Small aperture on lens caps – What’s it for?

[Petrol]

I have often wondered. I had a Newt reflector back in the late 70’s and that had one. Both my SW 200P & SW ST80 have small apertures in the lens caps. I can only assume it’s for aperture reduction?

Thanks for looking.

Pete

[steppenwolf]

I can only assume it’s for aperture reduction?

That is exactly what it is there for. Many of these 'scopes also have a 'false' cap as well - that is for parking the smaller cap on so you don't lose it!

[brantuk]

Hi Pete - yes you can use it to reduce aperture. Looking at the moon anywhere between full and half, can be very bright and blinding. I replace the main cap and take the small one off to reduce the glare.

It can also be used to change the focal ratio slightly - but I never worked out by how much lol

(edited "focal length" to "focal ratio")

[Hypernova]

I have this small aperture opening on the lens cap of my SW 200p F/6 dob which I assume is for fitting of a solar film filter for white light solar viewing. I am not 100% sure of this so any other's thoughts would be appreciated.

[John]

If you use the hole to reduce the aperture the focal ratio of the scope is changed but not the focal length.

Eg: you have a 200mm aperture scope with a focal length of 1200mm - an F/5 focal ratio. You put the end cover on the scope and view through a 50mm hole. You are now using a 50mm scope with a focal length of 1200mm ie: an F/24.

The downside is that your resolution, light grasp and contrast is reduced to that of a 50mm scope as well, so no "free lunch"

[steppenwolf]

I have this small aperture opening on the lens cap of my SW 200p F/6 dob which I assume is for fitting of a solar film filter for white light solar viewing.

It isn't specifically designed for that purpose but lots of people use it for just that - it's certainly convenient provided you can absolutely guarantee that the solar film is safely affixed.

[brantuk]

Thanks for the correction John - yes it's focal ratio. I know one or two people who use an aperture mask with 2 or 3 different sized apertures for that purpose. Mostly large dob owners

[John]

Thanks for the correction John - yes it's focal ratio. I know one or two people who use an aperture mask with 2 or 3 different sized apertures for that purpose. Mostly large dob owners

No probs

I think stopping down the aperture can be useful but the holes in the dust caps that I've seen are generally on the small side. The 150mm F/8 Skywatcher refractors have a more useful aperture cap which opens up a hole around 112mm in diameter giving an F/10.7 scope which is quite nicely colour corrected, although still with the bulk of the 150 OTA though.

[Hypernova]

It isn't specifically designed for that purpose but lots of people use it for just that - it's certainly convenient provided you can absolutely guarantee that the solar film is safely affixed.

If not for solar, what was the hole designed for apart from increasing the f/ratio and lowering the resolution?

[Merlin66]

In the ol' days this was used to reduce the aperture to allow solar observing with a Herschel wedge and eyepiece filter.

Nowadays a Baader solar film is a much cheaper and probably more effective solution.

[John]

...The 150mm F/8 Skywatcher refractors have a more useful aperture cap which opens up a hole around 112mm in diameter giving an F/10.7 scope which is quite nicely colour corrected, although still with the bulk of the 150 OTA though.

Doh ! - I've just realized how unnecessary the above highlighted line is - as if anyone is going to think that removing a small plastic cap will reduce the size of the whole scope

One too many glasses of wine here I think

[Rui]

a 50mm hole is way too small to be an advantage for observing anything but the Sun or Moon. Even for this purpose, the small apperture is a waste of resolution...

But in a larger scope, like a 12" or larger reflector, you can arrange a off-center hole, around 90mm or 100mm. When you rotate the cap so that the light path entering the hole misses the secondary mirror and spider vanes, this off-axis hole will work as a 90mm or 100mm long focus unobstructed telescope that will give you razor sharp views of planets or close double stars.

[steppenwolf]

what was the hole designed for apart from increasing the f/ratio

Nothing, just that! Increasing the focal ratio reduces the light capture making it easier to view bright objects - I use it to cut down the brightness of the Moon without using a Moon filter.

[Petrol]

Thanks guys for a very detailed and comprehensive explanation. Next time I look at the moon, I will give it a whirl. Steve - nice tip with the parking on the “false cap”

Once again, thank you.