What does this bit do? (hole in the lens cap)

[imakebeer]

A couple of questions actually:

 

First, as per the title, both my new SW 150PDS and our SW Capricorn refractor have a lens cap with a hole and a smaller lens cap. What's the point of that? I note that with only the smaller lens cap removed you can still see pretty well through the scope. Is it something to do with heating/cooling the scope, or dew maybe? Or something to do with limiting the amount of light you're capturing with bright objects maybe?

 

Second, the 150PDS comes with this mahoosive 2" eyepiece - "Eyepiece Supplied: 28mm 2" LET" according to the FLO blurb. I tried Googling LET but couldn't find what it means. Anyway, the rubber eyepiece shroud (?) screws in and out - it doesn't zoom but rather seems to change the field of view wider/narrower. Why's that then?

TIA 👍🔭

[wookie1965]

The little cups are for when you are looking at a bright object I. E the moon it doesn't let all the light in so you can view the moon without it being so bright. 

The let has a twist up eyecup to help if you wear glasses most eyepieces have this or a rubber eyecup. 

[fwm891]

The two holes when exposed will help you with focusing. As you rack the EP in and out two images of a star will move together/apart when together you are in focus (don't use a double star 🤔!) You can also use just a single hole for very bright (Moon) objects as a way of cutting down it's brightness. THE SUN will still need the proper solar film over the single hole.

[AstroMuni]

10 minutes ago, fwm891 said:

As you rack the EP in and out two images of a star will move together/apart when together you are in focus

I learn something new everyday 👍 So this could do the trick without need for a Bahtinov mask? I have always wondered about the 2nd opening which seems to be sealed.

Edited November 30, 2022 by AstroMuni

[imakebeer]

8 minutes ago, wookie1965 said:

The let has a twist up eyecup to help if you wear glasses most eyepieces have this or a rubber eyecup. 

Thanks 👍 Any idea what LET actually stands for by the way? I wear glasses but I can't immediately see how the twist up eyecup helps, early days yet though.

8 minutes ago, fwm891 said:

The two holes when exposed will help you with focusing.

It's actually just a single hole on the PDS - I think the second one is just something to grab. I just double checked though, and it's definitely fixed. But I get what you mean about focusing, a bit like a Bahtinov mask I suppose.

9 minutes ago, fwm891 said:

THE SUN will still need the proper solar film over the single hole.

Noted 👍 My eyesight is dodgy enough as it is without me going staring at the Sun though. Night time only for me 😁

[Knighty2112]

If it’s anything like my 250pds only one of the smaller caps come off on the large lens cap. The other one as far as I can tell is just to assist in taking off the whole large cap off the scope when in use. As others have said the one that comes off if for reducing brightness of the moon, but also useful for when viewing Jupiter and Venus as these can be bright too when viewed.

[imakebeer]

3 minutes ago, AstroMuni said:

I learn something new everyday

Me too. In fact if it ever stopped being cloudy I might learn 2 or 3 new things every day! 😂

[PeterC65]

9 minutes ago, fwm891 said:

The two holes when exposed will help you with focusing. As you rack the EP in and out two images of a star will move together/apart when together you are in focus (don't use a double star 🤔!) You can also use just a single hole for very bright (Moon) objects as a way of cutting down it's brightness. THE SUN will still need the proper solar film over the single hole.

On my 150PDS only one of these is a hole. The other is part of the moulding which, I assume, is meant to help remove the cover.

[Mandy D]

1 minute ago, Knighty2112 said:

If it’s anything like my 250pds only one of the smaller caps come off on the large lens cap. The other one as far as I can tell is just to assist in taking off the whole large cap off the scope when in use. As others have said the one that comes off if for reducing brightness of the moon, but also useful for when viewing Jupiter and Venus as these can be bright too when viewed.

The other fitting on the large cover is to place the small cap on when it is not covering it's hole. Try it, you will find it is a perfect fit.

[AstroMuni]

2 minutes ago, Knighty2112 said:

The other one as far as I can tell is just to assist in taking off the whole large cap off the scope when in use.

Atleast thats what I use it for 🙂

[AstroMuni]

And thinking a bit more, the number of openings should NOT create multiple images (in theory). I am just theorising here and someone with more knowledge of optics will probably prove me wrong.

