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Posts Made By: Michael Minnhaar

February 15, 2007 07:39 PM Forum: Star Parties

Mt St Helens?

Posted By Michael Minnhaar
Some folks of the Portland,OR RCA club at times went there a while ago - but I haven't heard of anyone going there the last 2 years. I will ask next Monday at the meeting if anybody knows.

Michael

http://www.rca-omsi.org
http://www.patch.com/astro/starsites/

February 26, 2007 12:58 AM Forum: Eyepieces

focal lengths of erfles?

Posted By Michael Minnhaar
I've seen also 16mm and 25mm celestron erfle's, both 80's vintage.

Michael

February 26, 2007 06:35 AM Forum: Celestron

Effective F-ratio with star diagonal and focuser

Posted By Michael Minnhaar
The SCT secondary is a divergeing element, if moved forward in the optical path it becomes more effective.
Assumeing the SCT primary is some f/2.5 and the secondary acts like a 4x barlow and I'd guess the f.l. increases about 4% for every 1% increase in the distance of the secondary to the final focal plane.
Thus 10% or 20% more f.l. would not surprise me.

Michael

March 9, 2007 05:03 PM Forum: Eyepieces

Barlows and field of view

Posted By Michael Minnhaar
Yes, its reduced to 50%.
Anything that increases the magnification for a given eyepiece will reduce the true field of view by that same amount.
Michael

November 1, 2007 05:21 AM Forum: Comets

Comet Watching

Posted By Michael Minnhaar
I tried a week ago for Loneos and it wasn't as great as I hoped for. The near full moon and some cloudbanks didn't help, either. But I have at least one wide angle shot that "detects" it. I could not make out any tail, neither in the photo nor in binoculars. By now it's setting within minutes of the sun here.

Michael

January 24, 2008 08:21 AM Forum: Equipment Talk

FotoSharp Canon t-ring focis indicator

Posted By Michael Minnhaar
Its even a while since you posted this - still want to have my 2 cents ?

I don't have that one but something similar.
In summary - its of limited use for fast scopes (
It works by telling the camera an autofocus capable lens would be attached, but the autofocus motor is actually turned off. Subsequently the camera does not try to command the AF motor in the lens to focus, but just uses its AF sensor to beep when the image happens to be in focus.
For far that sounds good - manually move the focus, press the prefocus on the camera until it beeps. A 3rd mag. star and a 6" aperture is enough light to get this confirmation.

But - these AF sensors measure focus in a way that the precision drops with the square of the F-ratio and around f/6 or slower the precision becomes useless for all purposes. And truely pinpoint star images require much more precsion than daylight photography. There is a book from Canon "EF Lens Works" that explains this in more detail.
So it works with my f/2.8 manual telephoto lens quite well, f/4 astrograph poorly and the f/6.5 refractor is reported in focus even if its still way off.

Michael

January 31, 2008 05:17 AM Forum: Eyepieces

Andromeda Eyepiece

Posted By Michael Minnhaar
I use a 4"/f6.5 refractor and 42mm GSO superview (some 65 deg AFOV)
- which results in 3.9deg true field of view.
But I'm still unimpressed by M31, the sight keeps being dominated by the
mostly featureless core. So to me m31 is photographically much more
impressive than visually.

Michael

March 15, 2008 09:49 PM Forum: Telescope Making

Need info on machinery

Posted By Michael Minnhaar
www.use-enco.com

One of the biggest resellers. Very good prices and selection on consumeables like drills/mills etc, less so on small machinery.

Michael

April 25, 2008 05:46 AM Forum: Telescope Making

Broadband coatings

Posted By Michael Minnhaar
Try l and l optical in Santa Ana, CA.

llopt.com

Michael


June 25, 2008 08:27 PM Forum: Star Parties

Golden State and Shingletown might not happen

Posted By Michael Minnhaar
Quote:

Due to the fires burning in the Shingletown area and the use of the airport as a fire fighting base for helicopters, SSP2008 will be postponed until at least the end of August 2008. Stay tuned for more information.

Michael

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