common settings I am using.

The amp glow builds up rapidly during the image read-out - ideally you should use a USB3 connection directly between camera and computer to get the fastest read-out. The problem will be worse with USB2 which is much slower to read out the large frames from the 1600. The newer 1600 Pro Models have onboard memory to speed up the read-out phase, but to be honest I haven't found it particularly problematic with my images using the standard version.

The bright edges all round are a bit more of a puzzle to be honest. If that is a single sub with no flat frame applied it can't be vignetting which is definitely an issue with such a large chip and 31mm / 1.25" filters (would have expected dark corners not light ones). If you have applied a flat to this, then be aware I have had similar looking frames due to the flats over-correcting and making the corners and edges brighter.

It could be some kind of reflection issue between any and all of the sensor cover glass (which is not AR coated in the 1600 or any other camera using this chip), the optical window (which is AR coated), the filter, any other nearby glass (like reducers or barlows) and the focuser drawtube or other elements in the train. It could also be light leaking in somewhere in the optical train. You can diagnose that by taking some dark frames with the camera, scope, etc. fully assembled but sealed in a light-proof box, and then take some in daylight and compare to see if there is any kind of leakage coming in.

If no, then I'd move to taking some good flat frames (using a diffuser and a cloudy sky to rule out any flat-box issues). Have a look at those and see what you are getting as it may begin to point in one direction or another in terms of optical issues.

[IanL]

You might also have a look at this thread regarding filters: 

https://www.cloudynights.com/topic/600525-flats-issues-with-zwo-filters-and-un-correctable-ring-artifacts/

It turns out that some filters (not just ZWO) don't have an AR coating right the way to the edges, and many have bevels or other issue than can cause reflection problems. Not saying this is the issue, but another possibility to consider.

[The Admiral]

I have no idea what could be going, nor can I suggest a solution. But, the edges look as though they are comprised of a series of overlapping straight-sided frames that have smaller and smaller sizes. Yet this is a single frame? May be I'm just stating the obvious, but I can't see the suggestions above explaning it.

You don't say what scope you used, but I could only see such an effect resulting from a stack of several frames where the focal length has successively slipped during the exposure, with the stacking software stretching the frames to align the stars. Were you using your zoom?

Ian

[IanL]

5 minutes ago, The Admiral said:

I have no idea what could be going, nor can I suggest a solution. But, the edges look as though they are comprised of a series of overlapping straight-sided frames that have smaller and smaller sizes. Yet this is a single frame? May be I'm just stating the obvious, but I can't see the suggestions above explaning it.

You don't say what scope you used, but I could only see such an effect resulting from a stack of several frames where the focal length has successively slipped during the exposure, with the stacking software stretching the frames to align the stars. Were you using your zoom?

Ian

I'm assuming that it is either:

- Just the stretch and / or conversion to a web-friendly image format posterising the levels in these artefacts.

- and / or A multi-level reflection around the edges of the optical window or cover glass; that would have the potential to create multiple straight edge artefacts.

[vlaiv]

10 minutes ago, The Admiral said:

I have no idea what could be going, nor can I suggest a solution. But, the edges look as though they are comprised of a series of overlapping straight-sided frames that have smaller and smaller sizes. Yet this is a single frame? May be I'm just stating the obvious, but I can't see the suggestions above explaning it.

You don't say what scope you used, but I could only see such an effect resulting from a stack of several frames where the focal length has successively slipped during the exposure, with the stacking software stretching the frames to align the stars. Were you using your zoom?

Ian

OP said it is single sub, so I suppose no stacking artifacts.

I did see such patterns before in CMOS sensors - bias signal sometimes looks like that - vertical and horizontal lines and bands usually close to edge, but this one looks kind of symmetric.

If OP did not mention it was a single sub, I would have thought it was a stack of subs with meridian flip - so vertical and horizontal bands being mirrored because of 180 degrees rotation pre and post meridian.

[topgearuk]

12 minutes ago, The Admiral said:

I have no idea what could be going, nor can I suggest a solution. But, the edges look as though they are comprised of a series of overlapping straight-sided frames that have smaller and smaller sizes. Yet this is a single frame? May be I'm just stating the obvious, but I can't see the suggestions above explaning it.

You don't say what scope you used, but I could only see such an effect resulting from a stack of several frames where the focal length has successively slipped during the exposure, with the stacking software stretching the frames to align the stars. Were you using your zoom?

Ian

Just to answer your post 1st, this is a single 10 minute frame, no stacking was done, this is a screen shot straight from SGP, with it using it's own stretch function.

Scope: TS 102 mm f/5.1 APO, ZWO 1600 mm-cool, ZWO Filter wheel with Astronomik 31mm Ha 6nm @ unity gain (139), -20C

It is the 1st photo shot with this scope, filter, mount and camera. So with so many changes It's hard for me to find the culprit or is it just amp glow?

Before this I used a Canon 450d with clip filter and a Skywatcher Evostar 80ED.  So this is my 1st time shooting with mono, with new soft and hardware.

IanL Thanks for your replies and will reply in a separate post.

[topgearuk]

2 hours ago, vlaiv said:

This is my master dark (x256 subs) 1 minute, unity gain, -20C for ASI1600, really stretched

So you see that there are couple of areas where amp glow is present, but it calibrates out nicely so it should worry you.

If you still have lighter areas after calibration, next thing to check out would be if there is light leak somewhere in optical train.

Thank you for your replies, I will try a master dark and see how much this helps

[topgearuk]

3 hours ago, IanL said:

You've got a number of different things going on there.

This is what I'm worried about

Quote

The large bloom to the bottom right is definitely amp glow if you haven't done darks yet. My darks are similar to Vlaiv's above, but there are two patches  both on the lower right side with the one in the corner being larger.  Amp glow is a fact of life with many cameras, including the ASI1600-MM. Is this the cooled or non-cooled version (which would likely be worse)? It will calibrate out fine if you use matching dark frames - same gain / offset, same temperature (ideally), same exposure time - I stack about 100 to make a master for each of the common settings I am using.

This makes sense and will try and do a dark library either tonight or tomorrow. It's the cooled version.

Quote

The amp glow builds up rapidly during the image read-out - ideally you should use a USB3 connection directly between camera and computer to get the fastest read-out. The problem will be worse with USB2 which is much slower to read out the large frames from the 1600. The newer 1600 Pro Models have onboard memory to speed up the read-out phase, but to be honest I haven't found it particularly problematic with my images using the standard version.

I'm running a 10 meter powered USB 3 cable.. It's not the newer Pro model as I brought the camera about 10 months ago, new, but this is the 1st time I have used it.

Quote

The bright edges all round are a bit more of a puzzle to be honest. If that is a single sub with no flat frame applied it can't be vignetting which is definitely an issue with such a large chip and 31mm / 1.25" filters (would have expected dark corners not light ones). If you have applied a flat to this, then be aware I have had similar looking frames due to the flats over-correcting and making the corners and edges brighter.

No Flats, just a single stretched frame using SGP stretch. 31mm unmounted filters.

Quote

It could be some kind of reflection issue between any and all of the sensor cover glass (which is not AR coated in the 1600 or any other camera using this chip), the optical window (which is AR coated), the filter, any other nearby glass (like reducers or barlows) and the focuser drawtube or other elements in the train. It could also be light leaking in somewhere in the optical train. You can diagnose that by taking some dark frames with the camera, scope, etc. fully assembled but sealed in a light-proof box, and then take some in daylight and compare to see if there is any kind of leakage coming in.

If no, then I'd move to taking some good flat frames (using a diffuser and a cloudy sky to rule out any flat-box issues). Have a look at those and see what you are getting as it may begin to point in one direction or another in terms of optical issues.

Thanks, I will try above 1st then try this.

[IanL]

The guys with the reflection issues over on Cloudy nights were mainly using unmounted filters, apparently there is less of a problem with mounted as the bevelled edges and uncoated areas tend to be under the filter mount stopping it happening with mounted ones.  I use the standard (V3) 1600mm with a 3 metre active USB extension and as I noted above, the Amp glow will calibrate out no problem (if you happen to use PixInsight at any point, turn off dark frame optimisation however as this doesn't work well for amp glow - use darks that match the exposure and temperature of the lights).

Change one thing at a time with a view to proving or disproving each suspected cause.

 

[topgearuk]

Screen shot of my work flow at the time using SGP, just in case there is something obviously wrong,  unfortunately you can't see the problem with the image in the background as it was not centred in the frame to show the problem at the time.

[The Admiral]

1 hour ago, IanL said:

Just the stretch and / or conversion to a web-friendly image format posterising the levels in these artefacts.

