· 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] Entrance · Main Index · Search · New user · Who's Online FAQ · Calendar Equipment Discussions >> ATM, Optics and DIY Forum Previous   Index   Next   Threaded   Pages: 1 KSINGHBOY newbie Reged: 01/19/13 Need some help regarding focal length       #5631414 - 01/19/13 11:49 AM Edit   Reply   Quote Quick Reply   Hello everyone, i am new here Am making 6" F/8 and My neighbour is also making 6" but F/6 mirror not F/8, i am sharing grits with him, but question is that my mirror when seen in sun its point is small sharp dot like playing marbel, but my neighbour mirror is giving sharp bigger dot. Bigger than my dot. Can you tell me why so difference? his dot is bigger than me. Who is wrong me or my neighbour? Thanks Post Extras:       gpelf super member Reged: 12/28/11 Loc: Kentucky Re: Need some help regarding focal length [Re: KSINGHBOY]       #5631472 - 01/19/13 12:23 PM Edit   Reply   Quote Quick Reply   Hi Ksinghboy, Even though I am a newbie here as well, Welcome to Cloudy Nights (You will love the people and the help you get here). Are you at rough grinding or fine grinding stage? If you are at the courser grits it may be nothing more than one mirror is closer to a sphere than the other, At this stage of grinding you cant be to critical about what you see optically. Have you tried directly reading the Sagitta (straight edge and feeler gauge method)? The focal length for a 6" F8 is 48" for a 6" F6 it is 36", Is this close to what you are seeing? Hope this helps somewhat, even if it does not, keep checking, I am sure some of the pros here will respond. Clear Skies, Greg Post Extras:       Mike I. Jones Carpal Tunnel Reged: 07/02/06 Loc: Fort Worth TX Re: Need some help regarding focal length [Re: KSINGHBOY]       #5631780 - 01/19/13 03:38 PM Edit   Reply   Quote Quick Reply   A couple of old ATM tricks for getting your focal length during roughing: (1) Spray WD-40 on the mirror, then use the sun to measure your focal length. Make sure the shadow of the tape measure is centered on the mirror. (2) You can also use shoe polish. Dab a little glob in the middle, then spread it evenly over the surface. Wipe off most, then buff polish to a nice sheen, and you'll get a good sharp sun image. Mike Post Extras:       Pinbout Post Laureate Reged: 02/22/10 Loc: Montclair Re: Need some help regarding focal length [Re: KSINGHBOY]       #5632443 - 01/19/13 11:36 PM Edit   Reply   Quote Quick Reply   Neither of you are wrong. it will all work out in the final figuring, that's where it counts. My 8in f4.5 isn't marble like and its fine. Post Extras:       KSINGHBOY newbie Reged: 01/19/13 Re: Need some help regarding focal length [Re: Pinbout]       #5632454 - 01/19/13 11:44 PM Edit   Reply   Quote Quick Reply   Thank you my friend. Can you all tell me that on long FL does point of sun image is small or bigger? My direct question is what is the diameter of short FL and long FL when seen in sun? Thanks Edited by KSINGHBOY (01/20/13 04:17 AM) Post Extras:       dan_h Carpal Tunnel Reged: 12/10/07 Re: Need some help regarding focal length [Re: KSINGHBOY]       #5632889 - 01/20/13 10:17 AM Edit   Reply   Quote Quick Reply   The size of the image at the focal plane is dependant on the focal length of the objective whether it be a lens or a mirror. For both the moon and the sun, which are nearly 1/2 degree,(30 arcminutes) in size, you can calculate the image size as 0.009 X focal lenght. For example, a 1000mm focal length scope creates an image of the sun that is 1000mm X 0.009 = 9mm in diameter. For a real quick estimate I use 1/100 the focal length so I can do the arithmentic in my head without thinking. For a 6" f8, focal length is 48", image of the sun is about 0.48". For a 6" f6, focal length is 36", image of the sun is about 0.36". Hope this helps, dan Post Extras:       KSINGHBOY newbie Reged: 01/19/13 Re: Need some help regarding focal length [Re: dan_h]       #5632954 - 01/20/13 10:51 AM Edit   Reply   Quote Quick Reply   Thank you Dan very useful informations you have provided. Post Extras:       Pages: 1 Previous   Index   Next   Threaded   Extra information 6 registered and 24 anonymous users are browsing this forum. Moderator:  ausastronomer, richard7  Print Thread Forum Permissions       You cannot start new topics       You cannot reply to topics       HTML is disabled       UBBCode is enabled Thread views: 338 Jump to *Announcements and News* -----   Cloudy Nights Announcements   Discussion of CN Articles, Reviews, and Reports   Celestial Events*General Astronomy* -----   Beginners Forum   General Observing and Astronomy   Outreach   Light Pollution*Equipment Discussions* -----   ATM, Optics and DIY Forum   Binoculars   Binoviewers   Cats & Casses   Classic Telescopes   Equipment   Eyepieces   Mounts   Observatories   Reflectors   Refractors    Video and Electronically Assisted Astronomy*Astrophotography and Sketching* -----   Beginning and Intermediate Imaging   Film Astrophotography   DSLR & Digital Camera Astro Imaging & Processing   CCD Imaging & Processing   Solar System Imaging & Processing   Sketching   Astro Imaging/Sketching Contest *Observing* -----   NEO   Planetary and Solar System Observing   Lunar Observing   Deep Sky Observing   Double Star Observing   Solar   Variable Star Observing and Radio Astronomy*Telescope Specific Forums* -----   Celestron NexStar    NexStar GPS / CPC Telescopes   Questar Forum   Meade ACF/RCX / LX / GPS    Meade ETX*Speciality Forums* -----   Arizona Science and Astronomy Expo   NEAF (Northeast Astronomy Forum) and NEAIC (Northeast Astro Imaging Conference)   AstroArt   Astronomy Clubs and Star Parties   Astronomy Software & Computers   AstroWebsites   Science! Astronomy & Space Exploration, and Others   Space Rocks   Stellar Media*Swap and Shop* -----   Shop and Swap Audio Components*CN Feedback and Vendor Announcements* -----   Vendor and Group Announcements*International Year of Astronomy 2009 - Forums are now closed.* -----   IYA Archive Info CN Forums Home

Cloudy Nights LLC Cloudy Nights Sponsor: Astronomics