The fov-coma balance

[Demonperformer]

1. As I understand it, coma is something that affects all newtonian scopes, but particularly fast ones. It also becomes more pronounced towards the edge of the field.

2. So, if I have 2 6" newtonians, one f4 and the other f8, (without coma correctors) the coma in the f8 scope will be less. But the fov of the f8 scope will also be smaller.

3. Now, if I select an object that is nicely framed in the smaller fov, and photograph it in both scopes in the same way, and then enlarge and crop the photo from the f4 scope so that it matches the fov of the f8 scope, assuming the mirrors are of similar quality (eg lambda/8), would I notice more coma in the f4 image or not?

Thanks.

[Knighty2112]

Good question. Not sure of the answer, so await enlightenment from others!  

[Stu]

Agreed, great question and I’m watching with interest too  

[vlaiv]

Yes you would.

Level of coma is function of third power of F/ratio for linear units (distance from optical axis in imaging plane in meters), or function of second power of F/ratio in angular units.

reference: http://www.telescope-optics.net/newtonian_off_axis_aberrations.htm

If you are interested in same FOV - you should look at second expression - one in angular units (70.2.1).

As you see, angle of coma free diffraction limited field is given by : F^2 / (90 * D), and since we are talking about scopes of same diameter, F/4 scope is going to have smaller diffraction limited field by factor of 16 compared to F/8 scope. This also translates that level of coma will be 16 times smaller for a given point off axis in F/8 scope compared to F/4 scope.

This is one of the reasons fast newtonians really need good collimation - as any offset from perfect collimation is going to have very big impact compared to more tolerant F/ratios (like F/6 - 4 times less sensitive, or F/8 - 16 times less sensitive).

[Demonperformer]

Thanks for that @vlaiv.

The document looks interesting and I will probably benefit from reading it from the beginning. The starting point on this page is a bit beyond my math, so I will need the bits before it to really understand it.

8 hours ago, vlaiv said:

level of coma will be 16 times smaller for a given point off axis in F/8 scope compared to F/4 scope.

That is a staggering figure, but I take your word for it (that that is what the math says). Certainly explains why everyone does not go out and buy a megafast newtonian.

Thanks.

[vlaiv]

Just realized that I've done calculations wrong

F/4 compared to F/8 = 4 times worse (not 16, I have no idea how I got to 16 when I was writing first post, probably were tired and did not realize that 8/4 is 2 rather than 4 ) at same angular distance from optical axis

F/4 compared to F/6 = 2.25 times worse (not 4)

But still, it is quite a bit in favor of slower scopes

[Demonperformer]

As you say, still provides a good reason to go for the longer scope - for general purposes at least.

Thanks.