The newtonian cap

[matt c]

Hi all just a quicky, whats the small cap for on the cap of a Newtonian scope. The trouble is i can't really think of a reason why you'd need a hole in it, why would you need such a small hole on such a large scope?

[yeti monster]

Is it about 1" diameter, in the OTA objective end cover?

That would be to reduce the aperture for when looking at the full moon, at least that's what i have on my Prinz newt.

[Helen]

The main reason I think is that it allows you to add a solar filter eg a Mylar filter for safe sun viewing - NEVER use either a filter at the eyepiece end or an unfiltered scope to view the sun.

Helen

[LukeSkywatcher]

Yep a cap within a cap sounds like its the same as on my Refractor. Its to step down the apeture so you cut down on the brightness of the moon. I cant think what else it is for.

[matt c]

Seems to be 50mm, just wouldn't think it would have much resolving power for to seeing anything interesting on the moon. What do you think would be the maximum magnification for 50mm?

[AndyH]

Hehehe.

Believe it or not, traditionally these were for viewing the sun with an eyepiece filter:icon_eek:

An absolute no no. Pretty bonkers if you ask me.

If the filter cracks with the heat...............

Tal's scopes, used to provide an extra cap and filter for this purpose.

Madness.

Best thing to do is buy some Baader solar foil and stick it to the inside of the cap. Just make sure it's stuck well down before every use.

Andy

[AndyH]

For the moon a grey or variable grey filter works well. Helps you stop going moon-blind in one eye.

Andy.

[AndyH]

I have a Tal Alcor 65mm newt(pics in my folder in my sig).

I remember getting it delivered to my work and at break time setting it up for a wee shot. I thought, what could the harm be. Stuck the sun filter on the eyepiece. Nice view, but moments later I hard a quiet cracking sound. Luckily for my right eye, I pulled my self out the way in the nick of time. Stupid, stupid boy.

Andy.

[yeti monster]

Try it with the full moon (if you can see the moon for the cleods), first without, then with. You notice quite a change in contrast with the aperture reduced.

[LukeSkywatcher]

Hehehe.

Believe it or not, traditionally these were for viewing the sun with an eyepiece filter:icon_eek:

An absolute no no. Pretty bonkers if you ask me.

If the filter cracks with the heat...............

Tal's scopes, used to provide an extra cap and filter for this purpose.

Madness.

Best thing to do is buy some Baader solar foil and stick it to the inside of the cap. Just make sure it's stuck well down before every use.

Andy

Now THAT would have been a hell of a lot easier then constructing the whole filter casing out of card.

[LukeSkywatcher]

Try it with the full moon (if you can see the moon for the cleods), first without, then with. You notice quite a change in contrast with the aperture reduced.

It was about 6 months after i got my scope that i noticed the whole cap comes out of the dew shield. I had been using my 90mm scope with only the middle cap removed. My first light of the moon with the whole cap off nearly blinded me. I've used a moon filter ever since. Image using a scope for 6 months with only the smaller inner cap removed.

[AndyH]

Try it with the full moon (if you can see the moon for the cleods), first without, then with. You notice quite a change in contrast with the aperture reduced.

Interesting

It would make your scope into a veeery long focal length one.

eg: My Tal 1 is 110mm F7.3(Fl=806mm) The hole on the extra cap is 45mm. This would make the Tal a 45mm F17.9 !!!!!!!!!!!!!!!

AND when the cap is fitted, twist it so the hole is between the secondary vanes. So basically it should perform like a refractor with no obstruction in front.

Never thought of doing that and Tal has no mention of using it for the moon in the manual, only solar viewing with the black filter.

Excellent tip

I shall try it tonight, clouds permitting and report back.

Cheers,

Andy.