Hole on Scope Cap?

[UnboxingReviews1997]

Hello, My name is Ben,

When i brought my Celestron NexStar 130 SLT Scope IT had dawned on me that the main scope cap has a hole in it with a removable cover, Would it be possible for someone to tell me what it is?. I have included a picture below.

Regards,

Ben Rolfe

[acey]

It's for viewing with a reduced aperture, set off-centre because the secondary would block it if the hole were in the middle. Usually used for solar viewing (with safe solar filter over the hole). The other raised part is just to make it all look symmetrical.

Stopping down the aperture can also be a way of reducing aberration and light scattering, though the hole in this case is very small and you may not get much of a view. No harm trying it on Jupiter, say, just for fun.

Edited January 23, 2014 by acey

[knobby]

Hi Ben... It allows you to view brighter objects without being to bright... The moon for example is pretty glarey when full.<br />

do not look at the sun though. You need a specific filter for the sun!

[UnboxingReviews1997]

It's for viewing with a reduced aperture, set off-centre because the secondary would block it if the hole were in the middle. Usually used for solar viewing (with safe solar filter over the hole).

Thanks very much, in what way would it help with solar viewing?

would it help to make planets and galaxies like andromeda much clearer? please can you explain?

[UnboxingReviews1997]

Hi Ben... It allows you to view brighter objects without being to bright... The moon for example is pretty glarey when full.<br />

do not look at the sun though. You need a specific filter for the sun!

Thanks alot!

Like i asked Acey, would it make viewing planets and galaxies like M31 easier to see?

[EBXL5]

It's for viewing with a reduced aperture, set off-centre because the secondary would block it if the hole were in the middle. Usually used for solar viewing (with safe solar filter over the hole). The other raised part is just to make it all look symmetrical.

The other raised part on the telescope blanking cap is usually for putting the smaller cap on so you don't lose it. Handy huh.

[rowan46]

Thanks alot!

Like i asked Acey, would it make viewing planets and galaxies like M31 easier to see?

it can help contrast a little and it can help with magnification as it in effect changes the focal ratio so it can help with brighter planets. but you still need apparture for dso's so stopping down for dso's is not such a good idea.  people do put baader solar film over it to look at the sun but thats not its primary function. If you do use it for such a purpose make sure the film is securely attached to the cap. A friend of mine used to do it until one time he turned to get an eyepiece and found it had come unstuck from the cap. fortunately it didn't happen while he was actually looking at the sun and he now uses a full apparture filter just to be safe. Moral be very careful when looking at the sun.

Edited January 23, 2014 by rowan46

[Bryan h]

I just brought a 12 dob and I also couldn't work why the the main cap had a hole in it.... You learn something new everyday..

Still new to this astronomy stuff.

[jnb]

The other raised part on the telescope blanking cap is usually for putting the smaller cap on so you don't lose it. Handy huh.

Though if the raised part were another hole then it would also allow the cap to be used as a hartmann mask.

[allcart]

For solar viewing you would cover the small hole with Baader solar film. The easiest way to do this would be to make a small tube to fit over the hole. This tube will have Baader solar film covering one end. You fit this over the hole on the scope cap so that you can observe the sunspots. Never attempt to look at the sun without the solar film cap on, and cover or remove the finder scope or it will melt very quickly.

[cotterless45]

Just using the small hole gives you cracking views of planets and is ace on double stars. Make sure that you line it up to give a clear view of the primary, avoiding the focuser,

Nick.

[jnb]

Just using the small hole gives you cracking views of planets and is ace on double stars. Make sure that you line it up to give a clear view of the primary, avoiding the focuser,

Nick.

Why should that be the case? Reducing the aperture reduces resolution so why should a smaller aperture improve the view.

[John]

Why should that be the case? Reducing the aperture reduces resolution so why should a smaller aperture improve the view.

The smaller aperture reduces the effects of atmospheric disturbance and in the case of newtonians the diffraction caused by the secondary and it's supports so the image you do see does seem sharp and steadier. You are quite right though that the resolution and light grasp of the instrument are reduced the the size of the hole in the cap so in most cases you now have a 2" or smaller aperture scope so the amount of contrast, detail and resolution you will see will be reduced to that level as well. There is no "free lunch" so to speak !

[rowan46]

it works better with big reflectors but it effectively takes any loss of contrast and diffraction spikes out of the equation as there is no lightspilling round the secondary

[jnb]

The smaller aperture reduces the effects of atmospheric disturbance and in the case of newtonians the diffraction caused by the secondary and it's supports so the image you do see does seem sharp and steadier. You are quite right though that the resolution and light grasp of the instrument are reduced the the size of the hole in the cap so in most cases you now have a 2" or smaller aperture scope so the amount of contrast, detail and resolution you will see will be reduced to that level as well. There is no "free lunch" so to speak !

So if I have a 6" scope it will show up diffraction spikes and atmospheric effects whereas a 2" scope won't?

Or put another way if I have a 6" scope it's resolution is 1" while a 2" scope has a resolution of 3" so it presumably removes effects smaller than 3"? If that's the case though then is the improvement real? For example if I was using a 150mm / 1000 scope with a dslr with 5µ pixels that is 1" resolution from both scope and camera. I could stop the aperture down to 50mm to overcome effects but couldn't I just as well use 3x3 binning to obtain the same effect but not lose the light gathering power?

