skyglow and imaging

[DJDD]

Hi,

[warning, screwy questions below ]

I have been following the thread "??????Increasing ED80 magnification????????", http://www.iceinspace.com.au/forum/s...ad.php?t=28833 ,
with interest.


If I had a choice between:
- skywatcher ED80, f/7.5
- orion ED102 f/7
- skywatcher ED100 f/9


What difference would the 900mm versus 700mm versus 600mm focal lengths have on the image, i.e. what difference in magnification would i see on the image? I would be using a DSLR at prime focus.

Should I be worried about the extra exposure time required for the skywatcher ED100 f/9 in light-polluted Melbourne skies?

Given the extra exposure time required for the f/9 ED100 over the f/7 ED102 telescopes, what effect would the skyglow, being an extended object, have on the final image? I am sure there would be differences between faint objects and stars. Also, LPS filters must make a difference. If I did not care about magnification of the image should I be worried about exposure time?


Would I gain much benefit in Melbourne (or in dark skies) by getting the Orion ED102 over the ED80? The magnification would be similar (700mm fl versus 600mm fl). In this case, will skyglow obscure the objects to be photographed given the ED102 has about 1.6 times the light-gathering power?



this would seem more coherent if I was speaking with someone...
this forum is great but sometimes voice communication would be easier. especially if the questions are screwy!



cheers,
DJDD

[Zuts]

Hi DJDD,

For planetary imaging people would go for a long FL. The longer the better. FL is not enough though and to get good resolution you need large aperture. This is why a bigger scope will give you a better view of a planet than a smaller scope. For a planet which is bright, light gathering is not a problem. A really great planetary imager would be a F20 12 inch Dal Kirkham.

Since you are talking refractors I can assume you are talking about DSO imaging. In this case I would always go for the biggest aperture with the lowest F ratio so in your case this would be the ED 102. Please note i am not talking about the optical or mechanical quality of the scopes as I dont know much about these brands.

As far as skyglow is concerned I dont think it is really relevant in your case. Any of the scopes you mentioned will under melbourne skies reach the skyglow limit after a few minutes. In Sydney I can only go for about two or three minutes before skyglow washes out everything. So i always image with a filter. In my case I use an Astronomic UHC filter.

There is a recent thread

http://www.iceinspace.com.au/forum/s...ad.php?t=28782

discussing the difference between other types of LPS/UHC filters.

To see the effect of FL on field of view (magnification) try this

http://www.newastro.com/wodaski/pick_a_camera.htm


Paul

[DJDD]

Thanks for the reply, Paul!
sorry it is a bit delayed. heh, sorry because with a post every three minutes i should have replied sooner!

Thanks for your clear info, as well.

anyway, i was looking at imaging DSO's and because I have not yet bought the gear I wanted to ensure I got the best equipment I can afford at the moment. I also like the idea of a fast telescope with larger focal length for imaging but as I would have to move the gear in & out of the house for every session it becomes impractical.

I have also been studying the Doublet vs Triplet debate, as well as FPL-53 versus FPL-51 ED glass debates. All very interesting.

maybe I should just go and buy the ED80 and be done with it!


thanks, again,
DJDD