Calculating image circle sizes?

[vineyard]

Hello,

Is there a formula that can be used to calculate how big an image circle a particular scope would show?  Or is the best way just to focus on a distant terrestrial object and let it fall on a piece of paper w bullseye style circles of different diameters?

Thank you!

Vin

[vlaiv]

I think it is best to measure it.

You can calculate it in principle - if you have 3D model of the telescope. Imaging circle depends on internal baffling.

With 3D model it is rather easy - you put it in ray trace software and it will show you illumination at focus plane (well - not easy in terms of calculations, but easy as it is solved problem and there is software that does it).

For simple designs - like newtonian - you could calculate it with a bit of trigonometry if you draw yourself a diagram (like size of mirrors, their spacing, focuser draw tube dimensions and position when focused and so on).

[vineyard]

Thanks @vlaiv - I'll measure it via the projection method!  Cheers

[vlaiv]

4 minutes ago, vineyard said:

Thanks @vlaiv - I'll measure it via the projection method!  Cheers

You can always attach a camera and look at the flat as well.

That will tell you where vignetting starts.

[vineyard]

2 minutes ago, vlaiv said:

You can always attach a camera and look at the flat as well.

That will tell you where vignetting starts.

That's interesting.  How could I calculate the image circle size in mm from that?  I guess if I know the sensor dimensions, then I can see the proportion of the central horizontal that is not vignetted and use that to guesstimate the image circle diameter?

[vlaiv]

3 minutes ago, vineyard said:

That's interesting.  How could I calculate the image circle size in mm from that?  I guess if I know the sensor dimensions, then I can see the proportion of the central horizontal that is not vignetted and use that to guesstimate the image circle diameter?

Yes - exactly like that. Use diagonal instead of horizontal.

With projection method you are using your eye to notice difference (error to about 7%) in brightness. With sensor you can be more precise. Only problem is - do you have big enough sensor. If your fully illuminated circle is larger then say 30mm and you have APS-C sensor - you won't capture vignetting drop off properly (~28mm diagonal depending on model).

[vineyard]

Thanks @vlaiv - I have a 294MCPro which I think has a diagonal of 23.1mm so if there's no vignetting on that, that will mean an image circle of at least 46.2mm which would be enough for me to know whether rigging something for 2" / T2 would be worth it or whether I should leave it as 1.25".  Let's see!  Cheers.

[pete_l]

4 hours ago, vineyard said:

Is there a formula that can be used to calculate how big an image circle a particular scope would show?

I asked that question (actually: what is the image circle from your model ... ) of Meade, quite some time ago when I was considering buying a 5-figure rig from them. The sum total response from their assembled experts was that nobody knew.

I didn't continue with the purchase.

Edited April 8, 2022 by pete_l

[vineyard]

Yowsa @pete_l that's quite disconcerting that they don't know that about their own kit?!

[vlaiv]

41 minutes ago, vineyard said:

Thanks @vlaiv - I have a 294MCPro which I think has a diagonal of 23.1mm so if there's no vignetting on that, that will mean an image circle of at least 46.2mm which would be enough for me to know whether rigging something for 2" / T2 would be worth it or whether I should leave it as 1.25".  Let's see!  Cheers.

No - using 23mm diagonal sensor and having no vignetting means that full illumination (different from imaging) circle is at least 23mm - vignetting can start at 12mm away from center (making diagonal 24mm - 1mm larger then what you checked).

Diagonal equates to diameter not radius.

[vineyard]

Doh, I'm an idiot @vlaiv you're quite right was getting my radius & diameters mixed up.  Hmm an APS-C DSLR would help but only up to 28.2mm.  Maybe I'll eyeball it w projection if its still bigger than that (just want to get a rough sense).  Cheers!

[ollypenrice]

Image circle issues come in two kinds, vignetting and stellar distortion. There's not much you can do about distortion but flats fix vignetting to an astonishing degree. You'd first need to know how much you're prepared to tolerate. On a full frame chip a certain Tak FSQ106N 😁 has about a 23% light fall off in the corners but flats completely correct that. I used it more for mosaics than anything else and putting them together was easy, showing that the illumination ended up even. The other way to confirm this is to stack a set of flats using flats to calibrate them.  I do this when demonstrating to beginners the purpose of flats. The output image should be (and is) almost perfectly flat.

Olly