#1  
Old 09-11-2012, 08:54 AM
graham.hobart's Avatar
graham.hobart (Graham stevens)
DeepSkySlacker

graham.hobart is offline
 
Join Date: Nov 2008
Location: hobart, tasmania
Posts: 1,996
changing Focal length

Just a simple question I hope-
When running a focus run with TCF (optec) in Maxim DL - the focal length/ratio has to be spot on. I am curious as to whether this is as advertised on the side of my scope (f7/900mm) or should I be changing this as I have the added length of the focuser between the scope and sensor?
I wondered whether this would affect then the accuracy of my focus and also image scale for calculations in Maxim as I would soon like to plate solve?
Cheers

Graham
Reply With Quote
  #2  
Old 09-11-2012, 01:44 PM
graham.hobart's Avatar
graham.hobart (Graham stevens)
DeepSkySlacker

graham.hobart is offline
 
Join Date: Nov 2008
Location: hobart, tasmania
Posts: 1,996
focal length question /(focal ratio)

Bump?
Reply With Quote
  #3  
Old 09-11-2012, 02:43 PM
mithrandir's Avatar
mithrandir (Andrew)
Registered User

mithrandir is offline
 
Join Date: Jan 2009
Location: Glenhaven
Posts: 4,161
Graham, your best bet is to take an image and plate solve it with something like astrometry.net which does not need an estimate of focal length, or UniMap for which a ball park figure will get it started.

That should either directly give you the arcsec/pixel or the FOV from which you can derive the ASP since you know the pixel size.

Then the true focal length is given by:

focal_length = 206.265 * pixel_size / arcsec_per_pixel

where:
focal_length in mm
pixel_size in um
Reply With Quote
  #4  
Old 09-11-2012, 03:15 PM
graham.hobart's Avatar
graham.hobart (Graham stevens)
DeepSkySlacker

graham.hobart is offline
 
Join Date: Nov 2008
Location: hobart, tasmania
Posts: 1,996
focal length question /(focal ratio)

Thank you sir, I have a suspicion that my calculations are wrong because of the added length of the focuser assembly. Will do what you say when the clouds are gone.
Reply With Quote
  #5  
Old 09-11-2012, 04:30 PM
mithrandir's Avatar
mithrandir (Andrew)
Registered User

mithrandir is offline
 
Join Date: Jan 2009
Location: Glenhaven
Posts: 4,161
Quote:
Originally Posted by graham.hobart View Post
Thank you sir, I have a suspicion that my calculations are wrong because of the added length of the focuser assembly. Will do what you say when the clouds are gone.
Graham it's not the physical length of the bits. It's the optical length. eg a C8 is physically ~450mm whereas it is 2000mm optically.
Reply With Quote
  #6  
Old 09-11-2012, 05:15 PM
Peter.M's Avatar
Peter.M
Registered User

Peter.M is offline
 
Join Date: Sep 2011
Location: Adelaide
Posts: 914
Just use pinpoint LE in maximdl, if it knows your RA and DEC you can solve an image in there and it tells you your focal length.
Reply With Quote
  #7  
Old 09-11-2012, 06:42 PM
gbeal
Registered User

gbeal is offline
 
Join Date: Oct 2004
Location: New Zealand
Posts: 4,301
Yeah, do the solve in Maxim, you'll need a few things to do this, but once you master it, you'll never look back.
That will then lead to auto-centering etc, all good fun, really.
Gary
Reply With Quote
  #8  
Old 12-11-2012, 09:56 AM
graham.hobart's Avatar
graham.hobart (Graham stevens)
DeepSkySlacker

graham.hobart is offline
 
Join Date: Nov 2008
Location: hobart, tasmania
Posts: 1,996
focal length question /(focal ratio)

Thanks chaps- did a solve in Maxim and it came out as 899mm! Many thanks for help.
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 10:32 PM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Advertisement
OzScopes Authorised Dealer
Advertisement
Bintel
Advertisement
SkyWatcher Australia
Advertisement
Meade Australia
Advertisement
Celestron Australia
Advertisement
NexDome Observatories
Advertisement
Lunatico Astronomical
Advertisement
Astronomy and Electronics Centre
Advertisement