New coma corrector!

I’ve been battling some odd colour fringes around stars with my photos, and I figure that the only part of the optical train that might be causing this is the Bintel coma corrector. I noticed that the lens has a slight rattle to it and that the locking nut was ever so slightly loose.

I decided to pick up a Baadar MPCC for several reasons – it seems quite popular for astrophotography, it is much lighter than the Bintel CC, and it is decently priced.

MPCC

Quite a significant drop in weight! It also sits flush against the focus tube, meaning I don’t have to worry about getting the right distance as much as I did with the Bintel CC. And since it sits closer, the weight of the camera has less of an effect on the overall balance of the telescope.

Some test shots show that coma is greatly reduced around the edges of the image (better than with the Bintel CC), and although it’s a bit early to tell, the colour fringing appears to have been eliminated.

NGC1365 – Great Barred Spiral Galaxy

ngc1365_final

Twenty-five 5-minute light frames, calibrated, registered, stacked, and post-processed using PixInsight. I was having a bit of trouble before this shot, my stars had colour fringes caused by the coma corrector. I have started performing my collimation with the coma corrector in place, which seems to have reduced this problem significantly. I think it’s just because the coma corrector isn’t as snug a fit as it should be in the focuser tube.

  • Date taken: 27/11/2014
  • Location: Adelaide, South Australia
  • Camera: Canon 450d w/ IR filter removed, GSO coma corrector
  • ISO800
  • Mount: HEQ5PRO
  • Scope: GSO 8″ f/5 Newtonian
  • Autoguider: Orion Starshoot AG
  • Imaging: BackyardEOS w/ PHD dithering
  • Guiding: PHD2
  • 50 bias frames, 20 dark frames, 50 flat frames.
  • Total integration time: 125 minutes

Pre-Processing details:

  • BatchPreProcessing, calibration only
  • BatchDebayer
  • Star Alignment
  • Image integration (using Winsorized Sigma Clipping)
  • Dynamic Background extraction, twice, first time to remove the majority of the light pollution, then a second run to reduce some of the gradients from nearby light sources.
  • Dynamic Crop to remove some of the dodgy edges, also centering the image on NGC1365
  • Extracted luminance from the RGB for later on

Processing the RGB Image:

  • BackgroundNeutralization
  • ColourCalibration
  • ATrousWaveletTransform to reduce noise
  • HistogramTransformation to reveal the galaxy etc
  • ACDNR to further improve noise
  • ATrousWaveletTransform a few more times with various masks to reduce noise and even out the background
  • Blurred the RGB image with ATrousWaveletTransform by removing all the detail from the first 4 layers!

Processing the luminance image:

  • Much the same as before, ATrousWaveletTransform to reduce noise
  • HistogramTransformation to stretch the image
  • More ATrousWaveletTransform, this time using a mask generated from a cloned image, HistogramTransformation used to auto clip highlights & shadows
  • Same mask, ACDNR.
  • Bit more HistogramTransformation now that there is a lot less noise
  • Pixel math: star_mask+range_mask, masking everything but the stars and galaxy
  • LocalHistogramEqualization to bring out some more detail in the galaxy
  • Tiny bit of bias in ATrousWaveletTransform to sharpen up the galaxy

Putting the images together:

  • LRGBCombination!
  • Saturation increased a tad, 0.350 or so, but otherwise just applied L to RGB
  • CurvesTransformation to tweak the saturation and luminance a tad, and increase contrast

Star field around NGC6556

starry-background.jpg

This is the first shot taken using my new coma corrector + adaptors! There is still some distortion around the outer edges of the image, but it’s a much flatter field compared to before.

This is a stack of twenty 5 minute exposures, but no dark/flat/bias frames were used. I have to take new flats since I’m now using the coma corrector, and I haven’t been able to take any darks at a cool enough temperature yet.

  • Date: 05/06/2014
  • Location: Adelaide, South Australia
  • Camera: Canon 450d, modded with IR filter removed
  • Mount: HEQ5PRO
  • Scope: GSO 8″ f/5 Newtonian
  • Autoguider: Orion Starshoot AG

Fun with coma correctors…

One thing I started to notice in photographs taken through my GSO 8″ f/5 newtonian was the stars towards the edge of the field were stretched out a reasonable amount. This is called comatic aberration or coma for short. I knew this would happen, but I decided I would try imaging without any corrective optics to begin with.

Eventually I decided that it was time to fix the comatic aberration in my images by using a coma corrector. I don’t like to spend money when it isn’t needed, so I figured that buying a cheap and cheerful coma corrector from Bintel would be the best idea. It says in the product description that it has a 2″ barrel, and it can be used with a DSLR. Perfect I thought! When I received the corrector, and tried to use it, I discovered things weren’t quite that easy.

Bintel Coma Corrector

Now, it’s really easy to make this mistake. If you look at the product description, it tells you that you can use a T-ring by unscrewing the 2″ barrel and screwing on a “wide” T-ring. Now here’s what tripped me up – I don’t have a “wide” T-ring. This appears to be some sort of “special” T-ring from Orion (and maybe others?) that is 48mm, versus the standard 42mm for a T-ring. I think the idea behind this size increase is full-size sensors in much more expensive DSLRs tend to experience an amount of vignetting when you use a 42mm T-ring. I’m running a 450d which is a crop sensor camera, and I don’t see any significant vignetting.

I was playing around with my current T-ring to 2″ barrel adaptor, and discovered that the barrel has a 48mm thread inside it for filters. It turns out this is the exact same thread that the coma corrector uses, and it screwed right in! I enthusiastically set up my telescope and tried to get it to focus. I failed, until I moved the combination of camera, tube and coma corrector out fairly far and racked the focus right out. Not good. I thought maybe it was because the distance between the lens and sensor was wrong (which it is), so I thought I could just get some spacers to correct this.

Back to Bintel – I found this product:
Orion T-Thread Spacer Ring Kit

This looks like it’ll do the job, but it’s for T-thread (42mm) which just isn’t going to help. The lens and sensor are already too far apart so this will just make the problem worse! I checked the Bintel site (and several other Australian telescope suppliers) but I couldn’t find a product to adapt a 42mm t-ring to the 48mm thread. I contacted Bintel about this problem, and it turns out they have a solution. They have a M48 to M42 step down ring for this exact problem. This, combined with the expensive T-thread spacers should allow the camera to mate up with the coma corrector at the right distance.

Fast forward a couple of days, the parts arrived. $50 for a few bits of metal that would hopefully make life easier. I tried several combinations of the spacers with the M42-M48 adaptor connected to the coma corrector, and I have found a combination that gives me roughly 70mm spacing. According to several sources on the Internet, this is the right amount of spacing to correct any comatic aberration in my f/5 newtonian.

As is always the case with these things, there hasn’t been a clear night since I received the spacers and adaptor so I haven’t had the chance to test this gear out. It’s looking likely that in the next couple of days there might be a few hours of clear skies during the night. I will attempt to get some before and after shots as soon as I can.