Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

sgl_imaging_challenge_banner_dslr_mirrorlesss.thumb.jpg.5b348d6a5e7f27bdcb79e9356b7fc03b.jpg

Stub Mandrel

Processing Short Exposure Images

Recommended Posts

This is a thread for those whose subs rarely if ever exceed 2 minutes long, often shorter, with relatively low total exposure time to discuss tips and techniques for image processing.

Whether your an Alt-Az imager battling field rotation, you have issues with polar alignment or you have a relatively small, unguided mount you will often be faced with the challenge of getting the most out of relatively sparse data. The No-EQ Challenge thread has shown that even twenty minutes of sub-minute exposures can give good results.

I think some of the things we could discuss are:

We can't get rid of noise, but how can we keep its effects under control

What are the best tools for short exposure images and what settings are best?

Different types of software and their pros and cons

What percentage of subs should we stack? (e.g. can we use a greater proportion of subs to reduce noise while letting sigma delta keep our stars round?)

Ways of boosting faint background detail

What sort of standard should we be aiming for

And anything else

  • Like 1

Share this post


Link to post
Share on other sites

Really good idea for a thread!

Can I offer a book recommendation?  Astrophotography on the Go: Using short exposures with light mounts  (Patrick Moore Series)

  • ISBN: 3319098306

a good reference with good info

Mike

 

 

  • Like 1

Share this post


Link to post
Share on other sites

Great idea to have a thread for short exposure (budget) AP. Here are a few of my experiences with a limited setup:

When imaging with my EQ3 Pro and being limited to short exposures, I found that the "law of diminishing returns" should be thrown overboard. The more subs you can stack, the better quality you will get as far as noise is concerned. Doing 90 x 1 minute exposures is still very little time on target, which can be done in one session.

And as you suggested, keeping even less than perfect subs in the stack can give good result if the stacking process is chosen with care. But obvious faults with tracking should be kept out of the stack. The addage "Garbage in, garbage out" holds true.

A good dithering scheme is all important. I have dithered by hand in -10 C cold weather, just because I knew it would be worth it. Dithering 15 pixels in both RA and DEC is THE single best way to decrease the effect of hot pixels and chroma noise (what Tony Hallas calls colour mottle), and lift detail. Noise reduction during processing can improve an image a lot, but never as much as dithering.

BTW, a simple rule for manual dithering: set the hand controller slewing speed to 1 x sidereal. To get 15 pixels dithering, you then slew the mount for as many seconds as the angular resolution of your setup is. E.g. if you have a 2 arcseconds/pixel resolution, then moving the mount at sidereal speed for 2 seconds will move the mount 2*15 arc seconds = 15 pixels. Keeping the slew rate to max 1 x sidereal, will also avoid backlash problems. (If you put the slewing speed to 0.5 x sidereal, if possible, you need to move the mount for 2 x the resolution in order to get 15 pixel dithering.)

While not photography related, but relevant to the EQ3:

When you use the EQ3 at higher latitude (like my 60 degrees), the weak alt adjustment screws are really stressed to the max. The design of the mount is also such that the alt axis will tighten itself more if you turn the mount upwards (towards higher alt). This is because the bolt that is in the alt axis tightens the fork in the bottom part of the mount. Taking off the black cover plates and slightly loosening the bolt will make it easier to do a polar adjustment. I have also made a small wedge for the alt adjustment screw to rest against. Polar alignment is easier with this wedge in place.

 

Hope this can be of use

  • Like 2

Share this post


Link to post
Share on other sites
19 hours ago, wimvb said:

And as you suggested, keeping even less than perfect subs in the stack can give good result if the stacking process is chosen with care. But obvious faults with tracking should be kept out of the stack. The addage "Garbage in, garbage out" holds true.

I've found my integrated image can improve when I actually reduce the number of subs. I now try and be quite harsh in what subs I allow through into integration to avoid poorer quality subs softening (at best) the final result. I would always recommend visually inspecting each sub at full screen size and then once again zoomed into a corner of each image. The first pass helps spot things like high clouds passing through (which can add unnecessary gradients into the image), the second pass helps spot subs where there has been trailing (poor tracking, vibrations from wind, etc).

I would never recommend using an arbitrary % of subs. I know this approach works when extracting frames from video for lunar and planetary imaging, but in these cases you can be taking 1000 subs from over 10000. I find % limits will either throw away good subs or include poor ones.

If you have good image analysis software, you could choose a parameter (signal to noise ratio, star eccentricity, FWHM, etc) and make a decision on an acceptable cut off, rejecting subs that fall over that limit. PixInsight also allows for subs to be weighted based on a user formula, so subs with higher SNR or smaller FWHM could be given more weight in the integration process.

As with many things, it is worth experimenting. Integrate everything, integrate everything except certain subs, etc and then compare and see which one provides the best result. Time spend on that initial stage could save a lot of time in post-processing trying to remove gradients or tighten star shapes, etc.

  • Like 1

Share this post


Link to post
Share on other sites

I must admit, I do a trawl and get rid of the clearly useless subs (cloud, triangular stars etc.) then use 80 or 90% in DSS, which apparently scores based on number of stars detected (i.e. clarity) and how round they are (eccentricity). I do experiment with different percentages when I hit problems or there's clearly a big variation in subs (especially when thin cloud strengthens the LP and means some subs are very light with few stars).

 

@FilrodenKen - specific question for you - what Ha filter do you have and what length exposure are you using - your results look like the 'next level' that I'm hoping to achieve.

Share this post


Link to post
Share on other sites
13 minutes ago, Stub Mandrel said:

@FilrodenKen - specific question for you - what Ha filter do you have and what length exposure are you using - your results look like the 'next level' that I'm hoping to achieve.

