Super Nooby Question - Dob Dust Cap

[Stu Ward]

Why does it have a removable smaller cap within it ?

Stu

[that_guy]

filters i think??

[bartman]

Good question!!!!!!
I thought it might have to do with reducing the aperture for viewing a full moon?????

[DavidU]

Caps with 2 holes are used for focusing
http://www.billyard-ink.com/Hartmann.shtml
Caps with one larger hole off set from the centre is to stop down a scopes aperture (ie: turning a 12" f5 into a 6" f12).

[Blue Skies]

If there is one off-centre hole then you can just use that to stop your aperture down when looking at a bright moon or the sun - you'd put the appropriate kind of filter over the hole first if you're doing the latter, though.

[Stu Ward]

Excellent !!!
Thank you very much, i will try that on the next full moon

Stu

[erick]

Turn it so the small hole opens over a gap between spider vanes. You should then see no diffraction spikes on stars, if I have my understanding right (I have never opened it over a vane). You will also be surprised at how much light still finds its way to your eye when you seem to be blocking off so much aperture.

[jjjnettie]

I covered the hole in mine with some Baadar solar film. Then using the lid from a spray can, made another filter to go over the working end of my finder scope.

[astro744]

Note reducing the aperture also reduces resolution. Not so much an issue on the Moon although smaller craters will no longer be seen. On planets fine detail will also be lost.

Stay with full aperture on the planets and during moments of exceptional seeing you will see much more detail. On the Moon a good quality neutral density filter is best and these are available in 13%, 25% and 50% transmision depending on your observing preference. I would not recommend variable polarizing filters as some (if not all) can also cut your resolution significantly. I have noticed small craters on the Moon actually disappear when using a variable polarizing filter to no filter, this on a C9.25. Not sure if there are better polarizing filters out there but a quality single density filter would not have such resolution issues.

See http://www.lumicon.com/telescope-accessories-list.php?cid=20&cn=Neutral+Density+Filters for more details.

[erick]

That is interesting information - thanks. Polarising filters have been my "moon filter" of choice - but usually only for public viewing. I find it best to go "au natural" for real observing - and put up with the purple spot in your eye when you turn away.

[astro744]

No worries!. I've got a Meade #905 variable polarizer and have read others have have had similar results as I have (resolution loss) with the same filter. Other variable filters may be OK and if you have one test it on the Moon next time. Pick some really small craters, say Armstrong, Aldrin and Collins and see if they disappear with the filter (provided your telescope is capable of showing them without filter). I cannot remember if these were the craters I used to test but pick some craters on the limit of your telescopes resolution without filter and then put the filter in with the same eyepiece.