ICEINSPACE
Most Read Articles
Moon Phase
CURRENT MOON Waxing Gibbous
99.1%
The Sun Now
Time Zones
Sydney*
8:34 am
Perth
5:34 am
Auckland*
10:34 am
New York
4:34 pm
Paris
10:34 pm
GMT
9:34 pm




  #1  
Old 13-12-2018, 11:14 AM
CalvinKlein's Avatar
CalvinKlein (Kelvin)
Registered User

CalvinKlein is offline
 
Join Date: Aug 2018
Location: Cudgen NSW
Posts: 25
How much coma in a 10" F4 Newtonian

I am considering buying an inexpensive 10" F4 Newtonian mainly for capturing the ISS and for tracked planetary imaging. Until now I've quite successfully used a 7" triplet refractor for each but need more aperture for the ISS.

I use a Televue Powermate 4x and ASI183MC for the imaging.

Can anyone give me an indication of how much of the Field of View is totally useless due to coma -ideally with sample images.

For manual ISS tracking ideally its good to utilise the entire sensor but if, say, the centre 30% remains sharp-ish and the rest coma affected - that might still be acceptable - any less would be too difficult to get centred.

Thanks
Reply With Quote
  #2  
Old 13-12-2018, 01:48 PM
multiweb's Avatar
multiweb (Marc)
ze frogginator

multiweb is offline
 
Join Date: Oct 2007
Location: Hinchinbrook
Posts: 17,459
A 10" F/4 newt would be quite a challenge to collimate and correct. I'd say ~70% of the fov would be coma free.
Reply With Quote
  #3  
Old 13-12-2018, 02:36 PM
Merlin66's Avatar
Merlin66 (Ken)
Spectroscopy Wizard

Merlin66 is offline
 
Join Date: Oct 2005
Location: St Leonards, Vic
Posts: 6,627
According to Rutten and Van Venrooij "Telescope optics" p 47 it would suggest for an f4 system that there will be noticeable coma beyond about 5mm off axis.
Reply With Quote
  #4  
Old 13-12-2018, 02:53 PM
CalvinKlein's Avatar
CalvinKlein (Kelvin)
Registered User

CalvinKlein is offline
 
Join Date: Aug 2018
Location: Cudgen NSW
Posts: 25
Hmmmmm - 5mm off centre vs 70% coma free is a big difference.

I read some theory on one of the optics sites talking about aberrations close to the centre axis but i assumed that it would still be tolerable much further out than that. That would kind of imply that a 10" F4 is pretty well useless without a corrector which makes you wonder if the product is really fit for sale - rather than advertise "coma corrector recommended" advertising should read "coma corrector mandatory" - in which case why isn't it built in ?
Reply With Quote
  #5  
Old 13-12-2018, 03:05 PM
multiweb's Avatar
multiweb (Marc)
ze frogginator

multiweb is offline
 
Join Date: Oct 2007
Location: Hinchinbrook
Posts: 17,459
Quote:
Originally Posted by CalvinKlein View Post
Hmmmmm - 5mm off centre vs 70% coma free is a big difference.
Not really. 5mm off center means you'll cover a 10mm diagonal chip. Depends on your sensor size . If it's a planetary camera your sensor will be tiny and you might not have any coma at all. Just got to figure out what your imaging circle is on an uncorrected 10" F/4 newt (I'd expect 15mm at least) then work your way back to your sensor size and work out the surface you can work with.
Reply With Quote
  #6  
Old 13-12-2018, 03:16 PM
Merlin66's Avatar
Merlin66 (Ken)
Spectroscopy Wizard

Merlin66 is offline
 
Join Date: Oct 2005
Location: St Leonards, Vic
Posts: 6,627
If you really want to read up on Coma:
https://www.telescope-optics.net/coma.htm

To quote from "Telescopes Eyepieces Astrographs" p172
For a 200mm f4.5 Newtonian ....only 7.5 arcmin off axis the coma has grown to over three times the Airy disk.

p175 they discuss wide field observing with an eyepiece (27mm) giving a 1.5 degree FOV (0.75 deg off axis) - the coma, towards the edge, would be 28 arcsec - "which is larger than the disk of Mars at it's closest"
Reply With Quote
  #7  
Old 13-12-2018, 03:18 PM
CalvinKlein's Avatar
CalvinKlein (Kelvin)
Registered User

CalvinKlein is offline
 
Join Date: Aug 2018
Location: Cudgen NSW
Posts: 25
Ahhh gotcha Lol i'm looking the other way at this - as in out the OTA into space :-) 5mm out of 254mm didn't seem right.

I'm using an ASI183 which is 13.2 x 8.8mm and about 20 megapixels so I might be okay then.
Reply With Quote
  #8  
Old 13-12-2018, 03:25 PM
multiweb's Avatar
multiweb (Marc)
ze frogginator

multiweb is offline
 
Join Date: Oct 2007
Location: Hinchinbrook
Posts: 17,459
Quote:
Originally Posted by CalvinKlein View Post
Ahhh gotcha Lol i'm looking the other way at this - as in out the OTA into space :-) 5mm out of 254mm didn't seem right.

I'm using an ASI183 which is 13.2 x 8.8mm and about 20 megapixels so I might be okay then.
You might get away with 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 07:34 AM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Advertisement
SkyWatcher WiFi Adaptor
Advertisement
OzScopes Authorised Dealer
Advertisement
FLI Cameras and Imaging Accessories
Advertisement
Lunatico Astronomical
Advertisement
Bintel
Advertisement
Interest Free Finance
Advertisement
SkyWatcher 2018 Catalogue
Advertisement
NexDome Observatories
Advertisement
Astronomy and Electronics Centre
Advertisement