Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Short Focal Length C Mount

NV
  • Please log in to reply
3 replies to this topic

#1 GrahamDFyffe

GrahamDFyffe

    Mariner 2

  • -----
  • topic starter
  • Posts: 243
  • Joined: 07 Apr 2016
  • Loc: Central Arizona

Posted 24 June 2020 - 05:18 PM

So I have a Mod 3C. Would it be possible to use a short (less than 26mm) C mount lens to effectively "speed" up the system? Kind of like using the 65mm Televue option with an Envis to double the speed. Could  say a 12mm C Mount objective have a similar effect (assuming you. Assuming you could  find some way of doing a focal with it.



#2 cnoct

cnoct

    Viking 1

  • *****
  • Posts: 906
  • Joined: 02 Sep 2010
  • Loc: Hawai'i

Posted 24 June 2020 - 10:59 PM

No not really, there needs to be decrease in f-stop (so called speed of the lens) which doesn't happen by just changing the focal length. One would need either seek focal reduction, decreasing of both focal length and f-stop or a lens with a natively lower f-stop. A shorter focal length lens, without an accompanying decrease in f-stop, just yields a greater field of view i.e. 16mm f/1.2 lens isn't any "brighter/faster" than a 27mm f/1.2 lens. 
 
Now if you reduce the native focal length of a lens through the use of a Speed Booster (aka: focal reducer), then yes, it could be thought of as being similar to using a long FL eyepiece to reduce the focal ratio e.g. 55mm f/1.4 prime lens + 0.71 "speed booster" yielding a fast f/1.0 lens with a focal length of 39mm.

 

So regardless of the prime configuration, whether pairing with a camera lens or telescope, the concomitant use of a focal reducer is needed to increase the "speed" of the lens. Consider the focal reducer or "speed booster" to be roughly equivalent to the focal reducing eyepiece in an afocal configuration i.e. > 27mm FL

 

FWIW:

 

16mm FL work a treat when view Sprites, other TLE's and meteor showers, even ones with considered slow i.e. f/1.8 to f/2.8

 

The increased field of view, ~ 62˚ over that of the native 40˚ makes a huge difference for those types of pursuits. 


Edited by cnoct, 24 June 2020 - 11:04 PM.


#3 GrahamDFyffe

GrahamDFyffe

    Mariner 2

  • -----
  • topic starter
  • Posts: 243
  • Joined: 07 Apr 2016
  • Loc: Central Arizona

Posted 25 June 2020 - 03:59 PM

Thanks I am obviously a little confused by this process! I am more used to basic visual rules like exit pupil being an indicator of brightness up to the limit of about 7mm for maximum pupil dilation (or much lower in the case of old codgers like me). Does the same rule of thumb work here except up to the sensor size, which is I think 18mm with these devices? I think exit pupil is Focal Length of the eyepiece divided by Focal Ratio of the Objective. Since the eyepiece for the Mod 3 is about 27mm that would imply that the maximum brightness would come from a Lens of about 1.8.



#4 a__l

a__l

    Vanguard

  • -----
  • Posts: 2,019
  • Joined: 24 Nov 2007

Posted 25 June 2020 - 11:33 PM

Consider the maximum hole diameter for c-mount. This internal thread is 1" diameter. The actual hole will be even smaller.

~22-23 мм


Edited by a__l, 25 June 2020 - 11:39 PM.



CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: NV



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics