Darks don't remove amp glow ASI 1600MM Pro

[wetcoaster]

I use SharpCap to capture my lights and to create master darks and master flats. I have never had any luck with flats and have sort of given up on them. However, till I bought the ASI1600MM Pro, I have never had a problem removing amp glow, albeit that was several years ago with the DSLRs then available. I have also created master darks by capturing a number of individual subs and then averaging them using Pixinsight. But it doesn't matter. After calibration, there appears to be little if any amp glow but then I stack the images it's back in full force.

My darks are captured at the same settings as my lights - 300 sec, gain 200, sensor temp -20°C. It doesn't matter whether I use master darks created by SharpCap or ones I've created from dark subs. I get the same results in Pixinsight and ImagesPlus.

 

Here is a link to a DropBox folder which contains a light, calibrated light and integrated image. 

<https://www.dropbox.com/sh/pgcxr1pibgmg2hu/AACY4EMPLq_9lFtx-cRroH0aa?dl=0>

 

I am looking for ideas.

 

Don

[simmo39]

Hi, yes im having the same problem. Not sure what else to try either.

[geordie85]

If I only use darks with my qhy183c I still have amp glow. The only way I can completely remove it is by using flats and darkflats as well as darks. 

[simmo39]

3 minutes ago, geordie85 said:

If I only use darks with my qhy183c I still have amp glow. The only way I can completely remove it is by using flats and darkflats as well as darks. 

I have tried flats and dark flats and still had no luck.

[vlaiv]

21 minutes ago, geordie85 said:

If I only use darks with my qhy183c I still have amp glow. The only way I can completely remove it is by using flats and darkflats as well as darks. 

Then it is not amp glow. If it gets corrected with flats then it is related to vignetting or some sort of light leak in the front of the scope.

If it looks like amp glow but is not corrected with proper calibration (in this case darks, flats and flat darks) - it can be light leak at the back of the scope or on the sides.

Prime candidate for light leaks is Newtonian because of construction - tubes are made shorter than they should be, and imaging newtonians have low profile focusers. Mirror cells at the bottom are often "ventilated" and not baffled properly.

For front light leak - use some sort of light shield extension to tube (similar to dew shield used on SCT) - make it out of non reflecting dark material. "Tube" part in front of focuser should be at least x1.5 tube diameter.

Back light leaks can easily be checked - put scope cover on, take single dark sub then take a torch and start another dark sub but this time flash a torch at back of the scope (you can move it around to change angle). If two subs have different amount of "amp" glow then it is due to light leak.

[brrttpaul]

Now this is interesting as I find I have the same problem (ZWO ASI 1600) tried everything without success and did wonder was it light leaking so will try it out ( im using an ed 80 scope )

[tooth_dr]

If it’s a common problem could the cameras themselves be leaking light?

[Laurin Dave]

I had similar issues and now always take the camera off the scope, put the cap on, and stand it on the cap to take darks and flat darks.  Seems to work.  I do them in the observatory or garage as indoors the camera won’t reliably reach its set point temperature. 

Dave

[vlaiv]

Good points above, I never thought of it, but I do also take my darks separate from the scope, I take the camera off (I have a mobile setup) and leave camera working in the basement to take darks - again capped and placed on table "face" down.

Never had calibration issues with it.

Flats and Flat darks I do while it is attached to telescope (short exposure, so it takes about 20 minutes to do a full set of 256 each). I need to build flat dark lib - but I never seem to note down exposure used for each filter in order to do so.

[Laurin Dave]

23 minutes ago, vlaiv said:

Good points above, I never thought of it, but I do also take my darks separate from the scope, I take the camera off (I have a mobile setup) and leave camera working in the basement to take darks - again capped and placed on table "face" down.

Never had calibration issues with it.

Flats and Flat darks I do while it is attached to telescope (short exposure, so it takes about 20 minutes to do a full set of 256 each). I need to build flat dark lib - but I never seem to note down exposure used for each filter in order to do so.

I use a Huion tracing panel on full for flats with varying numbers of sheets of  A4 paper to dim it, I have a flat dark library for various exposure between 0.1 and 1 sec and adjust the number of sheets so that I don't have to redo flat darks every time.  

[vlaiv]

I have simple panel (fits 200mm scopes or less) that is led based with a diffuser panel - it is pretty strong.

From what I remember, I use crazy short exposures for LRGB - order of 0.01s, and 0.3 - 0.5s range for narrow band filters.

I believe most will agree that longer exposures are needed for flats - but above worked fine for me from the start and never caused any issues (well once I had issues, but I traced that to 12V jack connection - it was not connecting properly and caused "flicker" in light intensity that was not obvious to naked eye but showed in such short exposures - a bit of soldering work and it was quickly fixed).

[Laurin Dave]

I haven't experimented with ultra short flats because I read somewhere that they needed to be long enough to avoid the ASI1600 wobbly bias issue..  (John Rista on CN probably)

Dave

[IanL]

If you have amp-glow or any other sort of large scale non-uniformity in your darks, you need to turn off dark frame optimisation when calibrating lights in PixInsight.

Dark frame optimisation in PI determines a global scaling factor based on minimising residual noise after dark subtraction. This works well for darks which are uniform and enables use of different exposure or temperatures between darks and lights.

For sensors with amp glow, the scaling factor will cause the amp-glow area to under or over correct. You need to turn it off and use well matched darks and lights (same temperature and exposure).

[vlaiv]

11 minutes ago, Laurin Dave said:

I haven't experimented with ultra short flats because I read somewhere that they needed to be long enough to avoid the ASI1600 wobbly bias issue..  (John Rista on CN probably)

Dave

These are not really ultrashort exposures in classical sense - I reach 80%+ histogram on unity gain with those - it is just my panel that is ultra bright

 

[brrttpaul]

at the point of sounding stupid here, would taking your camera off for your darks muck up your flats library?

[Jbro1985]

Yes - exwctly my thought when reading - screws your flats unless very very careful.  This is why I don't do it; if the camera doesn't go back on at exactly the same point of rotation, it's time for a new flats library.  

My rudimentary technique.  Dark room on a dark cloudy night with scope sitting on spare mount, imaging train untouched.  I then zip tie a black binbag or black rubble sack tight to the focuser covering camera, filter wheel, OAG etc with only enough room for the USB cable and power cable to pop through to prevent back end light leaks (my system is good anyway but belt and braces).  I then put the cap on the scope and to make 100% sure there is no light leaks, I put the BBQ cover over the top of the scope!

I then run darks on a SGP image sequence 30 10mins, 30 5mins, 30 2.5mins and go to bed...

Obviously, some like to take flat frames every session and if I had a permanent set up I might be tempted to do that too.  In which case, taking the camera off to do darks wouldn't make a jot of difference.

[vlaiv]

32 minutes ago, Jbro1985 said:

Obviously, some like to take flat frames every session and if I had a permanent set up I might be tempted to do that too.  In which case, taking the camera off to do darks wouldn't make a jot of difference. 

I do it every session - because I have to as I don't have permanent setup and I disassemble my gear for storage   - so it's opposite for me - I would build flat lib if I could

[Adam J]

I have never had a problem removing the amp glow. Do you have dark frame scaling enabled? Because you shouldn't.