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Lawrence Parchman

Sculptor Galaxy (NGC 253)

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Posts Made By: Doug Scobel

October 1, 2003 04:54 PM Forum: Telescope Making

YASQ (Yet another Secondary Question)

Posted By Doug Scobel
Well, Tom, I would think that 14 or even 15 inches would be fine. I own a homemade 13.1" f/4.5 dob (fl=58.7 inches). I use a 2.6" MA secondary in a Novak holder which reduces the MA a little, and a Moonlite Crayford focuser. All mounted in/on a 16" ID (solid, not truss) tube. Even at that size, I have fully and 75% illuminated field diameters of 0.44 and 1.58 inches (0.44/1.55 degrees) respectively, numbers from NEWT. Visually, the scope works wonderfully. So, you could probably go up to 15" to allow room for your filter slide and still be OK.

Just my $.02
Doug Scobel

October 3, 2003 12:56 PM Forum: Equipment Talk

Newt Fans

Posted By Doug Scobel
Cool (pun intended), Ron!

I'm in the process of installing a couple "blow across" fans in my 13" dob, which, even with a fan blowing on the back of the mirror, still takes hours to cool. This gives me more incentive to get it done. I'll let y'all know how it comes out.

Doug Scobel

October 6, 2003 08:11 PM Forum: Solar System Observing

Re: Saturn is back!

Posted By Doug Scobel
"Beautiful image."

Indeed

"Must... resist... Saturn..."

Resistance is futile...

October 7, 2003 09:22 PM Forum: Telescope Making

Secondary size for 10" reflector?

Posted By Doug Scobel
Joe, the 1.83" secondary with the f/5 is IMHO a little too small. At low and even medium power and the small .11" FIF, only a small portion of the field will be using all 10" of the primary. Go with the 2.14". Contrast wise I doubt that you'll see a difference between a 18% and 21% CO. CO doesn't become much of an issue until you're up around 30%, so it doesn't make a lot of sense to go to f/5.6 just to get an extra 3%. It would help with coma, collimation, etc., though.

Hope this helps,
Doug Scobel

October 8, 2003 01:09 PM Forum: Solar System Observing

Saturn 10.08.03

Posted By Doug Scobel
Fantastic drawing, Sol!

Say, how did you get such perfect ellipses for the rings? Do you use a template, or do you draw them free hand, or what?

Doug Scobel

October 15, 2003 05:03 PM Forum: Pictures of Me and My Telescope and........

Me and Mars Scope

Posted By Doug Scobel
Here's the top/rear, showing the fan vents and light/dirt/dew shield, and the plywood trim ring that does double duty as a reinforcing ring and annular baffle to block stray light from sneaking around the primary. The tube is painted with three coats of Krylon polyurethane (brush-on) enamel, which provides a completely waterproof, plastic-like coating.

October 15, 2003 06:07 PM Forum: Equipment Talk

Baffles in a DOB

Posted By Doug Scobel
Floyd -

I installed baffles in my 13" homemade dob a few years ago and indeed they helped improve contrast. I consistently get compliments at star parities regarding the high contrast images that scope provides.

There are 12 baffles (the tube is 16" ID), made from 3/16" foamcore board, which is both stiff and light. The problem is the thickness. I trimmed each one so that they were tapered to a sharp edge at the aperture. I calculated the size and spacing using a method I found off the ATM web site (I recall www.tiac.net/something) back then, but that site now seems to be defunct. I've looked but can't seem to find it. Rich Combs may have been the author, I find hyperlinks in various places but the site is gone.

I understand that baffles in a newt can force warm air into the light path, and I've always had pretty bad thermal problems in that scope. I'm in the process of installing blow-across fans w/vents at the mirror end and I'm hoping that this will help.

Good luck,
Doug Scobel

October 17, 2003 08:27 PM Forum: Pictures of Me and My Telescope and........

Me and Mars Scope

Posted By Doug Scobel
Thanks for all the nice comments, everyone.

Too bad we're all strewn across the planet. It would be cool to get everyone together in one place to look through everyone else's scopes n stuff. Still, it's cool to share in this venue.

Regards,
Doug

October 21, 2003 03:58 PM Forum: Telescope Making

1" secondary for an 8" f8 primary

Posted By Doug Scobel
Bob,

I just built an 8" f/8 this summer and was contemplating a 1" diagonal but went with a 1.3" instead. I'm using a 10" ID tube, .25" wall, 1.5" focuser, w/.25" spare in travel. A 1" secondary provides too small a fully illuminated field. With a planetary scope, having a too small fully illuminated field not only means that the object becomes dimmer off axis (maybe noticeably, maybe not), but also you're reducing resolution because you're not using all of the primary.

Another thing to caution you on is with a very small diagonal, a defective edge (not uncommon in all but the best quality diagonals) is just like having a bad edge on your primary, and your images can suffer. IMHO, as long as you're under 20% CO, going even smaller doesn't do a whole lot in the best of circumstances, and can maybe do mare harm than good.

Good luck,
Doug Scobel

October 22, 2003 01:12 PM Forum: Deep Sky Observing

Cave Nebula

Posted By Doug Scobel
Debbie,

I've seen only a hint of it in my 13" w/enhanced coatings and OIII under very dark skies. Perhaps he observed it from the Chilean Andes? Sometimes I think that Sir Patrick Moore is something of a sadist considering the extremely low surface brightness of some of the objects in his so-called Caldwell catalog. This is one of them.

Happy DSO hunting,
Doug Scobel

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