Aperture masks, focal lengths and mirrors

[bomberbaz]

Sorry about the title but trying to drag in the right response. I am aware that if I use an aperture mask taking my 12" scope down to 6", it alters the focal length and for the sake of argument its F5 to F10. ( I believe the science and maths is slightly different but lets keep it simple)

What I want to know is how much of the mirror is actually used on a reflector type scope, is it still the full 12" (less obstructions) that focuses the light back up to the secondary albeit from a much smaller light gathering aperture. Or is it just the mirror that is below the new, smaller aperture.

See picture for my new example of aperture, this is btw my new solar filter.

regards

Steve

[John]

Hi Steve,

The focal length is not altered actually. The effective aperture of the scope drops to the size of the hole in the aperture mask, assuming that there are no obsrtuations between the hole and the primary mirror, and the focal ratio also changes because that its the ratio between the aperture and the focal length.

In your example, without the mask the scope is a 12" F/5 with a focal length of 60". With the mask it's a 6" with a focal length of 60" so an F/10.

In practice I've only managed to squeeze a 4.25" aperture out of my 12" dob without part of the secondary or a support vane getting in the way.

[Moonshane]

Light comes in a parallel line Steve so it's just the but exposed that reflects the light.

To avoid the obstructions is best so perhaps 120 MM is the max

[Merlin66]

Steve,

The reduced aperture (say 6") will mean you are using effectively a smaller mirror ( can affect the resolution etc.), but the focal length remains unchanged. This means the f Ratio goes from f5 to f10.

[Peter Drew]

The off axis portion of a paraboloid is not a true figure of revolution so will not quite emulate an unobstructed refractor of equal aperture. Still pro rata better than an obstructed aperture. 

[bomberbaz]

Thank you gentlemen, I figured as much although I perhaps didnt explain it very well. Just measured the aperture and actually its 130mm, there will be a slight secondary obstruction. I will see how I go and if its a problem I can always shrink the aperture down with some stiff card from the inside so it won't spoil the look.

[russ.will]

The off axis portion of a paraboloid is not a true figure of revolution so will not quite emulate an unobstructed refractor of equal aperture. Still pro rata better than an obstructed aperture. 

Peter,

One thing I'd like to know - Say you have truss tube Newt, where you can either mask off-axis in front of the secondary, or centrally in front of the primary (I realize it's theoretically possible with a solid tube) which is better? A 4" off-axis mask at the secondary end, or a central 4" mask at the primary mirror?

I'm using the 4" value based on roughly the unobstructed diametre between the secondary and OTA wall of my old 12" Newt.

Russell

[Moonshane]

I don't think it matters Russell.

[ollypenrice]

What happens to resolution if you make two opposed holes in the mask?

Olly

[russ.will]

I don't think it matters Russell.

I'm not so sure and I'm sure I've read something on the subject somewhere, that suggests a mask nearer the primary is preferable - It can certainly be bigger, but then there's the central obstruction to consider as the flip-side to that coin.

I'll see if I can find the article, but with an oracle like Peter to hand, I thought I'd ask!

@ Olly - That's an interesting thought and it sounds very much like the Astro Engineering Focus Master which uses three off-axis apertures.

Russell

[John]

My understanding of the point that Peter is making is that the ideal off-axis newtonian will have a mirror that is cut from the off axis portion of a larger "parent" mirror. This gives it the correct figure for the job:

http://www.dgmoptics.com/oa_what_is.htm

That said, masking my 12" F/5.3 down to just over 4", off axis, gave views that were strikingly similar to the ones my Vixen ED102SS gives, ie: with those crisp airy disks and single faint diffraction ring. Resolving power seemed to match the 4" refractor as well.

My mask is a very sophisticated piece of corrugated cardboard with a hole cut in it that "misses" the secondary and secondary supports

[Peter Drew]

My off-axis aperture point was that a sample taken between the centre and edge of a paraboloid will have a variable slope across the diameter of the sample as a parabola changes slope from centre to edge, this being the desired effect at full aperture to correct for spherical aberration. An off-axis sample of a sphere would maintain the same charactristics of the parent sphere but would need to be of long effective focal ratio to suppress SA.

I've not noticed a difference in performance using masks at different distances from the primary but what I have noticed is that if the mask is close to or touching the primary, if you remove it after a while and then star test then you will see the area that was exposed imprinted on the image due to differential cooling. A lot of interesting effects on the cheap.    

[John]

Would it be possible that a mask close to the primary surface would impede the dispersal of the boundary layer of air just above the mirror ?

[Peter Drew]

Quite possible. Lots of scope for experiment!