probably a really stupid question...

[starstalker]

but can someone tell me why on my skywatcher 200p the main large lens cap on the end of the scope has a smaller removable cap built into it?

[fatwoul]

It's for viewing brighter objects like the Moon more comfortably (sometimes the moon can be unpleasantly bright through a telescope).

At least, that's what those are for on all the refractors I've ever had.

[LukeSkywatcher]

but can someone tell me why on my skywatcher 200p the main large lens cap on the end of the scope has a smaller removable cap built into it?

Its there to give you the choice whether to stop down the apeture or not for observing bright objects such as the sun and moon.

*Only ever observe the sun with the correct solar filter in place*

[darkly]

I don't classify that as a stupid question, I wondered too, and I'm sure there's plenty of others who like myself looked, wondered and then gave it 'a damn good ignoring'

As they say, the stupidest question is the one one you didn't ask!!!

[RayGil]

It's also there was Solar Viewing, they used to make a Baader filter that was placed over the opening?

On larger scope with a bigger aperture, it's better to have a smaller cap for Solar Viewing.

At least that's what I've been led to believe.

Ray

[just looking]

lol I Was going to ask the very same question Starstalker

as there are 2 smaller caps in the big cap on mine ;o)

[Merlin66]

The small 50mm (or so) cap is a throwback to the very early days of commercial telescopes and was included for reduced aperture observing of the Sun with a Herschel Wedge and ND filter.

Nowadays it can be covered with a Baader Solar film filter to do the same job.

[John]

The other posters have correctly suggested what the aperture in the tube cap can be used for. For solar work (correctly filtered of course) the smaller aperture is essential.

It's worth remembering that, for other viewing (ie: the Moon, Planets) that viewing through the aperture restricts the scope to the performance (resolution, contrast, etc) of the aperture diameter ie: around 50mm. Personally, apart from solar viewing, I could not see the point of owning an 200mm scope and then restricting it to a 50mm one ! - I'd rather use filtering (at the eyepiece end) on the Moon and planets to reduce their brightness if necessary.

[starstalker]

great!! thanks everybody, just another example as to why this is the best forum on the net for advice to noobies!!!

[LukeSkywatcher]

The other posters have correctly suggested what the aperture in the tube cap can be used for. For solar work (correctly filtered of course) the smaller aperture is essential.

It's worth remembering that, for other viewing (ie: the Moon, Planets) that viewing through the aperture restricts the scope to the performance (resolution, contrast, etc) of the aperture diameter ie: around 50mm. Personally, apart from solar viewing, I could not see the point of owning an 200mm scope and then restricting it to a 50mm one ! - I'd rather use filtering (at the eyepiece end) on the Moon and planets to reduce their brightness if necessary.

Good point. Forget i mentioned stopping down the scope to observe the moon. Use a ND/Moon filter on the EP end.

[hawk eye]

I too was going to ask the same question, the small lens cap on mine is well off centre though so I can't figure how the primary reflects the light to the secondary or am I missing something very obvious?

[just looking]

Thanks guys

Everything comes Clear you see when you ask a Q on here

very helpfull people.

well except the cloudy skies of course ;o)

[fatwoul]

Maybe that's another reason for the small cap in the middle. It means you only need a smaller hole in the clouds to look through.

[Gy Stargazer]

I wondered the same thing a while back while trying to observe Venus.

I had the full cap off and then found that if i'd only taken the middle bit out i'd have seen and probably imaged venus better than just being a really bright light in the sky that looked badly over exposed.

Silly me.

[Merlin66]

Hawk Eye,

The main mirror is designed to reflect any light coming in through the tube aperture to the secondary and hence to the focus.

It doesn't matter where the small opening is the light will still be reflected back.

Ken

[Macavity]

Yup. To help (perhaps?) with the "intuitive leap" - In 2-dimensions, anyway:

[themos]

The reason for off-centre apertures is avoiding the central obstruction of the secondary which diffracts light and reduces contrast. It turns a Newtonian into an APO refractor, effectively.

[starstalker]

oh ok, thats actually pretty cool,shame they didn't actually mention it in the instruction book that came with it!

[great_bear]

lol I Was going to ask the very same question Starstalker

as there are 2 smaller caps in the big cap on mine ;o)

No - there's only one.

You'll find that the second protrusion is designed as just a convenient holder for the cap that comes off the first protrusion - so that you don't lose it.