Go Back   IceInSpace > Equipment > Astrophotography and Imaging Equipment and Discussions

Reply
 
Thread Tools Rate Thread
  #21  
Old 05-05-2010, 03:33 PM
RobF's Avatar
RobF (Rob)
Mostly harmless...

RobF is offline
 
Join Date: Jul 2008
Location: Brisbane, Australia
Posts: 5,716
Its facinating to hear about the next stage of tinkering Brendan, but sometimes my wallet wishes I didn't read about upgrades that come out as a success
Reply With Quote
  #22  
Old 05-05-2010, 06:54 PM
Tandum's Avatar
Tandum (Robin)
Registered User

Tandum is offline
 
Join Date: Apr 2008
Location: Wynnum West, Brisbane.
Posts: 4,161
Surely you can measure the image circle with a flat in maxim? 8mm sounds ridiculously small and I thought 24mm was the max you'd get through a 2" focuser. I guess the FL of the scope comes into it too.
Reply With Quote
  #23  
Old 06-05-2010, 10:37 AM
bmitchell82's Avatar
bmitchell82 (Brendan)
Newtonian power! Love it!

bmitchell82 is offline
 
Join Date: Jul 2008
Location: Mandurah
Posts: 2,597
Im with you on that one robin, when i saw 8mm i said WTF... you do have to remember that a 10" dobsonian at F4.7 is pretty quick as it is. Being that it is actually a dobsonian and ment for viewing not astrophotography, they reduce the size of the secondary for ultra contrasty views furthermore they make the spider vanes super thin to reduce diffraction. I think the central obstruction on a 10" in its standard guize is 21% which is very low. with the new 70mm its upto just under 30% which still gives great contrast. RC's and the like are up near the 40-50%...

Just out of intrest, i tried to define how contrast is determined. and from what i could see its about light scattering and lightening the dark airy diffraction ring and darkening the light airy diffraction rings (used in star testing). hence the overall image is some what normalized.

You are right about the 2" focuser vingetting some of the light cone, but thats where things get better. some people think that you must allow upto the 75% ray cone to pass to the sensor/ocular, but this isn't the case, my 75% ray cone is nearly 100% vingetted, but it means nothing because the 100% covers the chip! and in any case who would want less photons hitting the sensor?

Just a side note, the standard SW 254mm Dob actually vingettes the lightcone from the front aperature! by a long way, to gain the full 75% i would have to open the ID of the tube to 310mm from its present 283mm!

Grab newt 2.5 and start putting your values in, you will be serprized.

Rob, I am a uni student, it is rare that i get much more than a hundred bucks or so to do something. so everything i do is done as economically as i can do it!
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 10:13 AM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Advertisement
Testar
Advertisement
Bintel
Advertisement