F-ratio Calculation for Telescope Cover Modification

[mishima]

Hi, I recently made a little cover for my telescope. It was just a piece of wood with a 15/16 inch (24 mm) hole in it. I was wondering how to calculate the new f-ratio.

Original focal length: 1370 mm
Original primary aperture 150 mm

Is it as simple as 1370/24=57? Just treat the hole like the new primary?

 

[Drakkith]

Is it as simple as 1370/24=57? Just treat the hole like the new primary?

Yep!

 

[sophiecentaur]

You could probably improve the image quality if you make a bigger hole in the wood and have a thin sheet to define the aperture. There is probably detectable flare due to the 'thick' iris that the wood is giving you.

 

[mishima]

Hmm, could flare be mistaken for out of focus? I might have had something like that. Any recommendation for the thin sheet material? What about something like very thin brass?

 

[sophiecentaur]

Hmm, could flare be mistaken for out of focus? I might have had something like that. Any recommendation for the thin sheet material? What about something like very thin brass?

I guess flare can produce poor contrast and edge effects (I believe). that could look like a focus problem, perhaps.
The material for your iris could be whatever you are happiest to work with, as long as it's stable. To get a good circular hole in a thin sheet, it could be an idea to sandwich (squeeze) the sheet between two thicker sheets (wood?) and drill through the lot. That will avoid the thin sheet getting buckled.
BTW, what is the purpose of this small aperture?

 

[pixel]

BTW, what is the purpose of this small aperture?

This? https://www.astronomics.com/aperture-mask_t.aspx

If so, how does the fact that the smaller aperture is offset from the optical axis affect the calculation of f-ratio?

 

[Drakkith]

This? https://www.astronomics.com/aperture-mask_t.aspx

If so, how does the fact that the smaller aperture is offset from the optical axis affect the calculation of f-ratio?

It doesn't. The standard definition of f-ratio is the focal length divided by the diameter of the aperture. The placement of the aperture makes no difference according to this definition, though it will affect how the final image looks, even if that effect isn't noticeable by just looking through an eyepiece.

 

[sophiecentaur]

This? https://www.astronomics.com/aperture-mask_t.aspx

Oh yes, of course. I have a similar hole in my Newtonian.
But, of course, that is not a sharp edge- there is a10mm lip around it. Perhaps the design is more for strength than for optics. Most people use a Moon Filter, for serious Moon watching, which is neutral density and let's through less than 10% and allows the full aperture of the scope.

 

[mishima]

Yep, it was for the observing the eclipse. I'll try it on Moon when it starts waxing again. I suppose that makes sense that aperture placement doesn't matter, its position doesn't appear anywhere in the equation.

 

[pixel]

It doesn't. The standard definition of f-ratio is the focal length divided by the diameter of the aperture. The placement of the aperture makes no difference according to this definition...

Agreed. I was thinking of the usual ray trace method of finding the aperture stop for an object on the axis and how it assumes circular elements centered on the axis. But yes, the standard definition of f-ratio would just use the diameter of the reduced aperture. Thanks.

 

[sophiecentaur]

Yep, it was for the observing the eclipse.

With some additional filtering, I hope - or you probably wouldn't be reading this.

I think the offset hole would probably have worse Coma in the image than if it were nearer axis. I never thought to look at stars through it to see. I guess a single hole needs to be offset somewhat or it would be right in front of the secondary! I must measure it up when I remember.
Talking about the Moon waxing. Needless to say, the skies have been really bad around the new Moon. Ain't that just the way?!

 

[Drakkith]

I think the offset hole would probably have worse Coma in the image than if it were nearer axis.

Hmmm. Perhaps, but it probably has better coma than the full aperture.

 

[sophiecentaur]

Using the end cap is certainly a lot more convenient than taking off eyepieces and inserting a filter. (that is if you don't have a wheel)

 

[mishima]

it probably has better coma than the full aperture

It's a Ritchey-Chretian so no coma on the full aperture. I really have no idea optically how introducing the small aperture affects aberrations in RC scopes and would love some insight there.

I guess a single hole needs to be offset somewhat or it would be right in front of the secondary!

Indeed, mine's just a 150mm aperture and the secondary covers a good percentage of that. I only had maybe 5-6 cm clearance to fit my little solar aperture in. I cut some eclipse glasses in half for the filter. Probably not the safest, but I was not doing visual observations, just DSLR. My camera is a cheap piece of junk so I took the risk for the once in a decade opportunity. Turned out to be ok and had a great experience with students.

 

[Drakkith]

It's a Ritchey-Chretian so no coma on the full aperture. I really have no idea optically how introducing the small aperture affects aberrations in RC scopes and would love some insight there.

As far as I know, increasing the f-ratio nearly always decreases any aberrations present in the system.

 

[sophiecentaur]

As far as I know, increasing the f-ratio nearly always decreases any aberrations present in the system.

I think that would only apply with an 'axial' aperture. The off axis contributions are worse, I think but can cancel if they are all there.

 

[Drakkith]

I think that would only apply with an 'axial' aperture. The off axis contributions are worse, I think but can cancel if they are all there.

Good point. Constricting the aperture near the edge is not the same as constricting it near the center.

 

[sophiecentaur]

Years ago I was involved with DBS and I remember that there was some advantage for offset feed dishes. Can't remember what but it could just have been no blockage by the feed. Same thing as the hole in the OP's cover! Lol.

 

[Drakkith]

Years ago I was involved with DBS and I remember that there was some advantage for offset feed dishes. Can't remember what but it could just have been no blockage by the feed. Same thing as the hole in the OP's cover! Lol.

Yes, I think that's the only advantage if I remember correctly.

 

[pixel]

As far as I know, increasing the f-ratio nearly always decreases any aberrations present in the system.

Except distortion (considering a centered system as per sophiecentaur).

 

[sophiecentaur]

Yes, I think that's the only advantage if I remember correctly.

There was something else about polarisation but it's all a bit fuzzy now. From the pictures you see around, the big dishes are all (?) centre fed and the small ones offset.

 

[mishima]

What if instead of 1 off-axis small aperture, I did something like 4 symmetric small apertures distributed around the axis? How would that affect f number and aberrations?

 

[Drakkith]

There was something else about polarisation but it's all a bit fuzzy now. From the pictures you see around, the big dishes are all (?) centre fed and the small ones offset.

I don't think it has anything to do with polarization. At least I don't remember hearing anything about that in my Air Force training on these kinds of dishes, but it's possible I'm just not remembering correctly. I think the only reason they're offset is to keep the dish size down. Large dishes can be center fed without blocking a significant portion of the dish, while small dishes can't.

What if instead of 1 off-axis small aperture, I did something like 4 symmetric small apertures distributed around the axis? How would that affect f number and aberrations?

Then you'd have to start calculating f-number differently. It wouldn't just be 4x the diameter of each aperture, it'd have something to do with the total area and the diameter of an aperture with the equivalent area. As for aberrations, it'd probably be better than a single offset aperture, as you'd have symmetry around the axis once again.

 

[sophiecentaur]

I don't think it has anything to do with polarization.

Cross polar discrimination, iirc. But I cannot remember if it was better or worse with offset. But we're not Stargazing here.