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Posts Made By: Grant Gussie

September 5, 2002 09:15 AM Forum: Telescope Making

How much does baltic birch plywood weigh?

Posted By Grant Gussie

I am designing a truss dob and the balance calculations require me to know what the density of my building materials are... I have them all except baltic birch plywood.

Now I could buy some sheets and weigh them, but I can't know how much I need to buy until I know the weight :-<. And there are no scales at the lumber yard to measure beforehand... I asked.

Does anyone have a value for the density of baltic birch plywood?

Or am I going to have to cart a scale to the lumber yard?

September 12, 2002 02:29 PM Forum: Equipment Talk

Orion Apex 90 for Wildlife

Posted By Grant Gussie

I have the f5.6 version, which I use mostly as a spotting scope, a 500mm telephoto lens. I would occasionally use it as a grab-and-go astro scope but magnifications > 30X requires I mount it on a surveyors tripod, which hardly qualifies as grap-and-go, more grab-and heft.

It has a large central obstruction as you would expect from a f5.6 Mak... just over 40%. It star-tests nearly perfectly for a telescope with such a large obstruction, with almost perfect spherical correction and a slight astigmatism/pinch that is not apparent until 200x.

The upper limit for this scope for truly sharp views is about 150X, making it surprisingly good on the moon, but so-so with planets. Splitting doubles down to 1.5 seconds is possible.

With a 16mm eyepiece (31X) it is a wonderful spotting scope. Razor sharp and as bright as the high-end 80mm refractors.

Photographs appear as good as those from any 500mm telephoto I've seen... meaning that if there is any problem with the pictures I can't see it. But it does not transmit a true f5.6... more like f8.

September 12, 2003 08:21 AM Forum: Equipment Talk

How to make an artificial star?

Posted By Grant Gussie

A reflective artifical star is much simpler to make than a tiny precision hole. Just shine a very bright (but very small) light at a chrome ball bearing or a silver christmas tree ornament. A dot-type laser pointer makes a good illuminator, but so does a bright halogen flashlight with a 1cm diameter off-axis aperture mask, and so does the sun.

July 5, 2004 07:07 AM Forum: Equipment Talk

Portable Planetary setup

Posted By Grant Gussie

An off-axis newtonian in the 6 inch range on a dob mount and an equatorial platform would make the lightest high performance planetary instrument (with tracking) that is currently available. If the eq platform replaced the dob ground board and the OTA had a handle it could be carried in one trip as well.

July 6, 2004 05:55 AM Forum: Equipment Talk

Does curved vanes improve planetary observation ?

Posted By Grant Gussie

No, there is no real improvement. The view with a curved vane is only more natural looking (aesthetically). The amount of light diverted from the image is a function of the total area of the obstruction, which increases when moving to a curved vane.

August 31, 2004 12:36 PM Forum: Equipment Talk

127MM Orion Apex vs 127MM Intes Micro Alter

Posted By Grant Gussie

I haven't used either telescope in question but have used the a 150mm Intes Micro and own a 90mm Orion Apex. The Intes Micro was a top quality instrument, while the Orion is a chinese knock-off with surprisingly good optics.

The Orion star tests with slight over correction and slight astigmatism, but I would still call it a diffraction-limited optic (the errors are small), maybe 1/5 wave PV error. The Intes Micro star tested very very slightly undercorrected, but then all spherical-optic Maksutovs must have some residual (5th order) spherical aberration and I believe I was just seeing that. I would call the IM optics pretty much perfect given the limitations of the design.

The light scattering (that is optical smoothness) favoured the IM as well, but again, we are talking the difference between very good (Orion) and excellent (Intes Micro).

Mechanicals in the focuser etc would be a greater win for the IM... the IM is excellent while the Orion is merely good. But then again, the Orion is "good enough", with a couple of stickier spots on the focuser but it still focuses fine.

If you already own an Orion Apex I personally would see no reason to buy a IM of the same size. The differences in view and smoothness of the focuser are small. You need to get a significantly larger scope to make an appreciable difference.

Grant Gussie
Salt Spring Island


September 1, 2004 07:56 AM Forum: Equipment Talk

Telescope sizes: inches vs. mm

Posted By Grant Gussie

The mix isn't likely to go away, as there are two many standards. For eyepieces we have 24.5mm, 1.25 inches, and 2 inches as standard barrel diameters. But in the US, the first size is refered to as 0.9651 inches! Lets drop that one, shall we? The others will just have to stay in inches, because, well, they ARE in inches.

Similarily, my 12.5 inch telescope is (embarrasingly) a 12.5 inch telescope and not a 3175mm one, since its based on a 12.5 inch mirror blank.

Thread measurements for screws, filters, visual backs, and other attachments is another nightmare mix.

Basically, if North America had joined the 19th century in the 19th century, and didn't wait until the late 20th century(Canada), or apparently chose to wait until the late 21st century (USA), then we wouldn't be in this mess. But we are.

September 9, 2004 08:10 AM Forum: Equipment Talk

Ad in a local Newsletter

Posted By Grant Gussie

Its an excellent deal. The mirror alone is worth more than that.

September 9, 2004 08:13 AM Forum: Equipment Talk

Dob equatorial platform

Posted By Grant Gussie

The yahoo eq platform group
http://groups.yahoo.com/group/eqplatformsis a great source of information on how to build them.
Specifically look at the "Sacred Files" folder in the Files section.

October 10, 2004 08:44 AM Forum: Equipment Talk

Quest for the ultimate "Quick Look" scope.

Posted By Grant Gussie

StarGazer Steve's 6" truss

http://stargazer.isys.ca/6inch.html

Awesome optics (SS deliberately understates his optical quality), cab be carryied in two hands, and there is no need to every take it apart unless you are packing it for air travel.