Quote:
Originally Posted by Paul Haese
Just some thoughts here on long focal length imaging.
Guiding and PA require greater precision with longer focal length imaging. Errors related to that become very obvious at longer focal lengths.
Focus often is the motto. I generally have my systems refocus (automated) every 60 minutes, no matter what size scope they are. Focus with steel tubes changes rapidly during the first few hours of imaging even if you give it an hour or so before commencing imaging. Even carbon systems will contract a little and on critical focus zoned instruments you get noticeable focus shift.
SCT's are best used for planetary imaging but can produce very good results imaging deep sky objects but I would rather use an RC, CDK or Newtonian for the task.
Collimation on an SCT is more forgiving but still needs to be controlled well to get excellent results.
Using a dew heater on an SCT can and does cause all sorts of issues with tube currents. All the planetary imaging I have done over the years, I have never used a dew heater, I use a hair dryer. That is not practical when doing DSO work.
Would I get a 9.25 or 8" SCT for DSO imaging? Hmm, nope. There are better tools to be obtained. A quality Newtonian with reasonable focal length will give you a good compromise between long and short focal length imaging. However if you really want to do galaxy work I would recommend an RC or a CDK. Using off axis guiding is a necessity too. You don't get any issues that come with SCT's. Dedicated tools for the job.
Best of luck with your hunt.
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Hi Paul...Sorry for such a late reply but I thought I had replied...one of those posts that I must have hit "previous thread" ...
Thank you very much for your post I greatly value your opinion.
I am setting up my eight inch with its own camera, filters etc and to hear your comments makes me very happy as it appears I have made a good choice.
Alex