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winensky
2007-Sep-06, 12:02 AM
I am looking at purchasing a Meade LXD75 S-N 8" for intermediate level astrophotography. At 8" the aperature is right and the mount should handle that size scope. It is fast at f/4 but will I get enough magnification at prime focus with only a 840mm focal length?

Kind regards
Matt

Siguy
2007-Sep-06, 12:26 AM
The Meade LXD75 SN-8 is perfect for astrophotography, but the SN-10 is too heavy for the mount. You will get a fair amount of magnification through afocal imaging, but high magnification isn't needed at all for deep sky imaging, except for some very small objects. You should still be able to get about .6 arcseconds of resolution, enough to do well at high magnification.

RickJ
2007-Sep-06, 03:11 AM
It depends on what you are trying to do. There are two main branches of astrophotography with quite different requirements, Deep Sky and Planetary imaging. For Deep sky you match the pixel size of the camera's CCD to the focal length of the local seeing. Scope aperture has little say here as it is the atmosphere that determines your resolution over the long exposure. This is 2 to 3 seconds of arc unless you have unusual conditions. You want a CCD that will have at least 2 pixels across 2" of arc which would be 1 second pixels. The formula is: Pixel scale = 206,265 arcseconds x pixel size / focal length. Or solving for pixel size it is Pixel Size = Focal length x pixel scale / 206265 arcseconds. To reach full resolution assuming you want a 1.5" pixel to match 3" of arc average seeing you'd want a camera with a pixel size of about .006mm or 6 microns. Since 7.4 is about the smallest commonly found you'd be slightly under sampled. This isn't bad at all and would mean a larger field of view and a brighter image at the penalty of a very slight loss in maximum resolution. It would be a good wide field system for things like M31, open clusters, most emission nebula etc. It wouldn't be perfect but quite acceptable for small planetary nebula and most galaxies. I used such a slightly under sampled system and got excellent results. Another big advantage of under sampling is your guiding errors are reduced making it easier to get well guided shots right from the beginning. If your are new to deep sky work starting with a short focal length is highly recommended. I started at 600mm and am very glad I did!

More critical to deep sky work is the quality of the mount. Nothing can overcome a weak mount. Guiders can only correct for so much. Unguided a good mount would go 10 minutes at that focal length under sampled but an average one only one or 2 at most. I'd consider the mount with this scope the weakest part of it as a deep sky system, not the focal length. That's working in your favor with this mount. Having not used this scope for deep sky imaging (only visual) I don't know how curved the FOV is. If strong it would limit the size of the usable field. Others may be able to answer that question. When I imaged at F/4 with a newtonian I found a field flatener/coma corrector necessary for a 35mm field of view. I'm just not familiar with Schmidt Newtonian optics to know if it needed there too.

Planetary work though is an entirely different thing. There you won't be using prime focus. You will use barlow or eyepiece projection to get the image scale way up. Since the image is bright you can over sample like crazy without harm. You may have to just to keep the image from being over exposed. F/25 to F/50 is the goal here. Long term seeing is meaningless. Now it is the resolution of your scope in those instants of perfect seeing that are important as you'll take several thousand AVI frames and let the software find those with best focus and stack them for making the image. Lousy tracking is good here as it actually improves resolution! The same spot hits a slightly different mix of pixels allowing the software to actually improve final resolution over what it would be if the image never moved. So the mount isn't of great importance here, now it is the quality of the optics. The better they are the better your result will be. Focal length is immaterial as you just increase the projection distance to get the image scale you want. But high quality optics are very important. Aperture past 8" may help if your best seeing can support it but at 8" you can get results far better than major observatories could in the days of film and it's short and light putting less strain on the mount. Avoid afocal imaging (lens still in the camera) if at all possible. It just adds a complication you don't need and makes distortion free high resolution imaging more difficult. While you can survive afocal imaging for planetary work it can't be used for deep sky work.

Kyle Edwards
2007-Sep-13, 12:53 AM
I do not know much about deep sky, but that scope would definately be great for planetary. To do much high resolution planetary work F/4, you will need a 3x or 4x barlow.