I got the Astrodon 3nm. I debated getting a set of Baader narrowband (which would have been much less than the sin,e Astrodon) but eventually decided I was likely to want to upgrade later so why spend twice?

I've only taken a few 30s and 60s subs so far but the difference in quality over my L filter is mind blowing. They are much easier to process to a good mono image though a little harder to then blend with LRGB (especially weak LRGB). You really need good L and R data to not be overwhelmed by the quality and quantity of Ha data. 

  • Like 1

Share this post


Link to post
Share on other sites

I agree, Ken. Number of subs should not be based on %, but on inspection. Inspection of single subs, but also inspection of final result. Increasing the number of subs will decrease noise in the background. But it can also, as you write, soften the final result. In case of very slight star trailing issues, I have found that the final stacked image can have rounder stars. But you can't expect improvement in terms of small scale detail. As always, experimentation and critical inspection is the best way to go. Sometimes you have to face the fact that a night's imaging only produced garbage that should go straight into the bin.

  • Like 1

Share this post


Link to post
Share on other sites

Thanks both.

I must admit I'm surprised that  you can use a narrowband filter down to 30 seconds!

I suppose ~95% of all that nebulosity is getting through, so i shouldn't be!

Share this post


Link to post
Share on other sites
4 minutes ago, Stub Mandrel said:

I suppose ~95% of all that nebulosity is getting through, so i shouldn't be

And very little else. I think you would need 60-120s for Oiii and Sii based on Gina's experience with wide field imaging so still within range of short subs! Note, this is with the ZWO ASI1600MM which has very low read noise and works really well with short subs.

Here's a stretched single 60s sub:

NGC2239_H_60sec_2017-01-23_185014_1x1_-20.0C_fpos_6847_0001.jpg

  • Like 1

Share this post


Link to post
Share on other sites

:-)

Well with my skies, guiding is hardly worth the bother without narrowband, so this convinces me that filter first guiding later is worth a try.

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Recently Browsing   0 members

    No registered users viewing this page.

  • Similar Content

    • By Phillips6549
      Complete beginner question regarding using this mount.
      When I have tried to set up and align the mount , the goto targeting is off by many degrees.  Here's what I do - can someone tell me what I'm doing wrong!?
       
      With the power off:
      Align the tripod and mount to North and, with the clutches released, set the declination to my latitude (52deg 28' N). and the mount in the weights-down home position.
      Lock the clutches and fine tune the alignment on Polaris using the mechanical adjustments on the mount.
       
      Turn the power on:
      Set up the date, time and location, elevation etc. in the handset (noting the American Date format and Longitude first (set to W 1deg 48')
      DST = Yes
      Select 1 Star align
      Pick a star, I usually try Procyon or Betelguese depending on the time.
      The scope slews to vaguely the right direction but actually nowhere near.  It can take 2 or 3 minutes of manual slewing to centre the star in the eyepiece.  Finishing, of course, with an up and right movement.
      Finally, press Enter to complete the alignment ==> Alignment Successful says the handset optimistically.  
      Now from the object menu pick another target.  Try, say, Polaris which was where I started and ... nowhere near it.   It's the same regardless of the selected target.
       
      I have tried 2 star alignment and 3 star alignment, a factory reset before starting,  but nothing seems to work.   Once i'm lined up on a target the mount tracks nicely - it just won't go where I want it to!
      Not yet at my wits end but - Help!  
      Mark
    • By cjdawson
      Hi everyone.
      A friend has an EQ3 Goto mount, I think it's the Sky-watcher one.    Currently, I know nothing about them, except that he's having issues getting it to work.  To the point that he's getting so frustrated with it he's seriously starting to thinking about throwing the mount away.
      He's described what's happening to me.    What he's saying is that the mount never points to the place where he's asked it to point.    He's saying that say the scope is supposed to point towards something in Orion, it'll end up pointing somewhere on the other side of the sky.     He's also said that it doesn't seem to want to track past a certain point in RA.
      I'd thought rather than simply jump in and try to help him out and get just as confused,  I was wondering if there's some check list of common EQ3 faults that I could work through to see if we can solve the issue.
       
      Once thing I do know is that he's running the mount from a dedicated tracer battery, so it maybe safe to assume that the power supply is good.   (althought it's something I'm going to double check as I don't want to miss anything)
    • By Ahgii
      Hi all!
      Few days ago i tried imaging a deep sky object for the first time, so i tried m13 as an easy first target, I used took 7  lights, out of witch i selected 3 for stacking, and used 5 darks. DSS only selected 1 frame out of the 3 and stacked that. The result wasn't that good but then again....it's the first time i'm doing this. Settings were iso 6400, 15 second exposure, auto WB. I have also been having this problem, when my telescope slews to a target, it's always to the left of the frame and that was a bummer since i had to crop my picture. Please give me your advice and feedback, Clear skies!
       

    • By Stub Mandrel
      34 2-minute subs, no darks. Exif says 6c so the sensor was probably somewhere between -15 and -20C!
      Begging for a few more hours!

    • By Stub Mandrel
      To my surprise last night wasn't a complete write-off. I managed to get another 135 minutes of decent 5-minute frames of M31, this time without my LP filter. Total of 3 hours 55 minutes with my cooled canon. There was some streaky noise caused by drifting very thin cloud.
      The subs were much brighter and no moon plus high elevation meant I got away with it. The mix of frames with and without filter stopped the core being too badly blown out. I think I've controlled my urge to over-process this time, but any suggestions for improvement welcome.

       
×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.