image circle

[blink138 (Pat)]

hello members i have been studying all of the latest crop of 70mm apo's and something is confusing me
why is is that if all have 2.5" focussers, how can their respective image circles vary so much?
for instance the WO star 71 f4.9 has an image circle of 45mm while the stellarvue 70mm f6 with their 0.8 fr/ff has an image circle of 27mm

what difference would it make as i have never thought about it before and now it is worrying me lol
pat

[multiweb (Marc)]

Originally Posted by blink138

why is is that if all have 2.5" focussers, how can their respective image circles vary so much?

They mean the corrected circle is the imaging circle which is independant from the ID of the focuser tube (vignetting). Better corrected more expensive refractor would have a larger imaging circle. Depends on your sensor diagonal. I use small sensors so it's easier to fit in smallish imaging circles.

[blink138 (Pat)]

ta mark ............... but what difference would a 45mm image circle have to an image from an apsc sensor? is there a lot of waste and what does that mean for imaging?
pat

[multiweb (Marc)]

Originally Posted by blink138

ta mark ............... but what difference would a 45mm image circle have to an image from an apsc sensor? is there a lot of waste and what does that mean for imaging?
pat

If the diagonal of your sensor fits within the imaging circle then you're good to go, otherwise you might get coma or other aberrations in the far corners.

[Wavytone]

45mm means you need to spend more money on a bigger camera - full frame, ideally

[blink138 (Pat)]

thanks guys
so if your sensor has a diagonal of 27mm and the image circle is 45mm as in the WO above, what would it do to your images or it is not worth going the large image circle if you only ever have an aps-c sensor, are you actually wasting a lot of the light?
pat

[RickS (Rick)]

Originally Posted by blink138

so if your sensor has a diagonal of 27mm and the image circle is 45mm as in the WO above, what would it do to your images or it is not worth going the large image circle if you only ever have an aps-c sensor, are you actually wasting a lot of the light?

You're not wasting light so much as you're not taking advantage of the larger FOV that would be possible with a bigger/more expensive sensor.

Cheers,
Rick.

[Slawomir (Suavi)]

From myself I will add that different companies have different standards relating to image circle or corrected circle. IMO a short fast 71mm refractor will not necessarily give pinpoint stars in the corners near the edge of the corrected circle when combined with a camera with smallish pixels.

[blink138 (Pat)]

ok thanks guys
so it seems wise to go the larger image circle, even regardless of whether you plan on getting a full frame sensor, and if the image circle is down to 27mm like the stellarvue, then when you attach the ff sensor it will vignette in the corners?
pat