Jump to content


Basic question about lens cap..

Recommended Posts



So, a basic-probably-stoopid beginners question. On my shiny brand new reflector 'scope, on the lens cap there are two small lens caps set into it.

What are they for?

Link to comment
Share on other sites

Hi there!

Assuming you mean the big dust cover that "closes" your main telescope tube, my understanding is that the smaller caps can be removed, and the bigger cover still left in place, to enable you to reduce the amount of light coming in.

For example, if you were looking at a full moon with the cover completely removed, it would be *very* bright when magnified by the Telescope, and would be hard to look at/see detail. By leaving the main cover on, and removing the smaller ones, you reduce the amount of light coming it, so the moon wouldn't be so bright and you can look at it easier.

On my reflector, only one of the smaller covers actually comes off, the other one is used as something to hold on to when removing the cover.


Edited by Jasonb
Link to comment
Share on other sites

Ahh, thanks. That make perfect sense and is such a clever idea. I'll give that a go next time I'm out.

I got my new telescope delivered last week but I've had a pretty nasty bug so have not been able to play yet... really frustrating but I guess it's building the excitement..

  • Like 1
Link to comment
Share on other sites

Ah yes, and usually it's cloudy for a week or two after you get something anyhow! Hope you're feeling better soon.

This is a great place to find out all you need to know, so ask away here! :)

Link to comment
Share on other sites

Hi Samop and welcome to SGL.

Rule 1 of SGL. There is no such thing as a stupid question.
We have all been there.

If you want to use the scope for basic solar viewing, you leave the cap on and fit an approved filter material over the small hole.
This significantly reduces the amount of energy that is put on the secondary mirror, and the eyepiece.
For example Baader solar film. WARNING...Solar observing is dangerous without the right equipment and your eyes don't get 2 chances!
Make sure the film/filter is secure and can't fly off in a wind.

For viewing a bright moon, I would use a filter to dim at the eyepeice end of things.
By keeping the full size of the mitrror, you can use high magnification.
You can buy a neutral density (ND) filter for fixed dimming.
My preference is a variable polarising filter.

HTH, David.



  • Like 1
Link to comment
Share on other sites

11 minutes ago, Carbon Brush said:

....Rule 1 of SGL. There is no such thing as a stupid question.....


Well there's a challenge!

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.