Image of the day

Captured by
Craig Bobchin

Coathanger CR 399

About Astromart

My Account

New to Astromart?

Register an account...

Need Help?

Posts Made By: Doug Scobel

September 22, 2004 12:50 PM Forum: Deep Sky Observing

NGC6888

Posted By Doug Scobel
Inge, NGC 6888 is one of my favorites. In my 13" with OIII, I find that it looks like a complete oval, with the brighter crescent shape of course. In some long exposure images you see the entire oval also, but I'm not sure if my brain is completing the oval for me or if I'm actually seeing it. :S

Doug

September 24, 2004 01:04 PM Forum: Equipment Talk

Strehl or Wavefront error...which to consider?

Posted By Doug Scobel
It depends on how the numbers are determined. If they're determined by interferometry, then they're probably more accurate than if they're derived from Foucault measurements. In the former case, I would trust the Strehl ratio more. In the latter, I'd trust the P-V more.

Truth be told, if the mirror is smooth, has no sharp breaks, and a good edge, then even a 1/7.8 (let's say 1/8) wave P-V wavefront is very, very good. You may see a difference with a better mirror when doing the star test (virtually no mirrors "pass" the star test 100%), but at focus it's highly unlikely that you'll see any difference at all.

My $.02

Doug

September 29, 2004 12:09 PM Forum: Telescope Making

Re: Once and for all..........what mirror coating?

Posted By Doug Scobel
Good, informed decision, Mike. That's what we're all here for, no?

Truth be told, all other things (besides whether the coating is standard or enhanced) being equal, it's how well you've honed your observing skills that will make the bigger difference.

Enjoy your new mirror. After all, the coating is the mirror - the glass just supports it.

Doug

October 5, 2004 12:20 PM Forum: Solar System Observing

Solar images from October 3, 2004

Posted By Doug Scobel
Another great, hi-res shot, Paul.

I see from your web site that you use a web cam. About how many frames do you stack to get these images?

Also, how does the view through the eyepiece compare with your final images (assuming you take the time to look)? I would guess that the former isn't as good as the latter.

Doug

October 5, 2004 08:47 PM Forum: Telescope Making

PDF? bearing material

Posted By Doug Scobel
I remember the article, Evan. I think it was "Improving Dobsonian Motion", by Martin Lewis, in the October 1999 S&T. He tested a whole bunch of materials, looking for a combination that gave best results when you only need to nudge the scope a little bit, like at very high magnification when it's all too easy to overshoot. I remember it being like a Teflon fabric, and it had lower "stiction" than solid Teflon.

Martin also has a more recent article "Better Dobsonian Bearings", in the October 2003 issue. Curiously, I don't remember reading that one, though.

I'll try to remember to look them up when I get home.

Doug

October 8, 2004 11:47 AM Forum: Telescope Making

DOB Tube material

Posted By Doug Scobel
Hi, Ed, a misconception is that Sonotube(tm) (the "real" Sonotube brand I mean, don't know about QwickTubes) is simply cardboard. It's fibers are made and treated to resist moisture, since it's original purpose in life is for casting concrete pillars. I've used Sonotubes for my scopes for years and if properly sealed and finished, they make a very sturdy and durable telescope. The tube on my 13" dates back to 1997, and has seen some extremely heavy dew and shows absolutely no signs of deterioration (see pic). They do tend to be a little on the heavy side, but when you consider the price they're quite a bargain.

Here's a link to more info: http://www.concreteacc.com/sonotube.asp

I'm not sure what you mean about "attachment points and overall fit". If you're worried about a perceived lack of precision re: mounting holes and such, I've never had problems with any of my Sonotube scopes not holding collimation. And sorry, I don't know about "Hollow tube" either. Gee, I guess I didn't answer your question did I?

Best regards,
Doug

October 8, 2004 08:28 PM Forum: Solar System Observing

Klaproth Sunset Lunar Ray

Posted By Doug Scobel
Cool, Dave. Any way to post the digital image Ted took?

Say, Ted Nichols II was the speaker at the BFSP that inspired me to observe Pluto and the outer planets' moons for the first time ever (see http://www.astromart.com/forums/viewpost.asp?forum_post_id=251015&poll_id=&news_id=&page=). Thank him for me, would you?

Doug

October 11, 2004 01:49 PM Forum: Equipment Talk

Thinking Caps, please

Posted By Doug Scobel
Lois, another thing to look out for is that LPR filters act like mirrors. Meaning any ambient light on your eyeball side of the EP can reflect back into your eye, actually reducing contrast. Whenever using such filters make sure you shield the EP from any stray ambient light.

Good luck,
Doug

October 12, 2004 08:28 PM Forum: Film Astrophotography - Imaging and Processing

Star Trails #2

Posted By Doug Scobel
Nice! It's raining Orion!

October 13, 2004 06:25 PM Forum: Film Astrophotography - Imaging and Processing

Barn Door Tracker

Posted By Doug Scobel
Mine uses a 1 rpm AC synchronous motor I bought from Edmund Scientific several years ago. When I'm away from a place to plug it in I use a 12V DC- 115V AC inverter plugged into the cigarette lighter.

Sorry, don't know if they still sell that motor or not.

Doug


Funding Member

Sponsors


View all sponsors

Search