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#1
15-11-2007, 09:38 PM
 netwolf Registered User Join Date: Jan 2005 Posts: 2,905
How to Calculate the projected Image Circle for a Telescope?

Hi All,

How does one calculate the Image circle for a given telescope? This appears to me based on the images i have taken to be smaller than what the FOV should be.

Regards
Fahim
#2
15-11-2007, 10:13 PM
 Kal (Andrew) 1¼" ñì®våñá Join Date: Nov 2006 Location: Sydney Posts: 1,798
First, depends on the narrowest section of the scope that the light path travels. Usually this is at the focuser, or diagonal if you use one. Secondly, focal reducer will shrink this down.

Roughly speaking, if you were to image through a 2" focuser, you would have an image circle (possibly with field curvature and some vignetting) towards the edges at a bit under two inches diameter. If you put a focal reducer in the path (such as the Meade .63 reducer you used) then the clear aperture of that reducer is 1.5" (if you look at the reducer 2" is the outer diameter of the metal casing, 1.5" is the clear aperture of the glass), and ontop of that it reduces that 1.5" down by approx 0.63 of that, so 0.945". If you use a 0.33 reducer you get half of this again, less than half an inch, which is why you can only use small CCD chips with these stronger reducers.

If you want to image on full 6cmx7cm medium format film, you will need a much larger focuser, which is why high end apos have 2.7" and 4" focusers and massive field flatteners.

I think this is what you are asking (based on what you asked me about vignetting the other day ) If not, image circle is based on the focal length of the scope and the size of the image sensor (or film) and someone else can give the mathematical answer
#3
15-11-2007, 10:47 PM
 ballaratdragons (Ken) The 'DRAGON MAN' Join Date: Jan 2005 Location: In the Dark at Snake Valley, Victoria Posts: 14,412
I went about it in an awkward way.

I look at the image on screen (even though it is rectangular) and then go into Cartes du Ciel and place the EP circle to cover the same area as the camera sees (the FOV on screen). By gradually adjusting the circle size by the numerical settings it tells me what size the image circle is.

Awkward and fiddly but it works

Here is a screenshot showing what I mean. All using an ED80 and M42 as the guide.
*I have the inner circle showing me my FOV in the Toucam.
*The next circle shows me the FOV of the Toucam and a 0.6x Focal Reducer
*The outer circle shows me what my finderscope sees.

On the left is the circle sizes of each.
Attached Thumbnails
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Last edited by ballaratdragons; 15-11-2007 at 11:03 PM.
#4
15-11-2007, 11:05 PM
 ballaratdragons (Ken) The 'DRAGON MAN' Join Date: Jan 2005 Location: In the Dark at Snake Valley, Victoria Posts: 14,412
I use this method above to see if what I want to image will fit in my imaging FOV before I even go to it
#5
16-11-2007, 05:08 AM
 iceman (Mike) Sir Post a Lot! Join Date: Sep 2004 Location: Gosford, NSW, Australia Posts: 36,709
I use the "FOV" setting in SNP which lets you enter the telescope (aperture/focal length) and the CCD (number of pixels, pixel size), and then it will draw a rectangle based on those settings.

It's very accurate.
#6
16-11-2007, 11:22 PM
 netwolf Registered User Join Date: Jan 2005 Posts: 2,905
Hi Guys thanks for all the answes. I already use CDC etc to work out the FOV. calculation. I was concerned more with the size of the secondary and its limitation on the FOV. The CCD Calculators etc do not account for the size of secondary or so it seems to me. For example A 8" F4 refractor scope and newt scope show the same FOV box for a DSLR camera regardless of the size of the secondary in the Newt. It seems we have to manually account for this. And I see this now thanks.

Regards
Fahim

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