Thursday, November 12, 2015

Image artifacts: Astrophotography is touchy

Once I tied the camera to the back of my telescope and set up guiding I found plenty of new problems to solve:

I've boosted that image up a bit to make the flaws really obvious; this is a narrow view of the Pleiades. Surprisingly you can see some of the blue wisps of nebula around the stars (ignore the horseshoe shapes, those are artifacts)!

My scope setup is:
  • Celestron C6
  • Celestron f6.3 focal reducer/corrector
  • Standard Celestron SCT-T2 adapter
  • T2-FujiFX adapter
  • Fuji X-T1
Tracking is:
  • Really cheap Orion 9x50mm
  • Even cheaper Microsoft LifeCam with the filters and lenses removed
  • PHD2 giuding software
  • GPUSB-ST4 box
Everything that's wrong:
  • The orange-brown glow is my light polluted skies. We get nights that are clearer than that, but I think a filter would go a long way.
  • Stars in the center are nice and round, but the farther from center they are they are radially stretched. I believe this is a testament to how well autoguiding works. The system is locked on to the center star, but the alignment was off and so after a long exposure (300 seconds) the scope was not rotating exactly in tune with the skies. This makes the field seem to spin.
  • This thing that's going on:

    is vignetting; light is being blocked by the sides of the scope's center passthrough tube. This might mean the camera is the improper distance away from the focal reducer (I don't have this without a focal reducer, but then my field of view is crazy narrow). The outlet of the scope is also narrow, which will force me to put the camera at a not-so-optimal focal point.
  • This one is light reflecting off of the inside of the telescope:

    looks like a mix of the main body (very pale, I might not bother) and the primary mirror passthrough baffle. It's black but a bit glossy, and this is apparently a common complain for the C6. There's even a very light reflection inside the reflection going on for the brightest off-axis star, that's probably the T2 adapter tube.
The solutions are going to be a mix of things:
  • Better polar alignment when imaging
  • Light pollution filter of some kind. I'm guessing 90% of sources are sodium based.
  • Move the camera back from the scope until I don't have vignetting; this turns out to be pretty far back. I've heard the "correct" number quoted at either 85mm or 105mm, the first place I found it vignette free was close to 140mm between the corrector and image sensor plane.
  • Black out the primary tube with protostar stick-on flocking. This might never go away completely, apparently SCT style scopes often have this issue. Sadly I don't have access to that crazy science grade paint that blocks 99.99xx% of light. If you do give me a holler.
Overall I think this is going pretty well, but I'm sure I'll find new problems once I fix some of these...

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