Image of the day

Captured by
Gary Taylor

M27 - Dumbell Nebula

About Astromart

My Account

New to Astromart?

Register an account...

Need Help?

Posts Made By: Alan French

November 29, 2002 07:02 PM Forum: Equipment Talk

Mirror Coating problem?!? (small pic)

Posted By Alan French
Coatings sometimes have lots of little pinholes. They look bad, but they should not have any obvious effect on the image. If the images are soft and will not focus above 120x and the scope is correctly collimated, and the seeing is reasonable, you may have a poorly figured mirror. It is also possible you have pinched optics somewhere. Put about 150x on the scope and carefully look at a star image just inside and just outside of focus. Is it round? If it is distorted, especially if there are sharp points to it, you may have the primary or secondary too tightly held.

December 2, 2002 02:52 AM Forum: Telescope Making

LEDs for Testing

Posted By Alan French
Thanks! I did find the discussion, and I think I will just go ahead and use the green LED.

December 16, 2002 10:56 PM Forum: Telescope Making

Building an Off-Axis Newtonian design?

Posted By Alan French
I had a long interest in off-axis instruments and looked at any I could find at conventions. On looking through most the owner would make a comment along these lines - "It's not quite collimated." At best, they seemed on par with a good Newtonian of modest focal length with a moderate central obstruction. It seems to me that it would be better to make a top quality 10" f/6 or 7 Newt than an 8" off-axis system, at least if your main goal is a high quality instrument. If you are looking for an interesting project, have at it.

December 28, 2002 03:36 AM Forum: Equipment Talk

Fully-illuminated field

Posted By Alan French
What type of telescope are you asking about? In a Newtonian, the fully illuminated field depends on the aperture, the focal length, the size of the diagonal, and the distance from the center of the diagonal to the focal plane. In a refractor is depends on the aperture, focal length, and the size and placement of the baffles.

December 31, 2002 11:03 PM Forum: Equipment Talk

Solar filters

Posted By Alan French
I would make a full aperture filter. Over the years I have had a chance to compare scopes up to 8" aperture with quality solar filters at Astrofest. The view does improve with more aperture.

While closed systems are not recommended for solar projection, a proper filter, like the Baader, eliminates worries about too much heat entering the scope. It is safe for you and the scope to make a full aperture filter.

January 7, 2003 11:33 PM Forum: Birding Optics and Photos

2" e.p.'s for birding?

Posted By Alan French
The advantage to 2" eyepieces is the larger field stop, which means you can get a wider field of view with a 2" eyepiece than you can with a 1.25" eyepiece of the same focal length. The downside is that 2" eyepieces are heavier and more expensive. Generally, you only need a large field stop in longer focal length (low power) eyepieces. Eyepieces shorter than about 17mm (going by the TeleVue line) will not have field stops requiring a 2" housing.

I use a small APO with a 2" mirror star diagonal for birding. I generally find I do not use 2" eyepieces - even a 17mm only yields 26x. Most of my birding is done at 64x with a 7mm Nagler (1.25") or higher.

January 20, 2003 02:38 PM Forum: Birding Optics and Photos

Pentax 100mm spotting scope

Posted By Alan French
At 5.75 pounds without an eyepiece, this is not a scope I'd want to carry around in the field.

January 28, 2003 01:20 AM Forum: Telescope Making

Nitrogen Filled Schmidt

Posted By Alan French
It would require a fair amount of work to modify an SCT to completely seal it so the nitrogen would not get out. Also, filling a scope with an inert gas is designed, I believe, to prevent internal fogging. The big problem with SCTs is that the corrector radiates heat to the night sky, cools down below ambient air temperature, and dew forms on the cold surface of the corrector. Filling the scope with nitrogen would not help prevent this.

February 19, 2003 08:57 PM Forum: Equipment Talk

Dobs planetary nudging?

Posted By Alan French
Hi Ron,

Although I started out with an equatorial mount for an 8" f/8 Newt, most of my scopes for a number of years were on Dob mounts. I thought they were fine - until my wife starting using equatorially mounted scopes. For me, high power viewing is much more enjoyable with a driven scope, and I would not go back to undriven Dobs.

It is also important to remember that Newtonians are only diffraction limited over a very small area near the center of the field (unless you add a coma corrector), so if you let a planet drift from one edge of the field to the other you are not getting the best possible view for a good part of the time.

February 20, 2003 12:47 AM Forum: Equipment Talk

Cool down time for a SCT

Posted By Alan French
The lens in a refractor is at the end of the tube where it can radiate very effectively to the night sky. The SCT corrector (or Mak front element) can also cool quite effectively, but the mirror inside the tube can not radiate its heat in a similar fashion, nor is there air moving through the tube to hasten cooling as there is in most Newtonians.

Funding Member

Sponsors


View all sponsors

Search