SCT in astrophotography - some thoughts

[riklaunim]

Since some time I'm using 8” Celestron SCT made in 1983 and is mounted on HEQ5 SynScan. The OTA is quite light (lighter than 150/750 SW Newtonian I had), but when you add some accessories it gets noticeably heavier. For planetary imaging it needs Cryford focuser or Cryford motofocus – which adds extra cost and weight. The OTA cools quickly and due to small size is quite stable on the mount, resistant to smaller wind buffets.

The OTA is old, however there are no problems with the optics or the mechanics. Nowadays if someone wants something catadioptric for DS imaging can choose from standard SCTs, slightly more expensive GSO RCs or extremely expensive ACF/HD/EDGE SCT or also Klevcow design (Vixen, TAL). I find standard SCT to be very good, it meets my needs.

For astrophotography I use:

f/6.3 Hirsch reducer

http://www.nauka.rk.edu.pl/site_media/resources/nauka.rk.edu.pl/images/reduktor.jpg

Bought this second hand. It should be a Celestron reducer clone. What I've noticed is that it adds some chromatic aberration, which is quite annoying when parfocal RGB filters aren't parfocal any more. The optimal range is stated as 105-110 mm and I'm somewhere close to that (few mm more or less depending how I would mount every pieces together). From replies I've got all SCT type reducers add some slight amount of chromatic aberration. With the f/6.3 reducer I have 1260 mm of focal length, which is a lot.

Simple filter threaded reducer doesn't add aberration (it's close to CCD) but it doesn't correct the field, and using this one would make radial guider unfocusable.

Celestron Radial Guider

http://www.nauka.rk.edu.pl/site_media/resources/nauka.rk.edu.pl/images/radial_guider.jpg

It was designed to guide analog cameras with 12 mm eyepieces. Sometimes it's priced low – and I got one of cheap ones. It's also modded by TS. On the telescope side it has SCT thread, and on the opposite side it has a T thread. Guide head has 1,25” eyepiece holder. You can move the angle of the prism, and move the whole head along the guider axis.

I'm using DMK21 to guide. It needs a short C/CS nosepiece to fit in the guide head. I also had to remove a spacer from the guide head so the prism would collect max possible light for it and of better quality (less elongated stars). The problem with Radial guider is to bring the guide cam to focus. When I have about 115 mm CCD to reducer distance it focus nearly at lowest cam position. Modded guiders by TS doesn't have the eyepiece holder but some T threads so guide cam can be even lower and focus for smaller distances reducer – CCD. It does work for me. Sometimes I have bright guide star at 0,1 - 0,2 sec exposures.

5 x 1,25" BS Astro manual filter wheel

http://www.nauka.rk.edu.pl/site_media/resources/nauka.rk.edu.pl/images/kolo.jpg

They made Atik wheels some time ago, and this one looks similar to Atik wheel. It's one of cheapest wheels and it comes with T threads on both ends, 1,25” T threaded eyepiece holder and nosepiece as well as T-ring with T-blocking ring for mounting a T-threaded camera. The T threaded on those items are short and doesn't go into the whee, so Baader or Orion filters aren't blocked. However standard T threads like in Radial Guider are longer and they will hit the high profile filters (I screw the wheel to the guider with filters opposite side to the guider.). The filter numbers on the wheel could glow in the darkness.

LRGB Orion filters and other

http://www.nauka.rk.edu.pl/site_media/resources/nauka.rk.edu.pl/images/orion_lrgb1.jpg

True RGB interference filters. Got them second hand much cheaper than new ones (LRGB loose on resale ). IR/UV blocked, parfocal. Also I have Castell O-III and Baader UHC-S. For planetary also Astronomik Pro Planet 742 and 807. They work very well. Didn't had any issues with them.

Bahtinov Mask

http://www.nauka.rk.edu.pl/site_media/resources/nauka.rk.edu.pl/images/bahtinov2.jpg

Farpoint Astro mask for SCT 8”. Makes focusing for DS imaging much easier.

DSI III Pro

http://www.nauka.rk.edu.pl/site_media/resources/nauka.rk.edu.pl/images/dsi-adapter.jpg

Mono camera with removed filter rail and added T/1,25” threads plate. It gives slightly to high resolution at bin 1, but bin 2 looks much better (bigger pixels or stronger reduction would be better).

JMI Motofocus

I'm using it for planetary imaging. Tuning focus without moving the mirror.

Planetary astrophotography

SCT does the job really well. JMI motofocus, filter wheel, and barlows (TS/GSO 1.5x/2x or 2.8x UO Klee Barlow). OTA holds collimation really well, and is stable.

DS imaging

I had mixed results with DS imaging, but I think I need more images to rate it better. Nebulas like M27 look really nice, but I do have problems with stars and their colors. The aberration added by the reducer is probably adding to the problem.

I'm also planning to use the big focal length to chase some small planetary nebulae with many short (few seconds) exposures (like on Astronominsk ). But looking at my results to my skills and the equipment type I say that results are very good

Future plans

- hunt for small planetary nebulae without reducer

- get nice star colors and shapes

- add a small ED piggyback (66-72 mm) for wide-field imaging, mega-finder and sometimes maybe guide

- upgrade DMK21 to newer ICX618 based camera (QHY or Basler) and then maybe think about some advanced filters like methane filter.