Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
· 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

Equipment Discussions >> ATM, Optics and DIY Forum

Pages: 1 | 2 | (show all)

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Eyepiece Aparent Field of View? new [Re: Mike I. Jones]
      #5845503 - 05/07/13 12:54 PM

For star drift determinations of true field, and one is satisfied with a result correct to 1%, the star's declination can lie within 8 degrees of the equator. To 2%, 11.5 degrees.

Similarly, the star is allowed to drift not quite centrally across the field. To 1% accuracy, the star can pass 0.13 of the field radius from the center, and to 2% accuracy, 0.2 of the field radius.

If one has a rough idea of the equator's location in some part of the sky (e.g., that it passes very near delta Orionis), a suitable star can be chosen and treated as having zero declination. And a trial pass or two with even a non-equatorially mounted scope to ascertain by eye alone a sufficiently close to central drift.

Post Extras: Print Post   Remind Me!   Notify Moderator  
David Knisely

Reged: 04/19/04

Loc: southeastern Nebraska
Re: Eyepiece Aparent Field of View? [Re: ricknau]
      #5846049 - 05/07/13 04:39 PM



Does it matter that much? 68 or 64 degrees?

False advertising matters. (If it is 64 or less and they know it.) It should matter to their competitors. Unless they're also lying.

Generally, I can measure the apparent fields to an accuracy of around half a degree or so with a little work. Much beyond that, it probably isn't really worth worrying about. What I have found is that most of the published eyepiece apparent fields of view are within a degree or two of what I get on the bench. There have been a few exceptions however. In the case of the variable Speers-Waler eyepieces (the 5-8mm and 8.5-12mm focal length models), the manufacturer/retailer apparent field claims for the 5-8mm model went from 82 to 89 degrees when the actual eyepiece had a *fixed* measured apparent field of view of 78 degrees. It was pretty clear in that case that all the manufacturer/retailer had done was work backwards from the old TFOV = AFOV/Magnification formula, so the claimed apparent field figures were useless. Also, some eyepiece "families" (the Orion Ultrascopics for example) have apparent fields of view that vary from eyepiece to eyepiece, yet some retailers just give one figure for the lot that can be more than two or three degrees off the figures for each different eyepiece. I would prefer that the retailers just give a single field stop figure for each of their eyepieces (or equivalent field stop figure based on a star-drift calculation) rather than playing games with apparent field of view (the Field Stop formula for true field is more accurate than the old AFOV/Mag formula anyway). However, with the exception of Tele Vue, this isn't generally being done, which is a little sad. Still, it doesn't take much work on the part of amateurs to discover the real truth behind the figures for their own eyepieces. Clear skies to you.

Post Extras: Print Post   Remind Me!   Notify Moderator  

Reged: 02/26/11

Re: Eyepiece Aparent Field of View? new [Re: David Knisely]
      #5851388 - 05/10/13 05:44 AM




Does it matter that much? 68 or 64 degrees?

False advertising matters. (If it is 64 or less and they know it.) It should matter to their competitors. Unless they're also lying.

When you can positively measure the AFoV to within a fraction of a degree and analytically demonstrate the validity of the techniques you are using, that might be the time to consider making claims of false advertising.

I did not make a claim of false advertising. I was asked if it mattered that much to me if the FOV was 64 instead of 68. I answered that question with specific caveats. Notice the word "if" in italaics. Don't read more into my post than is there. Admitingly I could have worded that better. How about: A few degrees of missing FOV wouldn't matter that much but receiving what was advertised does. I also said in another post that I'm sure the manufacturer has some way to defend their number.

I appreicate the help given here explaining various methods of measureing the AFOV. I have not had the time to attempt other methods. Maybe that says something about how much it matters to me. When I get the time I may give them a try.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | (show all)

Extra information
4 registered and 12 anonymous users are browsing this forum.

Moderator:  ausastronomer, richard7, Starman81 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled

Thread views: 1065

Jump to

CN Forums Home

Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics