Hole in my Dob's dustcap

reeaal (Mark) · #1 31-05-2009, 12:31 PM

Hi,
I was holding the dustcap for my 10" SW Dob the other night and the cap from one of the holders came off, leaving a 2" or so hole in my dustcap. As this looks like it is supposed to come off, what is the purpose? I've done a search and think it may be something to do with either solar viewing or somehow reducing aperature. Is this correct? How could I utilise this feature, if it is a feature?
I'm new to this.
Thanks for any illumination,
Mark

iceman (Mike) · #2 31-05-2009, 12:48 PM

Hi Mark

You're right - it's supposed to reduce the aperture like an aperture mask and give you a high focal ratio.

For solar viewing, you still MUST use a proper solar filter.

To be honest, it's not worth it at all and you'd never want to stop it down/take it off. Just ignore it.

sheeny (Al) · #3 31-05-2009, 01:00 PM

Mark,

Before the currently available solar films were available, an offset hole in the lid of newt was indeed used as part of a system for solar viewing.

The smaller hole served to reduce the amount of light entering the scope but more importantly, it was to reduce the amount of heat (IR radiation) entering the scope. This kep the heat below safe levels for either the eyepiece projection method or for the use of a herschel wedge.

In these days of excellent objective solar filters, I would recommend buying or making a full aperture filter from solar film

, or if you don't want to fork our the money for a full aperture filter, you could buy or make a solar filter to suit the aperture in your scope lid.

If you intend to try the eyepiece projection method (that is projecting an image of the sun onto a card or similar) use an old EP just in case

. The EP will probably get hot even with the reduced aperture, and if it melts anything in the EP, its ruined!

Never, never, never look through a scope at the sun without a proper objective filter!

Mike's advice is good - Ignore it.

Al.

reeaal (Mark) · #4 31-05-2009, 02:08 PM

Thanks Mike and Al. That helps heaps.

Robert9 (Robert) · #5 31-05-2009, 02:40 PM

You might also find that its very useful for moon observation.
Robert

reeaal (Mark) · #6 31-05-2009, 03:22 PM

Thanks Robert, I'll try that out if these clouds ever decide to move on.

Robert9 (Robert) · #7 31-05-2009, 09:06 PM

OK Mark. Know how you feel about clouds. We've got fog.

erick (Eric) · #8 01-06-2009, 11:03 AM

Cup holder?

reeaal (Mark) · #9 01-06-2009, 02:34 PM

Cup holder?

Haha, that could go in the pimp my scope thread

Liz · #10 01-06-2009, 02:41 PM

My previous Dob had them too, and they were very useful for the Moon and some planets.
Cup idea sounds promising too

reeaal (Mark) · #11 01-06-2009, 05:25 PM

Definitely going to use for the moon. Still waiting for those clouds to shift though.

astro_nutt · #12 01-06-2009, 07:13 PM

Hi!..another use is after viewing on a dewy night...leave the smaller cap off and cover the top with some cloth..so the condensation dries out without any dust sticking to a dewy mirror.
Cheers!

Merlin66 (Ken) · #13 01-06-2009, 08:22 PM

These small aperture openings are a throw back to the 1950's!!
Nowadays the answer, as mentioned, is a full aperture solar filter.
The original idea goes back to the Herschel wedge and ND3 filter. Someone (???) suggested that a 2" aperture + Herchel wedge+ ND3 filter was a "safe" combination for solar observing....
It's had it's day... cup holder sounds interesting... anything other than Solar observing!!!!!!!