Sunday, April 28, 2019

Telescope vs. Telephoto Lens for Photography

I have a bunch of intro gear -- none of which are particularly well suited to high magnification solar/lunar astrophotography. On my fourth attempt, I finally observed a solar transit of the ISS.



Unfortunately I forgot my telescope lenses and other hardware, so I resorted to my kit telephoto lens for magnification which was not enough. I may not have nailed focus, unfortunately, but I think the main source of image degradation was the intense crop on highly compressed video frames that had their format converted multiple times. I'm talking camera sensor data → .MOV video → JPEG frames exported → stacked, cropped, and re-exported as JPEG with minimal additional compression.

I know it's possible to perform a solid crop on raw still photos from my work on the August 21, 2017 solar eclipse, but clearly the video capture workflow is not cutting it.:

My shot of the Great American Solar Eclipse of 2017
So let's compare my DSLR + kit telephoto lens with my beginner telescope-mounted DSLR setup.

Gear

Camera

  • Canon T5 / 1200D DSLR
  • Canon 75-300 f/4-5.6 kit telephoto lens

Telescope

  • Celestron AstroMaster 114 EQ Reflector Telescope
  • 10 mm / 100x magnification eyepiece
  • T-adapter to mount an EOS camera body

Test Images

I'm observing a 500' FM radio tower located about half a mile from my back yard.

Telescope + DLSR

The telescope is 1000 mm @ f/9. The camera exposure is 1/30th sec @ ISO 800. Image size: 3457 x 5194 px. The projected image circle completely covers the camera sensor; and the image is magnified/cropped significantly through the camera when the T-adapter is added. This isn't my area of expertise, but I'd guess it has to do with added distance of the T-adapter instead of merely looking through the eyepiece. Maybe this is similar to a Barlow lens effect. I'll be glad to be corrected.


DSLR + Telephoto Lens

300 mm @ f/9, 1/125 sec, ISO 100. 5194 x 3457 px. Shooting raw. Un-cropped, and tuned with RawTherape for exposure, contrast, and some sharpening filters.


Significant crop added (7% of the original image -- 922 x 1385 px). Hello to an osprey that I didn't notice until I started reviewing these images. Not bad despite being one of the cheapest kit telephoto option available with notoriously poor image quality.



Very very tight crop (0.2% of the original image -- 152 x 228 px) only to match the telescope size; which clearly crosses the line line of not enough pixels to be of any quality other than "bad."


Telescope photo again for comparison:

Analysis

Telescope Benefits

  • Enormous magnification potential compared to the telephoto lens that I'm getting very used to. This is nuts.
  • Potentialy better (but not stellar) image quality on highly highly magnified subjects
  • Despite the weight of the camera body and metal T-adapter, the focusing mechanism is quite solid and seems to be able to hold the mass without slipping. It's not necessarily a benefit so much as it is adequate.

Telescope Drawbacks

  • It's challenging to focus. It's a basic rig, and hand-operated focusing results in a lot of image shaking so it takes several seconds for the image to stabilize. I spent 5-10 minutes attempting to adjust and review focus and I'm not sure if I'm at the limit of optical performance, the plane of focus is just that thin, or if I was simply not accurate enough. It's probably a bit of all three.
  • Earth's rotation will be a big issue. At this level of magnification, the Earth's rotation will need a lot of assistance to mitigate; and without a motor to keep the equatorial mount running smoothly, it will be a hard fought balance to keep an image in place (i.e. image shake).
  • Quite dark objective. I had to drag the shutter out to 1/30th @ ISO 800.
  • The image is still soft no matter what the cause is

Telephoto Lens Benefits

  • Less time to set up
  • Less equipment to transport
  • Easier to find an object

Telephoto Lens Drawbacks

  • It's challenging to nail focus in Live View in bright sunlight
  • 300 mm is useful, but on this kit lens it takes significant experience to recover image quality in post production
  • 300 mm is just not 1000+ mm. Dang.

Conclusions

  • There's not much else I can do (even just for basic astro work) with a dark objective, but I'll try anyway because it's fun
  • Thankfully I have experience recovering drab raw images from the kit 300 mm lens so I'm used to the shortcomings of intro gear
  • There should be remarkable (okay, at least pretty good) improvement from an image that fills the sensor. Not relying on such a significant crop could be a huge benefit to capturing a transit pass; though the camera body itself has limits (3-5 fps max burst photo rate, etc.)
  • I'll try another ISS transit again with the telescope next time; but if I can't overcome the limitations of a 1/30th sec @ ISO 800 exposure chasing an object traveling at 4.76 miles per second, no amount of practice is going to compensate for the shot I'm looking for here via telescope
  • The DSLR may fit but it's very large on top of the scope. It's secure enough when pointing in more of an upward direction, but it's not ideal. I have a cell phone mounting bracket but that has it's own drawbacks too (clumsy, no remote trigger so more touching, etc.)
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