Posts Made By: Michael Connelley

Helllo:
The easiest thing would be to re-mount the primary mirror farther up the telescope tube. This will have the affect of pushing the focal plane farther out of the focuser. You can estimate how much more in focus you need, when re-mount the primary mirror farther up the tube a little more than this value. Typically, this requires no more than drilling new holes in the tube for the primary mirror cell, and is a thing you can do in an evening.

Cheers
Mike Connelley

Hello:
Here is another picture of Jupiter's boring side, taken earlier this year. However, the picture is not boring.

Cheers
Mike Connelley

Hello:
First off, the telescope I use most is a 12.5" f/15 cassegrain on a G11 mount. I enjoy using both wide field and "regular" eyepieces. Since the maximum field of view is 30', wide field eyepieces are necessary for observing large DSOs. I usually use a 31mm and 17mm Naglers for DSOs. A wide field eyepiece is necessary to see the whole object, as well as to find them. A 17mm is around 300x, so again a wide field is useful. For planetary observations, I tend to use Plossls since my experiences show they have better contrast that Naglers. Both kinds of eyepieces are useful.

Cheers
Mike Connelley

Hello:
When I built my cassegrain, I found that the secondary mirror position and collimation are very sensitive. My scope is 12.5" f/15, with an f/3.8 primary mirror so the secondary mirror magnification is about 4. I used a set up similar to yours, but instead of the moon I used a collimated light beam from a zygo interferometer. Once I had the secondary mirror roughly set, I drilled the holes for the spider (I have a classical cassegrain, but the same rules apply to your scope), mounted the secondary, then put washers behind the secondary to carefully adjust it's location. If I recall correctly, in a cassegrain, the focus shift is equal to the secondary mirror shift times the square of the secondary mirror magnification. So, in my case, adding a 1/16" washer moves the focus 1". The optical bench set up you have should work well, as long as you carefully and precisely tranfer the measurements to the optical tube. As you can see, even a very small error will have a large consequence. Also, if the tube ends up a little short (i.e. the focus is too far out), you can always put blocks under the focuser to hold it farther from the back end of the scope. If you can estimate where you focus is now with the tube you have (such as by finding a sharp image of the moon on a scrap of paper), then determine how far the focus needs to move to put it where you want, divide by the square of your secondary mirror magnification, and you should be set. Getting the secondary mirror position is only a first step. After this, you'll have to collimate it.

Cheers
Mike Connelley

Hello:
For optical astronomy, gold coatings are not a good idea. Gold coatings have low emissivity (and high refelctivity) in the infrared, which is their main use. Gold has low reflectivity in the blue, so a gold mirror in your system would make white stars look golden. Also, the low blue throughput would make things like galaxies appear dimmer. What would work better than gold is silver. Silver has high reflectivity across the optical and through the infrared, which is why it's used on some infrared telescopes. The silver coating needs to be overcoated to prevent it tarnishing, and (at least at Gemini) there's an undercoating of nickel if I recall correctly. A friend of mine had his 20" mirror coated in silver, and the views are very nice (this, of course, could just be because it's a 20").

Cheers
Mike Connelley

Hello:
IF you really want to build yur own scope, then go for it. For a 10" dob, it probably won't come out much cheaper unless you make a lot of the components yourself. Just a good 10" mirror will cost a good part of the $650 you'd have to pay Orion. But if building your own scope is that you want to do, then you should do it. I did.
I don't think having a metal tube is a big deal. My scope has a tube that is just bare sheet metal, and it works perfectly well once it has a chance to cool down. In my case, the primary mirror is the limiting factor when it comes to cool down time. The idea of cork lining is to insulate the air in the tube from the tube itself. But, the tube has such a high surface area (through which it can cool) to volume (with which it can store heat) ratio, that it should get to ambient temperature fairly quickly. The light gathering ability of the scope is not affected by the tube material.
For imaging, you'll want a scope that tracks. If you have a dob, by the time you center a target and get it focused, it'll be out of the field of view of the camera. You can get the dob, have fun with it, and when you're ready for photography, just get a new mount (such as the Atlas or G11, or a tracking patform) to put the scope on. If you want to do this, then choose a dob that'll be very stable on which ever mount you think you'll choose in the future.

Cheers
Mike Connelley

Hello:
In my scopes, the primary source of astigmatism is me. The prescription for my glasses is old so there's quite a bit of residual astigmatism in the images. There are two easy ways to test. The first is to rotate your head, and see if the astigmatism rotates with you (best done with a star a little out of focus). Another way is to increase the magnification, and see if the astigmatism goes away. At higher powers, the exit pupil is smaller, so you're using less of your presumably astigmatic cornea+lens, so the image quality is better. If adjusting the mirror cell doesn't provide relief and rotating the mirror doesn't help, the problem may be you.

Cheers
Mike Connelley

Hello;
Why are there so many binary stars? The answer is that (practically) all stars form in binary and multiple systems. Recent research (such as mine) has shown that the fraction of stars with a companion increases for very young stars. Single stars like the Sun may have been ejeced from a tripple star system early in its history. There are a large number of tripple, quadruple, and higher order multiple stars. If I recall correctly, there are 6 stars in the Castor system. Systems with more components become systematically more rare, i.e. there are fewer tripples than binaries, fewer quadruples than tripples, etc. All systems are bound by gravity. There aren't that many tripple systems that can be resolved with an amateur scope since one component is often a spectroscopic binary (i.e. the binary companion cannot be seen, but we can observe the doppler shift of its primary star). The formation of close binaries may require the presence of a wider companion star. Easily resolved tripple systems are rare for two other reasons. 1) To be easily resolved they'll need to be far apart, which means they'd be easily stripped off in the cluster where they formed, so such systems are unlikely to survive and 2) The system will be unstable unless two of the stars are much closer than the distance from that pair to the third star. Thus, the closer pair will be hard to split. I have resolved such a star, gamma Andromidae, which is a ~10" binary and the B component is a 0.4" binary itself.

Cheers
Mike Connelley

Hello:
I did something similar, using welded alimunum tubing. The only problem I've had is with some flexture. The top end of the scope can "wag" from side to side a little, which can be annoying at high powers. Some corner braces would solve the problem. Lightening the top end is an easier way to solve the problem.

Cheers
Mike Connelley

Hello:
I think a good way to do this is to maintain a spreadsheet, and as you build parts you weigh them and update the spreadsheet. I plan to do this on my next scope. For my last one, I did these calculations on paper, built the truss tube without the bearings, then found the balance point by balancing the tube on the broom stick. The calculations on paper get you close enough to build the rocker box. Also, don't forget to include the weight of your favorite eyepiece! In my case, it's about half the weight of the top end unit.

Cheers
Mike Connelley