Lucky Imaging M13 Hercules Cluster
Posted: Mon May 02, 2022 12:11 pm
Tim's (@timh) work using ultra-short exposures to freeze the turbulent atmosphere inspired me to give it a try. Over the past two nights I enjoyed a good stretch of clear weather. I chose a tough target: M13, a very tight star cluster that benefits the most from Tim's technique.
I haven't got the best equipment like most of you, so if you give this a try you can expect better results. I've got a refractor but it is a doublet. I fell for the manufacturer's marketing that it was "APO-like" due to their use of "ED glass". Don't believe...buy an APO! I learned the hard way that the scope does a poor job at focusing blue light. As a result I don't allow blue light to enter my scope. For luminance I use a Wratten #12 filter; that is Red + Green but no Blue. For color I use a red and green filter. Later during processing I mix the red and green channels into Red, Green, and Blue. It works but sadly blue stars look cyan and red stars look orange. It's not ideal but good enough for government work as they say!
I have two cameras, a CCD and CMOS. The CMOS camera isn't cooled but it has low Read Noise (1.55e-) and small pixels (2.9um). The CCD is cooled but has high Read Noise (5.3e-) and large pixels (4.65um). For this project, I chose the CMOS camera: Altair 290M. Normally I use it for guiding, but now I was left without the ability to guide. I figured that since the exposures are short that I could simply rely on Periodic Error Correction (PEC) of my DIY Raspberry Pi + Stepper Motor of my 50-year old mount. The PEC reduced the uncorrected peak-to-peak PE from 90 arc-seconds to 15 arc-seconds over 8.5 minutes of time. It worked well, but I made sure to capture far more Light frames than I needed.
All of my frames suffered from severe Raining Noise as seen in the attached Wratten #12 stack. According to my calculations I needed a minimum of 22-second exposures to mitigate the Raining Noise problem, but that would have negated the "lucky imaging" objective. Thankfully I was able post-process it out using Astro Pixel Processor (APP). Tim can get away with 3-second exposures due to the fact that he has a substantially faster scope than mine, plus he suffers from more light pollution.
I captured a total of 3600 frames but kept only the best 2400:
W12: 1200x3s, R: 600x6s, G: 600x6s. Total integration time: 3 hours
I was very fortunate that none of the stars in the image is saturated, so there is no harsh over-exposure of the central core of the cluster. In this image you are seeing stars as faint as 17th magnitude, possibly 18th magnitude. My star catalog only goes down to 16th magnitude so I am only estimating how deep you are seeing.
Brian
The final image, zoomed in and cropped: Example of Raining Noise:
I haven't got the best equipment like most of you, so if you give this a try you can expect better results. I've got a refractor but it is a doublet. I fell for the manufacturer's marketing that it was "APO-like" due to their use of "ED glass". Don't believe...buy an APO! I learned the hard way that the scope does a poor job at focusing blue light. As a result I don't allow blue light to enter my scope. For luminance I use a Wratten #12 filter; that is Red + Green but no Blue. For color I use a red and green filter. Later during processing I mix the red and green channels into Red, Green, and Blue. It works but sadly blue stars look cyan and red stars look orange. It's not ideal but good enough for government work as they say!
I have two cameras, a CCD and CMOS. The CMOS camera isn't cooled but it has low Read Noise (1.55e-) and small pixels (2.9um). The CCD is cooled but has high Read Noise (5.3e-) and large pixels (4.65um). For this project, I chose the CMOS camera: Altair 290M. Normally I use it for guiding, but now I was left without the ability to guide. I figured that since the exposures are short that I could simply rely on Periodic Error Correction (PEC) of my DIY Raspberry Pi + Stepper Motor of my 50-year old mount. The PEC reduced the uncorrected peak-to-peak PE from 90 arc-seconds to 15 arc-seconds over 8.5 minutes of time. It worked well, but I made sure to capture far more Light frames than I needed.
All of my frames suffered from severe Raining Noise as seen in the attached Wratten #12 stack. According to my calculations I needed a minimum of 22-second exposures to mitigate the Raining Noise problem, but that would have negated the "lucky imaging" objective. Thankfully I was able post-process it out using Astro Pixel Processor (APP). Tim can get away with 3-second exposures due to the fact that he has a substantially faster scope than mine, plus he suffers from more light pollution.
I captured a total of 3600 frames but kept only the best 2400:
W12: 1200x3s, R: 600x6s, G: 600x6s. Total integration time: 3 hours
I was very fortunate that none of the stars in the image is saturated, so there is no harsh over-exposure of the central core of the cluster. In this image you are seeing stars as faint as 17th magnitude, possibly 18th magnitude. My star catalog only goes down to 16th magnitude so I am only estimating how deep you are seeing.
Brian
The final image, zoomed in and cropped: Example of Raining Noise: