· Get a Cloudy Nights T-Shirt · Submit a Review / Article    Click here if you are having trouble logging into the forums Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu.... uh, User   You are not logged in. [Login] Forum Archives · Entrance · Main Index · Search · New user · Who's Online FAQ · Calendar Astrophotography and Sketching >> Beginning and Intermediate Imaging Previous   Index   Next   Flat   Nocturnal Pooh-Bah Reged: 09/14/05 Posts: 1174 Loc: CT, USA Re: F-Ratio Myth (And You)       05/05/09 09:05 AM Edit   Reply   Quote   Quick Reply:   Comparing scopes with different focal lengths is difficult and gets misused in some cases. Look at advertisements for the hyperstar for example. It says exposure lengths are vastly reduced at F/2 compared to native F/10. The image scale is completely different as well so it means, IMO, nothing. To push it into the ridiculous you could cram all the light of M42 into 1 pixel and saturate it in a fraction of a second with the right optics. But you can't see any detail. Pointless. My take on the whole thing is that you decide what image scale you want and then get the largest clear aperture you can afford or carry on your mount. Image scale depends on focal length and pixel size. Clear aperture depends on actual aperture and the central obstruction. Twice the clear aperture (surface wise) at the same focal length will deliver twice the photons at the same image scale. *That* you can compare. *That's* why you don't see 66mm refractors mounted in those big Keck domes DSLRs make things more complicated because of the ISO settings. Still the physics (photons per pixel) are the same. How they translate into ADUs and noise gives me a headache so I won't attempt to understand it. CCDs are easy comparatively. -------------------- Best, Sander C11-HyperStar on Atlas EQ-Q driven by EQMOD William Optics M110 With FR-III/TRF-2008 DS2090 guide scope QHY-8, DSI-Pro and DSI cameras watec 802h video camera with KIWI OSD Astro stuff: http://www.tungstentech.com My Astro Photos Post Extras       Entire thread Subject Posted by Posted on F-Ratio Myth (And You) Kolenka 05/05/09 03:15 AM Re: F-Ratio Myth (And You) DonR   05/05/09 11:47 PM Re: F-Ratio Myth (And You) lernerda   06/05/09 09:57 AM Re: F-Ratio Myth (And You) freestar8n   06/05/09 11:23 AM Re: F-Ratio Myth (And You) Jimmy2K63   06/09/09 02:05 PM Re: F-Ratio Myth (And You) freestar8n   06/09/09 03:26 PM Re: F-Ratio Myth (And You) freestar8n   06/09/09 05:17 PM Re: F-Ratio Myth (And You) WarrenS   06/09/09 06:06 PM Re: F-Ratio Myth (And You) Jared   06/09/09 07:20 PM Re: F-Ratio Myth (And You) Jimmy2K63   06/09/09 10:10 PM Re: F-Ratio Myth (And You) NigelM   06/10/09 04:59 AM Re: F-Ratio Myth (And You) freestar8n   06/10/09 08:28 AM Re: F-Ratio Myth (And You) Jared   06/10/09 01:10 PM Re: F-Ratio Myth (And You) freestar8n   06/10/09 01:33 PM Re: F-Ratio Myth (And You) Jared   06/10/09 04:09 PM Re: F-Ratio Myth (And You) freestar8n   06/10/09 08:04 PM Re: F-Ratio Myth (And You) Jared   06/10/09 10:59 PM Re: F-Ratio Myth (And You) freestar8n   06/11/09 08:52 AM Re: F-Ratio Myth (And You) Jared   06/11/09 09:37 AM Re: F-Ratio Myth (And You) Galaxyhunter   06/11/09 01:16 PM Re: F-Ratio Myth (And You) DaemonGPF   06/12/09 09:22 AM Re: F-Ratio Myth (And You) freestar8n   06/12/09 11:40 AM Re: F-Ratio Myth (And You) Arkalius   06/13/09 03:56 AM Re: F-Ratio Myth (And You) freestar8n   06/13/09 09:18 AM Re: F-Ratio Myth (And You) Galaxyhunter   06/13/09 11:28 AM Re: F-Ratio Myth (And You) Arkalius   06/17/09 01:16 AM Re: F-Ratio Myth (And You) Jimmy2K63   09/23/09 09:43 AM Re: F-Ratio Myth (And You) saadabbasi   09/24/09 11:41 AM Re: F-Ratio Myth (And You) russ_watters   09/25/09 07:23 PM Re: F-Ratio Myth (And You) freestar8n   06/13/09 02:40 PM Re: F-Ratio Myth (And You) Galaxyhunter   06/13/09 04:26 PM Re: F-Ratio Myth (And You) freestar8n   06/13/09 06:41 PM Re: F-Ratio Myth (And You) Jared   06/12/09 10:23 AM Re: F-Ratio Myth (And You) Dean   06/05/09 10:59 AM Re: F-Ratio Myth (And You) lineman_16735   05/06/09 01:46 AM Re: F-Ratio Myth (And You) *DELETED* gavinm   05/06/09 01:26 AM Re: F-Ratio Myth (And You) DonR   05/06/09 01:41 PM Re: F-Ratio Myth (And You) gavinm   05/06/09 08:10 PM Re: F-Ratio Myth (And You) DonR   05/06/09 08:20 PM Re: F-Ratio Myth (And You) paulobao   05/18/09 04:17 PM Re: F-Ratio Myth (And You) Kolenka   05/18/09 05:47 PM Re: F-Ratio Myth (And You) gavinm   05/18/09 10:29 PM Re: F-Ratio Myth (And You) Kolenka   05/19/09 01:16 AM Re: F-Ratio Myth (And You) gavinm   05/19/09 01:55 AM Re: F-Ratio Myth (And You) paulobao   05/19/09 03:15 AM Re: F-Ratio Myth (And You) Kolenka   05/19/09 10:20 AM Re: F-Ratio Myth (And You) Qkslvr   05/19/09 10:42 AM Re: F-Ratio Myth (And You) Arkalius   05/20/09 02:27 AM Re: F-Ratio Myth (And You) Qkslvr   05/20/09 07:56 AM Re: F-Ratio Myth (And You) Galaxyhunter   05/20/09 06:45 AM Re: F-Ratio Myth (And You) gavinm   05/20/09 07:42 PM Re: F-Ratio Myth (And You) Arkalius   05/20/09 07:36 PM Re: F-Ratio Myth (And You) paulobao   05/20/09 05:51 AM Re: F-Ratio Myth (And You) russ_watters   05/18/09 10:05 PM Re: F-Ratio Myth (And You) jgraham   05/18/09 05:19 PM Re: F-Ratio Myth (And You) Kolenka   05/05/09 11:57 PM Re: F-Ratio Myth (And You) Nocturnal   05/06/09 12:07 AM Re: F-Ratio Myth (And You) lineman_16735   05/06/09 01:49 AM Re: F-Ratio Myth (And You) Arkalius   05/06/09 06:14 AM Re: F-Ratio Myth (And You) tiggere   05/06/09 08:14 AM Re: F-Ratio Myth (And You) lineman_16735   05/06/09 09:46 AM Re: F-Ratio Myth (And You) Dean   05/06/09 10:46 AM Re: F-Ratio Myth (And You) lineman_16735   05/06/09 01:04 PM Re: F-Ratio Myth (And You) tiggere   05/05/09 08:05 AM Re: F-Ratio Myth (And You) Miguel Lopes   05/05/09 08:47 AM Re: F-Ratio Myth (And You) Kolenka   05/05/09 11:22 AM Re: F-Ratio Myth (And You) Miguel Lopes   05/05/09 08:24 PM Re: F-Ratio Myth (And You) Dean   05/06/09 11:44 AM Re: F-Ratio Myth (And You) Jared   05/21/09 01:27 AM Re: F-Ratio Myth (And You) Arkalius   05/21/09 08:24 PM Re: F-Ratio Myth (And You) Jared   05/22/09 10:06 AM Re: F-Ratio Myth (And You) Qkslvr   05/22/09 11:29 AM Re: F-Ratio Myth (And You) Jared   05/22/09 05:36 PM Re: F-Ratio Myth (And You) Arkalius   05/22/09 07:31 PM Re: F-Ratio Myth (And You) Jared   05/24/09 01:17 PM Re: F-Ratio Myth (And You) Arkalius   05/24/09 07:47 PM Re: F-Ratio Myth (And You) Dean   05/21/09 10:03 PM Re: F-Ratio Myth (And You) Arkalius   05/22/09 12:21 