Wouldn't that also be evident throughout the rest of the image?

Ian

[topgearuk]

18 minutes ago, IanL said:

The guys with the reflection issues over on Cloudy nights were mainly using unmounted filters, apparently there is less of a problem with mounted as the bevelled edges and uncoated areas tend to be under the filter mount stopping it happening with mounted ones. 

That's interesting about the unmounted filters, wish I knew that 10 moths ago, would have saved a fortune.. I thought a bigger filter would be better at the time, I can live with some problems around the edges of the image I guess.

18 minutes ago, IanL said:

I use the standard (V3) 1600mm with a 3 metre active USB extension and as I noted above, the Amp glow will calibrate out no problem (if you happen to use PixInsight at any point, turn off dark frame optimisation however as this doesn't work well for amp glow - use darks that match the exposure and temperature of the lights).

Change one thing at a time with a view to proving or disproving each suspected cause.

 

Yes I do have a copy of PixInSight (whole new learning game), so will try that and thanks for the suggestions

[IanL]

12 minutes ago, The Admiral said:

Wouldn't that also be evident throughout the rest of the image?

Ian

Hard to say without seeing the original and the stretch that was applied. Posterisation usually only affects 'flat' parts of an image where just a few levels get spread over a lot of the histogram. That can happen at the dark end, tge light end or somewhere between depending on the image and stretch.

Similarly multi-order reflections can disappear or appear for similar reasons. We know this is a single sub now, so it isn't a stacking artefact. More experiments required to rule out some options.

[topgearuk]

Bingo!!

Thank you to everyone who contributed to this thread, Dark and Bias frames was all what was needed.. I'm so happy it's nothing more then that.

 

 

[Allinthehead]

Hi. Sorry but i don't think you have sorted the issue. I can tell you if i had a sub look like that I'd be wondering what's going on.

Your calibration frames have just masked the issue i can see the outline of the bright areas (only now they are dark) in your calibrated image. I use the same camera and I've never seen any of my uncalibrated subs look like that. I'm on my mobile now so I'll post one of my 10 min ha subs tomorrow so you can compare.

Can you upload a dark and bias frame?

[Allinthehead]

Notice the slight gradient on right of the image. That's amp glow. Have you tried that camera without the filter wheel on another scope?

 

[topgearuk]

On 1/12/2018 at 23:23, Allinthehead said:

Hi. Sorry but i don't think you have sorted the issue. I can tell you if i had a sub look like that I'd be wondering what's going on.

Your calibration frames have just masked the issue i can see the outline of the bright areas (only now they are dark) in your calibrated image. I use the same camera and I've never seen any of my uncalibrated subs look like that. I'm on my mobile now so I'll post one of my 10 min ha subs tomorrow so you can compare.

Can you upload a dark and bias frame?

Hi, interesting you say that.. To be honest I really not sure what's going on.. At first look at the processed subs it looked a lot better but yes I think you are right about it masking the problem. I do live in an extremely light polluted area.

Here is a single sub, dark and bias, if you wouldn't mind looking?

https://www.dropbox.com/s/0yj04ez864n2j3g/Target 1_600sec_1x1__frame1.fit?dl=0   Dark

https://www.dropbox.com/s/yz8gd8ar0a58fdo/Target 1_0sec_1x1__frame1.fit?dl=0   Bias

https://www.dropbox.com/s/bzqj7t2dw6nn2ya/Target 1_600sec_1x1__frame11.fit?dl=0  Light

 

[topgearuk]

On 1/13/2018 at 08:35, Allinthehead said:

 

Notice the slight gradient on right of the image. That's amp glow. Have you tried that camera without the filter wheel on another scope?

 

This looks a lot cleaner, I haven't tried another scope although I still have my Skywatcher Evostar 80ED I could use.

[Allinthehead]

I just had a look at your dark and my first instinct is that your camera is faulty. However your bias doesn't seem to have the same issue.

How do you take your darks? I cover telescope, camera, everything really and i only take them at night.

If i were you i would change the usb cable connecting the camera.

You could try taking the camera in doors with no scope or filter wheel attached, just camera with the cap on. Don't bother with cooling for now. Connect directly to laptop with a different usb cable and try a short dark say 60 seconds at lowest read noise gain. Do it with the camera covered and lights off. This should eliminate the possibility of light leakage. If you get the same result then i would send an email to your supplier with a dark frame attached.

Richard.