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Old 14-12-2020, 03:29 PM
kalon (Kevin)
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Join Date: Aug 2018
Location: Sydney
Posts: 46
My thoughts on the 8" RC - for beginners

I consider myself an amateur, but here's my astro journey so far: I started with a mirrorless camera (Fujifilm X-Pro1) and stock lenses (35mm and 12mm) on a tripod, to a tracking mount (SkyWatcher HEQ5 Pro), to a quintuplet apo refractor scope (Askar 400mm f/5.6), to a dedicated monochrome camera (ZWO 1600MM) and LRGB and Antlia 3.5nm filters.


I wanted to step up to my first "big boy" scope and increase my focal length without selling a kidney for a bigger refractor, or needing a new mount. The only thing that fit my payload capacity then was a Ritchey-Chretien 8" f/8 carbon fibre tube (for weight, but wow does it look schmexy) which would jump me up from 400mm to 1600mm focal length.


  • I read so many articles on how terrible it was for amateurs due to complexities and differences to a refractor
  • I was told that backfocus would require 3-decimal-point precision calculations for back focus, and the requisite adaptors to find it
  • I was dissuaded because of how slow imaging was compared to faster scopes
  • I read countless horror stories of never finding focus due to back spacing...
  • ...or never getting a useful image out of it due to collimation errors
  • I freaked myself out with YouTube collimating tutorials involving 38 steps
  • I was warned that I would never guide, because I would be off-axis guiding for the first time.
  • I was told that the focusing and collimation would be brief at best as constant corrections are needed due to tube flexing



All of these worries went out the window when I had my first night out with it.

I found focus! Whilst it was completely different to the daytime focus estimation I did, I found focus relatively quickly. I got my OAG focused to a decent-enough level that I could guide. And I even got images!


Were they perfectly in focus? No. Odd shaped stars? Slightly? Did I care? HELLZ NAW!



I have since carried out secondary mirror rough collimation using the "fuzzy donut" method - basically to eyeball the mirror occlusion to where the centre of an out of focus star is. My HFR went from 4 to 3, and my stars are round. There's room for improvement, and yes, I have a Howie Glatter laser/holographic collimator on order, but it isn't really needed. I do note that my sensor size is bigger than the "flat" field selling point for RCs, but I also have a focal reducer and flattener to put on next time the clouds clear.



Overall: I have a focal length I am in love with, reusing 50%+ of my existing equiment. I spent a quarter of what I calculated for the "next full step up" upgrade, and I couldn't be happier.
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  #2  
Old 23-12-2020, 01:47 AM
Lookingout (Bob)
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Join Date: Dec 2020
Location: australia
Posts: 14
Quote:
Originally Posted by kalon View Post
I consider myself an amateur, but here's my astro journey so far: I started with a mirrorless camera (Fujifilm X-Pro1) and stock lenses (35mm and 12mm) on a tripod, to a tracking mount (SkyWatcher HEQ5 Pro), to a quintuplet apo refractor scope (Askar 400mm f/5.6), to a dedicated monochrome camera (ZWO 1600MM) and LRGB and Antlia 3.5nm filters.


I wanted to step up to my first "big boy" scope and increase my focal length without selling a kidney for a bigger refractor, or needing a new mount. The only thing that fit my payload capacity then was a Ritchey-Chretien 8" f/8 carbon fibre tube (for weight, but wow does it look schmexy) which would jump me up from 400mm to 1600mm focal length.


  • I read so many articles on how terrible it was for amateurs due to complexities and differences to a refractor
  • I was told that backfocus would require 3-decimal-point precision calculations for back focus, and the requisite adaptors to find it
  • I was dissuaded because of how slow imaging was compared to faster scopes
  • I read countless horror stories of never finding focus due to back spacing...
  • ...or never getting a useful image out of it due to collimation errors
  • I freaked myself out with YouTube collimating tutorials involving 38 steps
  • I was warned that I would never guide, because I would be off-axis guiding for the first time.
  • I was told that the focusing and collimation would be brief at best as constant corrections are needed due to tube flexing



All of these worries went out the window when I had my first night out with it.

I found focus! Whilst it was completely different to the daytime focus estimation I did, I found focus relatively quickly. I got my OAG focused to a decent-enough level that I could guide. And I even got images!


Were they perfectly in focus? No. Odd shaped stars? Slightly? Did I care? HELLZ NAW!



I have since carried out secondary mirror rough collimation using the "fuzzy donut" method - basically to eyeball the mirror occlusion to where the centre of an out of focus star is. My HFR went from 4 to 3, and my stars are round. There's room for improvement, and yes, I have a Howie Glatter laser/holographic collimator on order, but it isn't really needed. I do note that my sensor size is bigger than the "flat" field selling point for RCs, but I also have a focal reducer and flattener to put on next time the clouds clear.



Overall: I have a focal length I am in love with, reusing 50%+ of my existing equiment. I spent a quarter of what I calculated for the "next full step up" upgrade, and I couldn't be happier.
Sounds like you have a good setup Kevin good luck if only we can get a clear sky.
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  #3  
Old 05-01-2021, 08:09 PM
kalon (Kevin)
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Join Date: Aug 2018
Location: Sydney
Posts: 46
I still love my RC 8".... however:
Not every photo will be perfect. Not every setup will be smooth... and then there’s this shot, the result of the adage “anything that can go wrong, will go wrong”. If you have children present or a weak disposition, abandon hope all ye that enter here:



  • I set up all my gear on a perfectly level, open netball court.
  • I then had to relocate everything because my battery box had mysteriously drained to 11.3V and I was forced to run from a 3-port cigarette adaptor from my car battery.
  • I needed to periodically turn on my ignition to stop the warning from my car that my battery wouldn’t be able to start.
  • I started a quick collimation to correct a small amount of oblong stars, I dropped the Allen key *in to* my RC. (No damage to mirror, thank The Flying Spaghetti Monster!)
  • When I took off my OTA to upend it to retrieve it, I failed to turn off my mount and remove the counterweights so it freaked out.
  • When I finally got to “Collimation Round 2: Optical Boogaloo”, I chased it for 45 minutes and ended up back where I started and no better collimation.
  • Because of the oblong stars, my HFR refused to drop below 7, I have previously repeatedly got ~3.5, so I know I was badly affected.
  • I had recently moved my prism in my off-axis guider to fix a small sensor occlusion, and the resulting focus change meant that I had to change adaptors - which feel in to either category: “too short” or “too long” and I had to use it held on by the very top of the brass securing collar, meaning my guide stars were “oddly shaped” and thus kept getting lost due to SNR changes.
  • NINA stubbornly refused to autocomplete the object I was imaging for coordinates and slewed to some random spot in the sky.
Icing on the cake: it was a full moon. I only started imaging at 10:45pm because of the above problems.



Cherry on the top: I had driven 6h from Sydney to Condobolin THAT DAY to find the only clear skies in the state. I ended up with just 64 minutes 8x 120” Lum/R/G/B before pulling the plug due to sheer frustration.
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