Hole on Scope Cap?

[acey]

I would recollimate it since the center of the aperture has moved. 

I wouldn't, since the optical axis hasn't moved.

[John]

I wouldn't, since the optical axis hasn't moved.

Exactly. 

[jnb]

It's just occurred to me what the real reason is for those holes is. It's used by manufacturers to fill new 'scopes up with clouds! Obvious when you think about it!

[Caldi]

On 25/01/2014 at 12:00, Alien 13 said:

Another effect already touched on is the increased depth of field when the scope is stopped down the same way  a camera works with different f stops on its lens.

If you where to focus jupiter at the scopes maximum aperature you may only get it 95% right but if you then stop the scope down the extra depth will bring the focus to 100%.

Alan

Could this be correct? using the average Earth-Jupiter distance and Jupiter radius I consider it to be equivalent to saying you could have a persons nose out of focus but their ears in-focus at 30 mt from the camera!

[Alien 13]

20 minutes ago, Caldi said:

Could this be correct? using the average Earth-Jupiter distance and Jupiter radius I consider it to be equivalent to saying you could have a persons nose out of focus but their ears in-focus at 30 mt from the camera!

Long time ago but what I probably meant was that it improves the depth of focus range making adjustments less critical.

Alan

[Caldi]

22 hours ago, Alien 13 said:

Long time ago but what I probably meant was that it improves the depth of focus range making adjustments less critical.

Alan

Thank you for explaining that, I was confused

 

[JOC]

I don't think I'd re-collimate - I don't use the reduced hole very often (some people might find it takes some of the brightness out of things like the moon, but I've never had issues with the moon anyway) and re-collimation is a lot of fiddling about if you immediately take the cover off as you decide you are probably better off without it.  I just drop the cover on and off again without worrying and don't see any difference that re-collimation would solve.

[domstar]

Funnily enough I've just been looking a Venus. It was a horrible mess and when I stopped the aperture down it improved significantly. Unlike the moon there was no detail to miss out on, so I think it worked well. (Shame about the snow storm that sent me inside).

[miguel87]

I cant wrap my brain around this! Need someone with a few more brain cells to explain.

I dont understand how part of the produced final image doesnt include the underside of the cap? With the aperture now being off centre, how does it still appear in the centre of the eyepiece?

If the small aperture still uses all of the primary mirror, how can the aperture make any difference at all? 😖

I guess what I am failing to understand is how the image on the primary (as seen from the secondary) relates to the final image. I presume with no eyepiece in, you could look down the empty focuser and see the off-centre, small aperture surrounded by the underside of the telescope tube cap? If the eyepiece forms its image from this information, how can we not see the cap.

My twisted logic assumes that a smaller aperture reflector has a more curved primary mirror to relate to the width of the opening.

Somebody help my brain!

Mike

[John]

It does work but I've had a couple of glasses of decent wine so I can't attempt to explain the optics behind it. But it does work !

 

[miguel87]

1 minute ago, John said:

It does work but I've had a couple of glasses of decent wine so I can't attempt to explain the optics behind it. But it does work !

 

You will have to attempt to explain it to me on a less eventful evening!

It got me onto thinking about the primary mirror and how when we use higher magnification we must only be looking at a small area of the mirror? 

If that is true, I dont understand how more aperture (adding extra unused mirror) makes that image any brighter.

It must all be down to how focussing works

[Paul M]

While you wait for a sensible answer, here is a brief primer on the nature of light.

Someone will pop along with a ray trace soon and all will be clear. The way I think about it is to consider the wave nature of light. The wave front of the light from the subject enters the offset hole in the scope end cap and as it passes through into the tube the waves act as though the hole was the source and spread out to illuminate the whole primary mirror. 

PS. don't quote me in your PhD

 

 

[miguel87]

I think some further googling has cleared my mind a little. It is wrong of me to think of the image existing on the primary mirror. The image exists at the focal point in the focussed tube and the eyepieces examine this image in different ways, they do not 'look' at the mirror.

All parts of the mirror can see the whole aperture (minus the secondary obstruction). So larger aperture just means any individual object is shining off a larger surface and focussed to a brighter point within the final image.

And as you said before, that is why recolimation is not needed, every part of the mirror sees all of the aperture, so...

stopping down the scope doesnt actually put any part of the mirror in the dark.

[miguel87]

My follow up question would be. If you continue to reduce aperture to silly amounts, say 1 inch or less, do you start to limit TFOV?

Because the mirror physically cant see past the narrowing tube. I imagine being a primary mirror looking at a tiny hole at the other end of the scope. I cant imagine fitting the seven sisters into that view?

[Paul M]

2 minutes ago, miguel87 said:

My follow up question would be. If you continue to reduce aperture to silly amounts, say 1 inch or less, do you start to limit TFOV?

Because the mirror physically cant see past the narrowing tube. I imagine being a primary mirror looking at a tiny hole at the other end of the scope. I cant imagine fitting the seven sisters into that view?

I'd expect that as stopping down of aperture increases it will introduce increasing amounts of vignetting.

 

[miguel87]

13 minutes ago, Paul M said:

I'd expect that as stopping down of aperture increases it will introduce increasing amounts of vignetting.

 

That makes sense. I think I have it, probably wont last long tho!

Thanks for the replies

[Captain Scarlet]

1 hour ago, miguel87 said:

...  thinking about the primary mirror and how when we use higher magnification we must only be looking at a small area of the mirror? 

...

It must all be down to how focussing works

No. The primary mirror projects an image in the focal plane.  Every part of the mirror contributes to every part of the image*. The eyepiece is essentially a very sophisticated magnifying glass on that image.

So when you increase magnification, you’re simply zooming in further on that brightened image.

I.e. the bigger the aperture, the brighter the image you’re zooming in on becomes.

M

* ... for the fully illuminated field around the centre of the image, but that’s beside the point here 

Edited April 17, 2020 by Captain Magenta

[Captain Scarlet]

On 23/01/2014 at 14:39, John said:

... I have an aperture mask with a 4" hole which is the largest I can use with my 12" dobsonian. Using this I get the effective performance of a 4" unobstructed scope with a focal length of 1590mm so F/15.5. 

As I already have a rather nice 4" ED refractor which is actually much smaller and lighter than my 12" dob, it makes more sense for me to get that scope out if the seeing conditions are not good enough for the dob to make use of it's full aperture.

John I apologize for quoting you 6.5 years on, but seeing as you have the gear, that would be a fabulous side-by-side comparison to do (please?)

M

[John]

1 hour ago, Captain Magenta said:

John I apologize for quoting you 6.5 years on, but seeing as you have the gear, that would be a fabulous side-by-side comparison to do (please?)

M

Actually, since then I have done that comparison on a few targets. The image of, say a tight double star, looks pretty much identical through the 12 inch dobsonian stopped down using an off axis cardboard mask with a 10cm aperture. The aperture can be positioned so that it avoids both the secondary vanes and the secondary support so it is an unobstructed 100mm scope.

I would expect the mirror system to transmit a bit less light than the objective lens of the eyepiece - perhaps 90% rather than the refractors 98% so the performance on DSOs might be a little less with the stopped down newtonian.

 

 

[jonathan]

Just as a bit of an aside to this topic, the other night we were observing Saturn from a dome as it set, the scope was pointing so low that only half of the aperture was exposed to sky, the rest was pointing at the inside of the dome, yet the views through the eyepiece were perfectly fine.  It's a bizarre thing to try to get your head around, totally different to our normal concept of vision, even using binoculars.