Jump to content

Narrowband

M31 Defect


Recommended Posts

Hi All

Recently imaged M31 with a 130pds and Canon 1100D with no coma corrector.  Comprises 19 x 8min subs 10x 2min darks and 10 bias.  I was guiding at the time. I  used DSS and then Pixinsight for post processing.  I have attached the result below.  I noticed on this occasion and previously that there was a band of 'noise' running along the edge of the picture.  You can see it on the attachment below running along the right hand edge.  I am at a loss to know what is causing this effect and it appears to be repeatable !

Not sure if its related to my set up perhaps  - stray light ?  

Any comments on this would be greatly appreciated

post-36401-0-76624700-1416131537_thumb.j

Link to comment
Share on other sites

Hi

Doesn't seem to be apparent in the individual subs.  I used full length darks last time and the effect was still there. Never had this before and I have used shorter exposure times for darks often in the past as well.  Haven't done too much imaging with the 130pds before it is relatively new.  

What is amp glow?

Thanks

Link to comment
Share on other sites

It could be stray light entering from the viewfinder? Can you see it on your Darks if you stretch them?

Amp glow is caused by a high gain amplifier at one side of the sensor that converts electrons to a voltage that can be measured by the analogue/digital ( A/D) converter that can, bizarrely generate a small amount of light as well as thermal noise.

Link to comment
Share on other sites

I used full length darks last time and the effect was still there. Never had this before and I have used shorter exposure times for darks often in the past as well. 

Hopefully someone will correct me if I'm wrong, but I believe if the exposure time of the darks doesn't match the lights it won't improve the final image. Might be worth stacking with no darks to see if that makes a difference.

Interesting that it doesn't show up in the subs, have you tried stretching one to see if it is present but faint?

As steppenwolf says, have you tried using a viewfinder cover?

Nice looking M31 by the way.

Link to comment
Share on other sites

Hi again

Thanks for your comments.  

Funny concept to stretch a dark frame but understand the logic - good thinking !  I had a go but doesn't appear to be anything obvious there.

I think the camera viewfinder was illuminated so may have been a source of spurious stray light

I also had a go at stretching an individual sub and I can see a darker band right at the edge - there was some background gradient defect which I removed with Pixinsight's DBE tool .  Its almost like the background gradient  removal wasn't effective at the very edge and left some of the defect behind causing the band of noise.  Perhaps I will have another go and place the samples in a different place going along the offending  edge.

Wrt amp glow noise  I guess this would be a random effect - I was using ISO 400 so the gain wasn't very high.

Interesting about the length of the dark frames - perhaps I should persevere in future and keep them as long as the lights.  Its just that when you are guiding, the amount of light time increases and I have a tendency to want to pack up without doubling the imaging time at the scope - more patience is required !

I will report back when I have had another go with the DBE tool.

Thanks

Link to comment
Share on other sites

You can always take darks from other sessions if the temperature and gain is the same and create dark libraries for different temperatures and exposure times.

I know what you mean though as when you start taking 10 min subs it's annoying to have to take 10 min darks to go with them.

TSED70Q, iOptron Smart EQ pro, ASI-120MM, Finepix S5 pro.

Link to comment
Share on other sites

I guess I need to get more organised with my Darks !

Anyhow I have tried with DBE to no avail so have cropped my previous image to remove the noise band at the edge at the expense of a bit of M31 and re processed !  

I guess I have taken some artistic license with the re processing to tart it up !!

 Looks like I will never be uncertain why the noise band came about  

post-36401-0-59684000-1416172666_thumb.j

Link to comment
Share on other sites

Thanks Jack - I may try and calibrate and stack in Pixinsight to see if it makes a difference

You should definitely learn how to do that; can't understand why people buy PI and then continue to use DSS for calibration and stacking (other than familiarity I suppose).  The Batch Preprocessing script in PI is just as easy to use as DSS and should give good results - you can do better if you manually calibrate and stack with individual processes but there are a lot of buttons and controls to learn and it is a bit easier once you have become familiar with the BPP script.

One major benefit of PI calibration is dark frame scaling - it will compare your master dark to your light frames and try to scale the dark to match the light.  It does this purely based on noise statistics - PI doesn't need to know about exposure lengths, ISO settings or camera temperatures (which is especially good for Canon DSLR users as there is no reliable way to measure the average sensor temperature - I can, and am, in the process of writing an essay on the subject but believe me when I say the 'temperature' recorded in the RAW file bears no relation to the dark current or dark current noise).

Instead it tries different scaling factors (increasing or decreasing the effect of the dark frame) until it creates a calibrated light frame with the least amount of noise in it.  If your darks don't match your lights in any respect it will throw a 'no correlation between dark and light' error and not apply the dark at all, since that would make the image worse.

Link to comment
Share on other sites

ok you've convinced me  - I will give it a go.! 

 I think it is familiarity that creates a lack of invention sometimes.  

I think also the Canon RAW format subs  need converting to FITS for

Pixinsight ?

Link to comment
Share on other sites

ok you've convinced me  - I will give it a go.! 

 I think it is familiarity that creates a lack of invention sometimes.  

I think also the Canon RAW format subs  need converting to FITS for

Pixinsight ?

No it will read them fine. You need to specify the RAW CFA format hints when reading them. See this for details:

http://pixinsight.com/forum/index.php?topic=3118.msg21348#msg21348

Link to comment
Share on other sites

i had that on images taken with my 1100d. it's caused by the flip mirror when it's in the up position interfering with the light cone. i removed the mirror completely from my camera and no more artefact

here's one of my early images before i modded my camera. the artefact is always at the 'bottom' of the image but as it's reversed... well, you get the drift.

M33_zps5e12db94.jpg

Link to comment
Share on other sites

Wow looks to be very similar ! Makes sense as the mirror in the up position would affect one edge ie the edge where the mirror was located.  Well done for working it out.

Funny thing is that I only appear to have noticed it recently but it was the same in the last two images.   I have recently started imaging with a !30PDS reflector wheras I was using a ST80 previously.  Perhaps thats to do with it.  

I guess I will carry on and take some more images to see how repeatable it is and decide whether I can live with it by cropping or do as you have done.  

Im not really into day time photography so it wont be too much of a sacrifice to remove the mirror - did you follow a written instruction from somewhere to do it ?

Thanks again for taking the  trouble to share this - appreciated.

Link to comment
Share on other sites

OK thanks IanL - ive never used flats before only darks and bias - I will experiment with them and see what happens.  

There are a couple of clear nights coming up in this area soon so may take some pics of the Pleiades or M33 and see if the noise band appears and what effect flats have.  

Perhaps I will post the results next week if the clear skies appear and I can overcome the dew !

Cheers

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.