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Post by Deleted on Dec 23, 2014 16:05:35 GMT
Was wondering about this. On a small DSO can a large aperture scope, say an 18" work as well at a longer FL than a 4" scope using the same camera due to collecting more light? Seems a 3" works best around 300-400mm. What is best for the 18"? 1200mm? Or an 8" , 800mm? I understand resolution will be better in the larger scope, I am thinking about object brightness only with say a 15 second exposure in each case. Not taking into account FOV which gets smaller with increased FL. I guess I am trying to determine resolution verses brightness. Is there a sweet spot for each aperture to give pleasing views? Do you need a better camera (larger chip) the longer the focal length at a given aperture? Or am I thinking all wrong about this?
What would be interesting is seeing pictures using same camera , same exposure time on different scopes at same focal ratio. Say f4. Same object. Would I find that a lower end camera would work well on all or just the smaller 3"-5" scopes?
Thanks,
Richard
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Post by Deleted on Dec 23, 2014 18:34:08 GMT
im rather new at all the lengths myself richard but id be thinking at a shorter focal length you would achieve more field of view say wide field and as with the longer focal length at 8" would put you closer on the target and less fov also the longer length giving you lower tracking times than a 300mm id be thinking the camera of choice would achieve good results in both focal lengths just a different with one being wider than the other ? i may be of the mark my self with my idea of it and a pro may come along and give you better information but well as i think im going to do experiment and enjoy it si
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Post by Deleted on Dec 24, 2014 2:39:31 GMT
The common way to compare different aperture scopes is by focal ratio.
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Post by Dragon Man on Dec 24, 2014 3:33:18 GMT
Aperture and focal length do play a major part. I don't have a collection of comparison photos as I've never though to do it, but I do have one which does show the difference in two Refractors that aren't very different from each other. It shows the difference in FOV between a 150mm (6 inch) f/5 Achromat and an 80mm f/7.5 ED80. Our minds tell us that they should be about the same FOV because although the ED80 is smaller it has a longer focal length because it is f/7.5 compared to the 150mm's focal ratio of f/5 so they should sort of match each other. But the evidence shows different. It is a great example of how much effect Foal LENGTH has over Focal RATIO. Notice how even though the 150mm is only f/5 compared the the ED80's f/7.5 the 150mm FOV is still way smaller. This is why I keep mentioning the most important factor in Video Astronomy is Focal LENGTH, not ratio.
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Post by Deleted on Dec 24, 2014 6:34:21 GMT
Ken, your ED80 amazes me! Got a new camera did you? What do you think so far? How long exposure on that shot? I understand about focal length, but thinking aperture verses camera quality.(along with how long it will expose for).
Richard
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Post by Deleted on Dec 24, 2014 7:14:56 GMT
Was wondering about this. On a small DSO can a large aperture scope, say an 18" work as well at a longer FL than a 4" scope using the same camera due to collecting more light? Seems a 3" works best around 300-400mm. What is best for the 18"? 1200mm? Or an 8" , 800mm? I understand resolution will be better in the larger scope, I am thinking about object brightness only with say a 15 second exposure in each case. Not taking into account FOV which gets smaller with increased FL. I guess I am trying to determine resolution verses brightness. Is there a sweet spot for each aperture to give pleasing views? Do you need a better camera (larger chip) the longer the focal length at a given aperture? Or am I thinking all wrong about this? What would be interesting is seeing pictures using same camera , same exposure time on different scopes at same focal ratio. Say f4. Same object. Would I find that a lower end camera would work well on all or just the smaller 3"-5" scopes? Thanks, Richard Hi Richard,
Basically two telescopes at f/4 will show images of the same brightness, the difference will be the 80 mm image will be very small compared to say a 12" scope. Resolution will also improve as the aperture size increases. Basic lens optics.
Regards,
Shevill
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Post by Dragon Man on Dec 24, 2014 9:01:46 GMT
Ken, your ED80 amazes me! Got a new camera did you? What do you think so far? How long exposure on that shot? I understand about focal length, but thinking aperture verses camera quality.(along with how long it will expose for). Richard Thanks Richard, yes the ED80 scopes are amazing. I used the same scope when I was doing Astrophotography. No, it's not a new camera. Had it for a few years now, but it was new back then when I used it for these views What do I think? I think it is spectacular. No regrets on getting it at all. I honestly can't remember the integration times on those images. At a rough go at my memory I'd say the 6" Achro was about 30 seconds, the ED80 about 40 seconds. I tend to get a lot of Amp Glow and noise in my Video Screen Grabs because I run AGC at the maximum level of 8. I can get away with it because of my dark skies. If I had some light pollution I would probably dial it down to about AGC 4, but I don't have any. I have never actually done Astrophotography or Video Astronomy away from dark sky.
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Post by Deleted on Dec 24, 2014 18:23:45 GMT
Shevill,
Thanks, you confirmed what I was thinking. So am I right in thinking that a lower end camera's which work well on small fast scopes are not going to be very useful on medium to large scopes without massive focal reduction (if possible)due to fov issues and exposure limitations? Watching NSN a lot and I am seeing a pattern. Seems once you get near 1000mm fl or over a higher end camera is needed (longer exposures and fov)for DSO to get really good results?
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Post by Deleted on Dec 24, 2014 18:33:04 GMT
I have never actually done Astrophotography or Video Astronomy away from dark sky. What are you, 75 miles west of Melbourne? You are spoiled! So do you ever really need/want to use anything else? (Camera wise).
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Post by Dragon Man on Dec 25, 2014 5:04:07 GMT
I have never actually done Astrophotography or Video Astronomy away from dark sky. What are you, 75 miles west of Melbourne? You are spoiled! So do you ever really need/want to use anything else? (Camera wise). Yes Richard, way out west in the state of Victoria in a tiny country town with zero light pollution. As far as wanting/needing another camera, I have the camera I wanted. My Imaging started with a Phillips ToUcam Webcam about 11 years ago. I have 2 versions. One standard and one modified for long exposure so I could do DSO's. Then after doing amazing results with them for about 2 years I realised I needed a better camera so I purchased a Canon 350d DSLR. I also had a Meade DSI-c and a couple of cheapie Planetary cameras, and I was given a second-hand Starlight Express MX-5c. I preferred using the DSLR so I carried on with that for a few more years but then I discovered Video I bought a Samsung SCC-A2333 (SCB-4000) until I could get the Mallincam Xtreme I have now. If anything, I would love a better scope. One of these: (droool) www.astronomyalive.com.au/catalog/product/gallery/id/515/image/2041/
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Post by Rick in NWArk on Dec 31, 2014 18:04:35 GMT
The image scale (the size of the object - and whether it will fit on the chip) is determined by the effective focal length of the telescope. Its calculation is a little more complicated than that for visual magnification (lens focal length divided by eyepiece focal length). I use the following website: www.12dstring.me.uk/fov.php to do my calculations for me. With that, I can play with all sorts of different scopes, barlow/FR combos, and even different cameras to see what different objects would look like on the monitor.
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