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Sculptor Galaxy (NGC 253)

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Posts Made By: Doug Scobel

October 24, 2003 01:51 PM Forum: Deep Sky Observing

Caldwell Objects? Do you do them?

Posted By Doug Scobel
Yeah, I've done them some years ago, before the book, when it was just a list published in S&T.

I agree that the renaming of everything with his name was bogus - I pretty much ignored those names. But they were a good starting point after observing all the Messiers. Some of the objects are real stinkers, they need such perfect observing conditions that they're near impossible to see for many of us. Plus, I have little hope of ever finishing the list unless I can arrange a trip to South America or Australia.

Doug Scobel

October 28, 2003 01:47 PM Forum: Solar System Observing

Picture of the sun Today(10.27.03)

Posted By Doug Scobel
You're killing me, Vlad, you're killing me! It's been cloudy for weeks and I haven't had a chance to take a gander myself. It's driving me crazy!

Thanks for the excellent shot.

Doug Scobel

October 31, 2003 05:21 PM Forum: Equipment Talk

Blistering paint inside sonatube

Posted By Doug Scobel
"it's called 'randomly spaced micro-baffling' "

Hee hee, funny until your micro baffles end up on your primary mirror! grin

Seriously, moisture must be getting in there and lifting the paint. Both my homemade scopes use Sonotube, and I seal the inside with a couple coats of shellac before painting them. The tube on my 13" is over six years old and has no signs of deterioration. You already have paint inside yours, but if you remove all the loose paint my guess is the shellac will still seal it well. Then repaint with Krylon ultra flat black. The outside of the tube is probably letting moisture in as well. I use Krylon polyurethane enamel which is extremely moisture resistant.

Be sure to remove the optics before you do all this! wink

Good luck,
Doug Scobel

October 31, 2003 05:35 PM Forum: Solar System Observing

Mars Observation (October 30, 2003)

Posted By Doug Scobel
Very nice indeed, Carlos.

I had my 8-inch f/8 in the backyard last night, but the seeing was poor and there was a significant breeze jiggling the scope around, so I could not see a lot of detail, and no sketch this time. But to me, the SPC looked larger than the last time I saw it a couple weeks ago. Does it seem bigger to anyone else?

Doug Scobel

November 4, 2003 01:38 PM Forum: Equipment Talk

aperture mask

Posted By Doug Scobel
Just out of curiosity, Dale, why do you want to stop down a 6-inch reflector? Are you trying to accomplish something in particular or are you simply experimenting?

Doug Scobel

November 4, 2003 01:55 PM Forum: Solar System Observing

New at this.

Posted By Doug Scobel
Ted, how much magnification are you using? If the planets (like Jupiter) are "too bright", then you may not be magnifying them enough. Also, try colored filters. I've found that a #21 orange works really good on Jupiter. Others probably have their own favorite filters. Local atmospheric seeing can be causing you problems as well. You say that you're in the city. Are you observing over houses, buildings, or parking lots?

Comparing planets to the moon is like comparing apples to oranges. The moon is so big and has so much detail and it’s all very high contrast, so virtually any scope in virtually any conditions will show a lot of lunar detail. Planetary detail is inherently more subtle and more difficult to extract.

Good luck,
Doug Scobel

November 5, 2003 03:50 PM Forum: Telescope Making

focusing accuracy

Posted By Doug Scobel
Very interesting, Vlad, but the situation may not be all that bad for those of use with fast dobs. Typically, fast dobs are used for deep sky observing and also typically not at really high magnifications where you can see 1/6 or even 1/4 wave mis-focusing. At low to medium magnifications very slight mis-focusing of that order is not noticeable. Using your fast newt for planetary and/or close double splitting is another story, but that's the exception rather than the rule.

Still, I like my Moonlite dual-rate Crayford, as it is far superior to the old helical I put up with for more than 15 years on my 13". It makes focusing a "snap". grin

Best regards,
Doug Scobel

November 13, 2003 03:36 PM Forum: Deep Sky Observing

Dob in Michigan

Posted By Doug Scobel
Daryl -

I observe with my homemade 13" f/4.5 dob in the Ann Arbor area. I observe at Peach Mtn. near Dexter (where the University Lowbrow Astronomers observe, see our web site listed on the AM home page) and at Lake Hudson WSW of Adrian. Where are you? You'd be more than welcome to visit and "give it a spin" as you say. I think "try before you buy" is a really good idea.

Doug Scobel

November 13, 2003 03:40 PM Forum: Equipment Talk

Clear day!

Posted By Doug Scobel
Don't get me started....

November 14, 2003 03:25 PM Forum: Telescope Making

testing a fine old coulter

Posted By Doug Scobel
Phil,

The program I use shows that your wavefront error is about 1/3.8 wave at a focus position with a maximum transverse aberration of 1.9 times the theoretical Airy disk radius, or about 1/2.8 wave at a focus position with a maximum transverse aberration of 1.4 times the Airy disk radius. The first pair of numbers represents a focus position for the best wavefront, the second for the smallest transverse aberration (i.e., smallest Airy disk).

I made the following assumptions:
Active diameter of 201mm (allowing for bevel)
Moving light source tester
Mask zone openings 12mm (~1/2") wide

I use a program named Genmir I found and adapted in the old Telescope Making magazine some years ago. It might have been done by Dick Suiter, but I'm not sure. It was published in Basic (!) and I have my own C version.

Hope this is not too discouraging but it is reasonable. At f/4.5, mirrors must be very precisely figured, and it doesn't have to be off by much to get it out of tolerance.

Best regards,
Doug Scobel

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