EDIT: They might create diffraction lines

Edited November 30, 2022 by AstroMuni

[Tomatobro]

5 minutes ago, Mandy D said:

place the small cap on when it is not covering it's hole

Well I never.........So it does

[Cosmic Geoff]

Most of the Sky-watcher scopes have this feature in the lens cap - a single central hole +cap for a refractor, and an offset hole + blank holder for a reflector.  But what's it for? Clearly it is meant for stopping down the aperture to around 2 inches (otherwise why offset it to avoid the secondary mirror of a refractor?). 

But what is it for? Nothing to do with focusing, as there is only one hole, not two.

Reducing the brightness for Moon viewing? This is a dumb idea, as you don't need to reduce the brightness for Moon viewing (it is about as bright as a black asphalt road in sunlight) and if you think you do, you could accustom your eyes to the glare, (as discussed in a recent thread elsewhere in the forum), or use a Moon filter.

Reducing the aperture to improve the image when the seeing is poor?  Another fairly dumb idea. Why turn your expensive scope into the equivalent of a cheap long-focus one of 2 inch aperture? If you think this works for you, carry on.

Another thing you can use it for is to stick some solar film behind the hole. This is quick to do, and the cap is not going to fall off at the wrong moment.  The disadvantage is that you have reduced the aperture to 2", which may cost resolution even with poor seeing caused by the heat of the sun.

[vlaiv]

24 minutes ago, AstroMuni said:

And thinking a bit more, the number of openings should NOT create multiple images (in theory). I am just theorising here and someone with more knowledge of optics will probably prove me wrong.

EDIT: They might create diffraction lines

It will. Here is why:

It will create double image when out of focus, but single image when in focus.

[vlaiv]

Just to add - I'd never focus like that . Too imprecise in my view.

[Louis D]

1 hour ago, imakebeer said:

Thanks 👍 Any idea what LET actually stands for by the way? I wear glasses but I can't immediately see how the twist up eyecup helps, early days yet though.

Actually, it helps if you don't wear eyeglasses at the eyepiece.  It twists upward to reduce usable eye relief to place the eye at the correct distance to avoid blackouts.

I've always presumed the LE of LET stands for Long Eye (relief).  Maybe relief starts with a T in some other language?  There are multiple long eye relief eyepiece lines labelled as or with LER, but this is the only one using LET.

[AstroMuni]

2 hours ago, vlaiv said:

It will create double image when out of focus, but single image when in focus.

Thanks. Sounds like it would need to be badly out of focus to create 2 distinct images is my guess. And it would help if there was only 1 star in the FOV.

As you approach closer it would create close overlap and diffraction spikes?

You have piqued my curiosity and I have to now try it out and see what it looks like vlaiv. 🙂

Edited November 30, 2022 by AstroMuni

[vlaiv]

4 hours ago, AstroMuni said:

As you approach closer it would create close overlap and diffraction spikes?

Not sure about spikes.

Yes, it will grow closer and it will do some sort of interference, but I have no idea what sort of pattern one would get.

We can run some simulations to find out, or - it can be tested in the field - whatever is easiest

4 hours ago, AstroMuni said:

You have piqued my curiosity and I have to now try it out and see what it looks like vlaiv. 🙂

Yes, it is interesting topic - especially if you consider that similar (or even the same) approach is used in speckle interferometry.

https://en.wikipedia.org/wiki/Speckle_imaging

Quote

Speckle interferometry

In 1970, the French astronomer Antoine Labeyrie showed that Fourier analysis (speckle interferometry) can obtain information about the high-resolution structure of the object from the statistical properties of the speckle patterns.^[8] Methods developed in the 1980s allowed simple images to be reconstructed from this power spectrum information.

One more recent type of speckle interferometry called speckle masking' involves calculation of the bispectrum or closure phases from each of the short exposures.^[9] The "average bispectrum" can then be calculated and then inverted to obtain an image. This works particularly well using aperture masks. In this arrangement the telescope aperture is blocked except for a few holes which allow light through, creating a small optical interferometer with better resolving power than the telescope would otherwise have. This aperture masking technique was pioneered by the Cavendish Astrophysics Group.^[10][11]

One limitation of the technique is that it requires extensive computer processing of the image, which was hard to come by when the technique was first developed. This limitation has faded away over the years as computing power has increased, and nowadays desktop computers have more than enough power to make such processing a trivial task.