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Old 14-08-2015, 01:33 PM
johnnyt123 (John)
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Focal lengths for deep sky objects.

Hi all.

I have been out of the game for a while but want to get back into astro imaging..

Can anyone suggest what the best focal length and F-Ratio are for imaging various Deep sky objects and the recommended exposure times...
Does any know of such a guide that may exist..?
Also same question for planetary imaging.

My armamentarium is as follows:

Hep5 pro
WO Megres ED 110mm
GSO RC8
Celestron CPC 925
QHY5 -L
QHY8 single shot colour
QSI 683wsg-8
Imaging source DMK 41BF02

Any help or advise would be really appreciated.

Thanks

Regards

John
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Old 14-08-2015, 03:20 PM
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Slawomir (Suavi)
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Hi John,

My limited understanding tells me that you should be more concerned with imaging resolution in arc seconds per pixel rather than just with the focal length by itself. With HEQ5 I would stay above 2 arcseconds per pixel for DSO imaging (although it is possible to successfully image with this mount at higher resolutions). I would aim for 2.5-3 arcseconds per pixel to start with, as it puts less demands on your mount and thus makes imaging more enjoyable. You can always apply drizzle process to your data to increase resolution of the final image.

As for the F-ratio, the lower the number the faster the system and thus the shorter the subs, but there are implications in terms of increasing demands on proper optical alignment and colour correction (refractors) for faster F-ratios, not to mention taking into account vignetting and wavelength shift for filters making narrowband filters less effective at extremely low F-ratios.

Exposure times depend on many factors, but some software estimates optimal exposure times for given target taking into account your camera's characteristics, resolution etc. for example SGP gives such estimate after capturing every frame, allowing for adjusting and experimenting with exposure times. So perhaps your 110mm refractor with a reducer and QSI683 would make a nice imaging apparatus

Planetary imaging is out my my "comfort" zone..., so I won't comment on that.
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Old 14-08-2015, 07:23 PM
rally
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Why not download Ron Wodaskis CCD Calculator and have a play with that
You can select a bunch of different telescopes and cameras and get a good idea of the field of view using the various popular DSO targets he has in the programs library and then you can see for yourself what works best for each particular object.
You can see the full object and then a rectangular box around it representing the size of the image using your choice of equipment.

As can determine for yourself - there really isnt a one size fits all.
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Old 14-08-2015, 07:45 PM
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lazjen (Chris)
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This site: http://www.12dstring.me.uk/fovcalc.php# might help too for scopes, cameras, etc combos.
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Old 14-08-2015, 09:48 PM
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For planetary imaging, as long a focal length as your seeing will support. I imaged Jupiter with an 11" SCT at f/40 once. 11200mm focal length.

For deepsky. I've imaged everything from 300mm focal length through to 5600mm and f/ratios from f/2.8 through f/20. What is best? That's a matter of personal taste. Do you like wide fields? Narrow field? Something in the midrange?

Exposure times. Go with the longest you can manage with your mount and sky conditions. Some targets (m42 is a prime example) require long and short subs to properly expose.
At home, I run 5 minute exposures due to sky conditions. Under dark skies I run the longest exposures I can stomach loosing Usually 20~30 minutes. I don't like going longer because should something go wrong you don't want to lose a 1hr exposure.

Total exposure per target, this varies with sky conditions, object brightness, speed of the optical system etc. Anywhere from 1 to 150 hours depending on your goal with the image. The more the better is the general rule. I like to dedicate 5+hrs per image.


There are no real answers to these sorts of questions.
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Old 18-08-2015, 12:04 PM
johnnyt123 (John)
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Thank you all for your replies..

I tried deep space imaging last Friday night but unfortunately I had one of those nights where nothing was working, Usb devices not recognized, focuser slipping.....and when was ready to image, the clouds rolled in....

Will have to wait till next time.

Last edited by johnnyt123; 18-08-2015 at 10:55 PM.
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Old 18-08-2015, 01:58 PM
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We have all had those nights. I lost half a night over new moon weekend because I couldn't have the sky with eqmod controlling the mount with ccdsoft autoguiding at the same time. If I changed to cdc controlling the mount with eqmod everything worked fine. But I didn't have the catalogues I needed for the targets I wanted to image.
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