AM Re: F-Ratio Myth (And You) Dean   05/22/09 11:46 AM Re: F-Ratio Myth (And You) Qkslvr   05/22/09 01:16 PM Re: F-Ratio Myth (And You) Dean   05/22/09 03:09 PM Re: F-Ratio Myth (And You) Qkslvr   05/22/09 07:42 AM Re: F-Ratio Myth (And You) DonR   05/05/09 01:06 PM Re: F-Ratio Myth (And You) Dean   05/06/09 10:25 AM Re: F-Ratio Myth (And You) lineman_16735   05/05/09 12:07 PM Re: F-Ratio Myth (And You) Nocturnal   05/05/09 01:06 PM Re: F-Ratio Myth (And You) DonR   05/05/09 12:59 PM Re: F-Ratio Myth (And You) WarrenS   05/05/09 12:27 PM Re: F-Ratio Myth (And You) gavinm   05/05/09 04:51 PM Re: F-Ratio Myth (And You) freestar8n   06/04/09 12:20 PM Re: F-Ratio Myth (And You) HunterofPhotons   06/05/09 01:11 PM Re: F-Ratio Myth (And You) freestar8n   06/06/09 07:45 AM Re: F-Ratio Myth (And You) HunterofPhotons   06/06/09 01:02 PM Re: F-Ratio Myth (And You) freestar8n   06/06/09 03:13 PM Re: F-Ratio Myth (And You) HunterofPhotons   06/07/09 01:17 PM Re: F-Ratio Myth (And You) freestar8n   06/07/09 02:06 PM Re: F-Ratio Myth (And You) HunterofPhotons   06/08/09 11:37 AM Re: F-Ratio Myth (And You) freestar8n   06/08/09 12:06 PM Re: F-Ratio Myth (And You) Kolenka   06/08/09 12:39 PM Re: F-Ratio Myth (And You) freestar8n   06/08/09 01:38 PM Re: F-Ratio Myth (And You) freestar8n   06/08/09 04:28 PM Re: F-Ratio Myth (And You) Jared   06/08/09 07:02 PM Re: F-Ratio Myth (And You) freestar8n   06/08/09 11:04 PM Re: F-Ratio Myth (And You) Galaxyhunter   06/08/09 07:42 PM Re: F-Ratio Myth (And You) Dean   06/08/09 08:31 PM Re: F-Ratio Myth (And You) Galaxyhunter   06/08/09 10:31 PM Re: F-Ratio Myth (And You) Dean   06/09/09 12:42 AM Re: F-Ratio Myth (And You) gavinm   06/09/09 01:24 AM Re: F-Ratio Myth (And You) Dean   06/09/09 11:24 AM Re: F-Ratio Myth (And You) Qkslvr   06/09/09 09:12 AM Re: F-Ratio Myth (And You) freestar8n   06/09/09 10:07 AM Re: F-Ratio Myth (And You) Jared   06/09/09 12:35 PM Re: F-Ratio Myth (And You) freestar8n   06/09/09 01:41 PM Re: F-Ratio Myth (And You) Jared   06/09/09 05:58 PM Re: F-Ratio Myth (And You) Dean   06/09/09 12:02 PM Re: F-Ratio Myth (And You) freestar8n   06/09/09 01:10 PM Re: F-Ratio Myth (And You) Qkslvr   06/09/09 11:53 AM Re: F-Ratio Myth (And You) Qkslvr   06/08/09 05:00 PM Re: F-Ratio Myth (And You) freestar8n   06/08/09 05:07 PM Re: F-Ratio Myth (And You) Doubleglaze   06/07/09 02:53 AM Re: F-Ratio Myth (And You) freestar8n   06/07/09 09:21 AM Re: F-Ratio Myth (And You) Galaxyhunter   06/05/09 02:55 PM Re: F-Ratio Myth (And You) freestar8n   06/06/09 07:56 AM Re: F-Ratio Myth (And You) Galaxyhunter   06/06/09 01:26 PM Re: F-Ratio Myth (And You) Arkalius   06/06/09 01:27 AM Re: F-Ratio Myth (And You) bbasiaga   05/05/09 06:23 PM Re: F-Ratio Myth (And You) Arkalius   05/05/09 06:19 PM Re: F-Ratio Myth (And You) Kolenka   05/05/09 08:18 PM Re: F-Ratio Myth (And You) Kolenka   05/05/09 12:32 PM Re: F-Ratio Myth (And You) lineman_16735   05/05/09 12:11 PM Re: F-Ratio Myth (And You) Nocturnal   05/05/09 09:05 AM Re: F-Ratio Myth (And You) DonR   05/05/09 09:32 AM Re: F-Ratio Myth (And You) tiggere   05/05/09 10:47 AM Extra information 8 registered and 7 anonymous users are browsing this forum. 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