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M27 - Dumbell Nebula

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Posts Made By: Alan French

May 13, 2003 10:39 PM Forum: Telescope Making

Can I do it ?

Posted By Alan French
If you are really interested in making a really good mirror, I suggest making two. That way you can figure one as well as possible, and then try to do better with the second. By going back and forth, you could probably get a darn good mirror. However, the time you spend could easily be spent making enough money at some part time job to buy a top quality mirror.

May 17, 2003 11:45 PM Forum: Equipment Talk

How much color is too much?

Posted By Alan French
A lot depends on the individual. Some folks seem more sensitive to the out of focus red and blue that form the purple halo. When using our club's C-102, which has a focal ratio a bit less than 10 I believe, Jupiter shows an obvious purple halo. But as I use the scope, my eye/brain tends to forget the out of focus color, and it becomes much less obvious. Nice to have some built in image processing. You and your girlfriend should give the scope a good chance to prove itself - worry more about the detail you see through it and less about that purple halo.

You scope probably has the normal secondary color typical of any achromat. The secondary color is determined by the glass choices the company made. The only real way to check it is to measure the variation of focus across the spectrum using narrow band filters.

May 22, 2003 10:58 PM Forum: Equipment Talk

Is it just me or is Al sucking us dry($)....

Posted By Alan French
I think quality eyepieces are a very reasonable value, and Al Nagler has certainly done a lot for us in terms of making high quality eyepieces available. Amateurs need, however, to control themselves. You don't need to collect the complete set of every design ! Seriously, most folks probably have considerably more eyepieces than they really need.

May 29, 2003 01:34 AM Forum: Telescope Making

Mirror silvering.......anyone?

Posted By Alan French
A high school student in our club silvered a mirror so he could have it done in time for Stellafane, the telescope maker's convention. It worked well for him, so I suspect you can do it to. Be careful, though.

May 31, 2003 11:36 PM Forum: Telescope Making

Why f-5.5 is better than f-4.5

Posted By Alan French
For many eyepiece designs, the astigmatism of the eyepiece swamps the coma in the mirror, so what is often taken for coma, is not.

June 5, 2003 10:02 PM Forum: Telescope Making

It's been a real eye opener for me

Posted By Alan French
To me, the discussion was very odd. I am skeptical about claims of many mirror quality claims, and expressing such skepticism would hardly seem to warrant discussions of "lawsuits."

June 7, 2003 02:32 AM Forum: Telescope Making

Folded Refractor Light Loss ???

Posted By Alan French
A folded refractor can work very, very well. John Gregory had an 8" APO folded to work with a Springfield mount at TSP one year, and the views of Saturn were quite astonishing. Quality flats, however, are not inexpensive. Neither are they beyond the abilities of a careful amateur optician.

June 9, 2003 11:10 PM Forum: Telescope Making

Difference between a 13" and a 14"

Posted By Alan French
The visual difference between a 13" and a 14" is not going to be very obvious. Just make sure you are getting a good mirror, support it well, and make a scope that will cool well. Actually, a 13" mirror will cool somewhat faster than a 14", and you will get away with a somewhat lighter scope.

Where are you looking at mirrors?

June 9, 2003 11:15 PM Forum: Equipment Talk

The truth, finally: Conventional Wisdom

Posted By Alan French
I was going to jump in on this, but I decided I'll go down and do some work in my shop instead. I'm tired of telescope arguments, and think I'll get back to spending my time on ATM projects. Have fun!

July 19, 2003 01:48 AM Forum: Telescope Making

10" Dob Problems?

Posted By Alan French
What do you mean when you say the trees are not sharp? Are they completely out of focus? If they approach being in focus, and then get worse as you continue moving the focuser in the same direction, the issue is not focus, but image sharpness. I suspect you do mean you actually can not reach focus.

If the focus is down below the focuser top, find a 1.25" tube that will slide down inside the focuser. Cut a disc of thin paper to fit inside the bottom of the tube. Aim the scope at the Moon, and slide the tube into the focuser until the Moon forms a sharp image on the paper. Now that you know where the focal plane is, you can figure out how far to move the mirror to push the focal plane out where you need it.

If the focus is outside the tube, you can reverse the tube so the paper disk is on the end sticking out of the focuser, and slide it out until the Moon forms a nice image on the paper.

IMHO, this method beats the heck out of trying to measure everything and guess where the focal plane is.


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