Aperture mask question

[AlcorAlly]

I enjoy experimenting with various aperture mask sizes, particularly on double stars.

My question is, which placement is better?

A. Aperture mask goes over the dew shield
B. Aperture mask goes close to the objective lens cell and the dew shield then goes over the mask

I imagine A is better for light scatter because it creates a darker chamber inside the dew shield. However, I'm not sure that this placement would alter the optical characteristics of an instrument in the same way that an aperture mask over the objective lens does. I would appreciate any opinions – based on practical experience or theoretical. 

 

Image Credit: Jeff B on cloudynights

 

Edited March 21 by AlcorAlly

[vlaiv]

When focused at infinity - I think there is zero difference between the two.

There might be slight difference in what you suggest - but only if you have significant source of light that is in front of telescope - but not directly - so that light is able to hit interior of dew shield.

[AlcorAlly]

@vlaiv sorry do you mean position A is better if there's significant source of indirect light that is in front of telescope e.g. street lamps? As in better for reducing light scatter?

As for reducing the actual aperture, you don't think that the effects of an aperture mask are more pronounced when the mask is positioned at the location where the effective aperture is defined i.e. at the objective lens?

 

[vlaiv]

1 minute ago, AlcorAlly said:

@vlaiv sorry do you mean position A is better if there's significant source of indirect light that is in front of telescope e.g. street lamps? As in better for reducing light scatter?

As for reducing the actual aperture, you don't think that the effects of an aperture mask are more pronounced when the mask is positioned at the location where the effective aperture is defined i.e. at the objective lens?

 

Right. Position A - better if there is street lamp near by (although if scope is properly baffled and dew shield has mat black paint on the inside - difference will be minimal if at all visible).

No difference between two positions as far as view goes when there are no street lights involved. Sources at infinity don't really care if mask is a bit away from the lens.

This is strictly not true for very wide field use, where edge of the field stars / objects won't hit center of the lens if aperture mask is further away - but for normal use and few degrees of distance from optical axis - everything is the same (here is diagram to explain this last bit):

If incoming light rays are at very high angle and aperture mask is further away - you might end up using edge of the lens rather than center - red arrows in above diagram. For few degrees - this is generally negligible effect (you can calculate the offset - as it is sin(angle) * distance - so for 2 degrees of axis, or 4 degrees of FOV in total and 20cm of distance of aperture mask - offset would be 6.9mm from center)

[AlcorAlly]

Thank you @vlaiv 

Does the size of the offset make any difference relative to the size of the aperture being masked and the size of the aperture stop? 

Example:

Let's say we have a 75mm and a 150mm aperture telescopes and we apply 80% aperture mask to both at a distance of 200mm from the objective. If the focal lengths of the telescopes are the same and we use the same 30mm 100 degree eyepiece we will get about ≈5.24mm offset for both with a 3 degrees FOV. 

Would the 5mm offset make more of noticeable difference to the smaller aperture telescope because it would take a larger proportion of the overall available aperture? What about reducing or increasing the size of the mask? Do we still expect the difference to be negligible regardless of those factors?