[rowan46]

You line the hole up so that it does not cross the spider its the spider that causes the diffraction spikes. So if you had a 12" scope and a 4" hole you would in effect have a 4" scope with no central obstruction in effect a 4" apo this technique is usually used on planets visually to aid with contrast

Edited January 23, 2014 by rowan46

[John]

So if I have a 6" scope it will show up diffraction spikes and atmospheric effects whereas a 2" scope won't?

Or put another way if I have a 6" scope it's resolution is 1" while a 2" scope has a resolution of 3" so it presumably removes effects smaller than 3"? If that's the case though then is the improvement real? For example if I was using a 150mm / 1000 scope with a dslr with 5µ pixels that is 1" resolution from both scope and camera. I could stop the aperture down to 50mm to overcome effects but couldn't I just as well use 3x3 binning to obtain the same effect but not lose the light gathering power?

I don't know how it applies to imaging as I only do visual observing.

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.

[itmo]

"stopping down" a telescope loses some resolution, but it also makes it easier to focus. IIRC the "large scopes suffer more from atmospheric effects" is [removed word]. The problem is that usually the large scopes are dobs which are fast (difficult to focus and collimate) and miscollimated. Stopping them down makes it easier to focus and makes collimation less critical. Also most 10 or 12 inch scopes have so good resolutions that you would have to be on top of mauna kea to get full use of the resolution all the time. So stopping down a 12incher to 6 or 8 doesnt necessarily lose you too much resolution in already so-so 1.0 to 2.0 arcsecond weather. 

[cantab]

Stopping a scope down also increases the f-ratio. That might reduce eyepiece aberrations, especially if the scope is fast normally.

[itmo]

But remember that stopping down also requires recollimation of main mirror AFAIK.

[itmo]

http://www.skyandtelescope.com/howto/basics/3304176.html?showAll=y

Here is one of the articles analyzing seeing effects on scopes of different size. And there is the most important part too, usually with bad seeing you still have short moments of very good seeing in between. If you stop down the scope you will lose these brief moments of clarity. 

[John]

But remember that stopping down also requires recollimation of main mirror AFAIK.

Why ?

I don't seem to need to re-collimate my 12" when I stop it down to 4".

[acey]

To expand a little on my earlier post - stopping down the aperture of a refractor or reflector is a traditional way of reducing the effects of aberration and light scattering. It may be used for various reasons:

1) If the objective lens or primary mirror is not properly figured overall, or has major imperfections, you can reduce to using a part of it that is of sufficiently good quality. In a refractor this means blocking off the outer ring of the objective, in a reflector it means using any part of the primary that is not obstructed by the secondary or spider (which would reduce light or cause diffraction).

2) By reducing the aperture you effectively increase the focal ratio, so if there is coma in the objective or primary then the effect of this will be reduced.

3) If the target object is very bright then scattered light in the telescope can obscure the view. Reducing the aperture reduces this effect and can sometimes give a clearer view.

4) A small bonus in the case of a reflector is that there will be no diffraction spikes due to secondary/spider obstruction.

The obvious downside is that you are reducing the aperture, hence reducing the light grasp and resolution of the scope. In some cases the above points outweigh the downsides - a scope with larger aperture and better theoretical resolution may in practice give poorer views because of light scattering. Put a top quality 4" refractor alongside a basic 10" dob and you may well find that the 4" gives better planetary views than the 12". In practice I suspect there are few cases where the upsides outweigh the downsides, and in general you would anyway want a bigger hole (larger percentage of available aperture) than the one provided in the dustcap.

The idea that larger apertures suffer more from atmospheric turbulence is an old myth, and has nothing to do with the present discussion.

The presence of these holes on dust caps is, I think, largely a throw-back to earlier times, and I reckon most people never use them. Solar observing can be done with a filter (e.g. Baader solar film) across the whole aperture or the smaller aperture - the latter saves you money on solar film, and may give sufficient resolution for a satisfactory view. Full aperture will give higher resolution. In either case the film needs to be secured in place with tape so it doesn't blow off. If you ever do suffer the misfortune of solar film coming off while viewing the sun, your blink reflex will almost certainly save you from anything more than a nasty fright. Far more perfidious would be a dodgy filter which reduces visible wavelengths but is letting through infra-red and ultraviolet, damaging your eye without you noticing. I'm not aware of any dodgy filter film out there but you never know.

In the case of a very bright target like the Moon or Jupiter, the small hole might give a better view if there is a lot of light scattering in the scope. Though in that case perhaps there might be a better way to deal with light scattering, and a neutral density filter ("moon filter") might give better results anyway.

I suppose one other benefit of using the little hole would be that you won't get any dewing on your optics, but that wouldn't be sufficient reason for doing it. The reduced aperture means that the technique is completely useless on faint objects (i.e DSOs) - you would only ever consider using it on something bright, and only if it were going to offer any kind of advantage, which in most cases it won't.

[Alien 13]

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

[itmo]

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