Message posted by gary on March 30, 2001 at 11:24:43 PST:

I forgot to mention that the shot was done using a T-barlow, not eyepiece projection. This T-barlow is a tube with T-threads on the end and a 2x barlow mounted inside. [The T-barlow was $35.]

The set-up is
telescope->lar->visual back->###->tbarlow->camera

The LAR is needed to convert the ETX to standard SCT threads. The visual back is basically an eyepiece holder for use in the primary (out the back) path.

The ### is a piece of an eyepiece projection unit that I don't know if it is sold sperately. [### is just some placeholder since I don't know what this thing is even called. On one end it is a 1.25inch tube, which is the same diamter as an eyepiece (occular). The tube has threads on the inside to hold a standard filter, since the T-barlow does not make provision for a filter. If you read the desertsecrets.com web site, the red filter is explained. The other end of the ### has T-threads.]

The barlow produces a sharp image at the edge of the 35mm frame, while eyepiece projection only does this if you use an orthoscopic occular. I think the T-barlow is the way to go because you simple can't use any more magnification at this distance. I only use a barlow because the focal length of the etx-90 is 1280mm, and you need to be around 2500 or so to shoot Groom Lake, based mostly on the atmospheric degradation, or as the astronomers call it "poor seeing".

One problem is the exposure times are much longer than the "sunny 16" indicates, especially with the red filter. A red filter (#25) has a filter factor of 8, meaning it will reduce the amount of light by a linear factor of 8. However, what you see though 26 miles of atmosphere is very very blue, which is the opposite of red, and thus the attenuation seems to be higher.

In Reply to: Re: Travis AFB photo (practice for Tikaboo) posted by Joerg (Webmaster) on March 30, 2001 at 8:10:49 PST:

Replies: