Front Dust Cover

[DemonicOmelette]

My front dust cover (lens cap? What's it called?) has two smaller caps on it. Why's this? It's the same as the cap visible in the photo in this thread:

http://stargazerslounge.com/showthread.php?t=64137

-eli

[crepitis]

One of the caps is detachable and covers what is known as an "aperture mask" , others more knowledgeable than me will explain what that does, and the other "cap" is fixed to the cover and is just for putting the detachable one on so that you dont lose it...I think.

Heres a link to what an aperture mask does. http://www.birdbino.com/main/definition.asp/catalog_name/Astronomics/category_name/tegugu99x3m6f9lghe8/Page/1

[Merlin66]

Most of the covers seem to have a 2" capped opening.

Over the years this has been used by amateurs to provide a reduced aperture when observing the Sun. You still need a ND5 filter before the eyepiece.... don't recommend it; a White Solar Screen (ie Baader) is much much safer nowadays.

[Terry]

Its also useful for reducing the glare when obsering the full moon. I tried it and it does work.

[DemonicOmelette]

Right, that makes sense. Perhaps because I have no working knowledge whatsoever of optics, but I am baffled by the fact that you can twist the cap all the way around, with no effect on the image. I would think that if the hole is at the top, there would be a darker area at the bottom, and vice versa. Hmm..

[Merlin66]

When the aperture stop is smaller that the telescope's objective or Mirror, it doesn't matter where it is radially, the light will always come to the same focus. The Hartmann focus mask uses 3 holes at 120deg apart and when the optics are in perfect focus all three "dots" merge into one! Magic!!

[Astro_Baby]

The aperture cap serves three purposes really

1/ provides a smaller objective for solar observing with the right film and filters NEVER look at the sun without safety filters sold to you by someone competent.

2/ Reduces glare (sort of) because of (3)

3/ The cap increase the focal ratio of the telescope. Lets say you have a Sky-Watcher 130PM. The scope has a primary optic ( its main mirror ) of 130mm and a focal length of 650mm. So - 650/130=5 so the scope has a ratio of f5.

Now put the front cap on and uncover the small hole. The hole is probably about 40mm so now do the focal ratio of 650mm divided by the effective size of the primary (now reduced to) 40mm and you get 650/40=16.25 so you have a ratio of f16.25