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Hello all. Later this summer I plan to purchase a planetary setup (with the goal of deep space imaging later on - that is why I am going with the EDGE) and I want to get the right setup. I know about the pixel size x 5 rule. So for the 678MC it would be 2x5 = 10 which would match the F/10 ratio of the Edge 8. What happens when I throw a 2x Barlow on it for better magnification? Does it distort the image or something running at F/20? I am new to planetary so I don't really understand how the math matters. As I look at the images on astrobin the 678MC has a lot cleaner images than the 224MC with more detail (probably because the pixel size is so small). But I am wondering if the larger pixel size of the 224MC would be a better match if I want to use a Barlow. Any help would be appreciated. Scott |
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| Makes no difference. The main factor, apart from quality optics and collimation, is the seeing. In my very long experience I'd say the 224 fares at the top in terms of quality images produced. The 678 is a very good camera as well, at least on paper. Any quality barlow will not degrade the image at all. |
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Seeing is the main limiting factor. The formula you cited seems to be optimal for typical seeing conditions. If you use a barlow to go up to f/20 with 678 and your seeing does not support it, you will be introducing more noise for no additional benefit. But if your seeing is above average or exceptional, it may help your image achieve better details. The 678 is a convenient camera for SCTs because under most circumstances, you can use it natively and achieve the maximum details from your optics without a barlow for most seeing conditions. |
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| With the 678 there’s little to be gained resolution wise from a Barlow unless as Ashraf points out you have exceptional seeing. |
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| Then there is the even tiny pixel asi715mc. 1.45um |
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The good rule for sampling is resolving value of the telescope divided by 4 to 5. In the C8 case, 0.57/ 5 =0.114 arc-sec per pixel. This your maximum. 0.14 seems to be good for most use. Getting a lower sampling will result in over sampling and you'll only get blured images. For C11, we use 0.1 to 0.085. AND this is regarding of a good to very good seeing. The more the seeing is bad, the more you need to lower your resolution. Notice that this sampling value is to calculate WITH the use of a barlow. If you choose a camera pixel size calculated without barlow, the day you add a barlow in your train image, you'll be oversampling. I've used a ASI 178 MC with a C8, and its pixels size is too small or to use only a F10. Using it with a barlow will give you poor result. Instead, I'd pick up an ASI 662 MC (or a used 462 MC for a lower price) with their 3.75µ pixels. I use and used them both on C8 and C11. https://www.astrobin.com/full/16llsr/0/ |
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| I do have both and prefer running the Asi678 on my EdgeHD11 (despite oversampling) as I appreciate the slighty larger chip diagonal of 8.8 vs. 6mm. Not that I need the space for imaging, but it is handy ie. For collimating to have the star not jump too easily out of the image … |
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I went through the same dilemma before new years and got the 678mc as it perfectly matches the Edge 8 without needing a barlow. It's a relatively cheap camera but has high resolution and low noise. I usually do DSO imaging so I wanted to keep my planetary equipment light at first, so it seemed like the perfect fit for me. One thing to keep in mind though is that the camera doesn't seem to handle regular DSO very well. I know it's not the purpose of the camera but still something to keep in mind if you like to experiment. I haven't had the chance to use it for planetary yet as the planets are too low - but I did try a couple of different DSO targets with my Esprit 100ED (hence the remark about DSO's). |
