Posts Made By: Doug Tanaka

David,

One other thing is to make sure your mirror is cooled down enough to give good images. Even if the seeing is excellent you won't see it if the mirror is giving off heat. You can judge seeing pretty accurately by centering a moderately bright star in the FOV of an eyepiece giving about 200x and defocusing the image. You should see some clear, concentric rings when slightly out focus if the seeing is very good. Atmospheric turbulence will be visible as waves in the rings. With practice you can also see the difference between atmospheric turbulence and heat from the mirror.

Doug Tanaka

Tie a cloth bag (or a large sock) to where the weight would go and fill with sand. When you're balanced, weigh the bag. Be sure to keep the sand out of your eyepiece case ;-)

Doug Tanaka

Take a look at . Good luck!

Doug Tanaka

I don't know about knowledge (didn't someone once say something like, "100 anecdotes still isn't data?"), but I live on an island, too, and have what I believe to be salt damage. When the mirror was new I went with the common practice of not cleaning the mirror too often, and keeping it covered. After a few months I noticed what I thought were mildew spots. They were about 1/8" -1/4" diameter and looked almost exactly like miniature sunspots. Another e-mail acquaintance who lives on an island described his spots as miniature Julia Set Fractal patterns. I have since come to the conclusion, after looking at them through a magnifying lens, that it isn't mildew, but corrosion from salt that is eating the coating from the back having entered through a pinhole. My coating is Beral, but our club's scope is coated aluminum and shows almost exactly the same pattern of spots. Before I saw it on the club's scope I was thinking that the SiO overcoat would protect and that I should eventually get mine recoated. But apparently even the overcoat can have pinholes for salt to intrude through. The spots don't wash off or even get better with washing - they are permanently etched.

My solution was to just clean the mirror more often, like just about once a month. I know that it's frowned upon in general, but since I've been doing that the spotting has stopped. I figure that as long as I'm careful I won't do much damage to the mirror, and it definitely can't be as bad. Just another anecdote ;-) but I hope it helps.

Doug Tanaka

You might also think about two of the Sky Instrument (Antares) eyepieces, the 30mm modified Erfle, 2", 72* AFOV, $130, or the 24mm Speers-Waler, 1.25", 67* AFOV, $124.

Doug Tanaka

Blow off dust using a photographic blower (or a clean ketchup bottle works well), follow with a soft lens brush. For a lens solution I like to use a 50/50 solution of isopropyl acohol (rubbing alcohol) and distilled water with a drop or two of dishwashing detergent. Apply with 100% surgical cotton balls. As long as you're reasonably careful you should be OK - the optics on my ETX are pretty tough. However, they haven't been subjected to the frosted mini-wheats....

You can find more detailed instructions at http://www.company7.com/library/clean.html

Doug Tanaka

When you say raise your Newtonian spider, do you mean raising the whole secondary cage? If you only raise the spider but the focuser remains in the same position, it will no longer be centered on the secondary (or the secondary will no longer be centered in the focuser). With a truss-type Dob you should move the whole secondary cage. In a sonotube Dob you'd need to move the focuser. As Dan mentioned, make sure you don't go too small (and if you do, make sure you get a smaller primary mirror as well ;-)

Doug Tanaka

Craig, I tried sending this out earlier today but looks like it didn't go through. I originally made curved spiders for my scope and later decided to try a three vaned string spider using super thin archery string. After six months I still didn't like the diffraction spikes so went back to curved.

I think the bad press on curved spiders is that they traditionally had to be made from fairly thick material to support properly, especially when apertures got above 8" or so. My scope is 12.5 but I made the vanes from carbon fiber, pressed in a wood mold so they come out to about .04" thick, probably thinner than most commercially made four vaned spiders. I don't, however know of a source for any curved spiders.

Doug Tanaka

You can get pretty cheap foam, fairly thick at Home Depot or Lowes. You could also try a place that sells foam for matresses to see if they have cut offs.

Doug Tanaka

I think cement would be too heavy. Sand, because of the movement of the individual grains would damp vibration better, but still weigh a lot. Before going with wood (probably the best choice) what about trying some of that medium expansion spray-in foam contractors use to seal houses?

Doug Tanaka