Posts Made By: Allen Tracht

Starbuckets, a sponsor, has some nice, small dobs. I don't know if their 6" is out of your price range. The 8", unfortunately, is too expensive for your budget.

Allen

http://www.mcmaster.com/ has ingots and bricks for < $3/lb. Other shapes are more expensive.

http://www.nuclead.com/lead_products.html?gclid=CJ-RxNf5yIUCFTROGAodB0nBsw might also be useful, but you'll have to call them for more info.

Allen

I took a rather different approach:

http://www.astromart.com/forums/viewpost.asp?forum_post_id=375267&poll_id=&news_id=&page=

The USPTO (www.uspto.gov) has online access to recent patents. For example:

http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=6,317,279.PN.&OS=PN/6,317,279&RS=PN/6,317,279

There you'll find a link to view images of the patent. You'll need a TIFF plug-in for your browser. See the help at the images page.

I've grabbed a diagram from the patent that shows how it's done with mirrors.

You could try focusing on a nearer object and then correct for the focus shift. With an 18" f4.5 mirror, your focal length is about 74". Say you go to the football field of your local high school and focus on an object 100 yards (3600 inches) away: The focal plane would be displaced by 74 * 74 / 3600 = 1.52". So, you'll have to move the upper cage 1.5" closer to the primary to focus on the stars.

I hope this helps.

Have you considered a C14? It is probably not in the same league (optically or mechanically) with most of the other suggestions, but it will give you more aperture, lower cost, and with hyperstar can be used for wider fields.

Here's some real detail:

http://www.dsp.ece.rice.edu/cs/cscam-SPIEJan06.pdf

The impressive thing here is that instead of capturing all of the pixels and then compressing them to a much smaller dataset, they've found a way to capture much less information and still have an equivalent-quality final image.

I'm really surprised that this discussion is focused on sensitivity. Roland is certainly correct that modern sensors can detect most every photon that arrives (leaving little room for improvement) and that the system described, with its single, small, photodetector, deliberately discards most (99.9+%) of the photons.

So, of course, it's not applicable to astrophotography, where you are trying (and need) to collect every photon from the FOV that arrives any time in a period of minutes or even hours.

Read their paper:

http://www.dsp.ece.rice.edu/cs/cscam-SPIEJan06.pdf

It's all about how, if all we want is a compressed image, we don't really need to initially acquire a full-detail original image. It's really an application some complex math, not a realistic imaging system.

Still, it might be valuable for systems in which detectors are difficult to fabricate. Perhaps millimeter waves or some other exotic radiation.

Alternatively, with extraordinary optics (holographic?) and a multi-pixel detector, you might be able to take all of the measurements at once, achieving the goal of using a smaller detector to get an acceptable image quality. It might make cell-phone cameras even smaller, lighter, and less expensive.

Jim, I think you have some valid questions. The mirror state is clearly necessary to reconstruct the image and so must be recorded (or computable) on a per-measurement basis. As the mirrors can be driven with a known pseudo-random (calculable) sequence, this isn't too hard. Also, I'm sure the math requires that the image not be time-varying: the noise of a faint astronomical signal would cause noise and/or artifacts in the reconstructed image.

Allen

George,

Perhaps you can find help from a local astronomy club. The Clear Sky Clock shows a number of clocks in Maryland:

http://cleardarksky.com/csk/prov/Maryland_map.html?Mn=meteorites

Clicking on them, I find one in your area operated by the Howard Astronomical League:

http://www.howardastro.org/

Harford Community College and the Harford County Astronomical Society

http://www.harford.edu/ and http://www.harfordastro.org/

There may also be other clubs in your areas.

I'm certain that they can help you (or know someone who can).

Good Luck,

Allen

Some discussion of emissivity with links:

http://edhiker.home.comcast.net/telescope12inch.html