Posts Made By: Gary Carter

Don,

Either scope will work...ultimately the mount is going to be the bigger factor in choosing which to use. The 8" newt may yield better resolution, however, if it is on a non-motorized dob mount it may be difficult to track and photograph objects.

The Celestron Neximage is primarily geared for Lunar and Planetary imaging. You will want to use it in conjunction with a good barlow to provide the magnification necessary to bring out Lunar and Planetary detail. You will also find that a motorized focuser is a good investment for achieving fine-focus.

Paul,

You might try joseph nastasi at http://www.parallaxinstruments.com or Ken Dauzat at http://www.kendauzat.net

Both are makers of rings and one of them is sure to have what you need.

Greg,

In his book "Star Testing Astronomical Telescopes" Harold Suiter does a great job of describing many of these these terms and how to recognize them through star testing. The book is a great read. See http://www.willbell.com/TM/tm5.htm

On the web there are a few resources. For some background information on your questions about RTA, P-V, and Wavefront RMS see: http://www.telescope-optics.net/aberrations.htm

Regards,

Mike,

Collimation in the daytime will help you understand the process. Hang a round silver Christmas ornament up across your yard (or in the neighbor's yard if necessary) such that you can see the reflection of the sun from the position of your scope. Focus on this reflection and center it in the FOV using a high power eyepiece, a green filter (Note 1) if you have one, and NOT using a diagonal. Next slightly defocus the image to see the expanded diffraction ring pattern (three to five rings. Note 2). The image in the eyepiece should appear to be a bullseye; the dark center is the secondary position. *Slightly* loosen the three collimation screws, one at a time, noting the effect to collimation. Don't loosen them too far...just enough to provide a bit of room for +/- adjustment in each. The goal is to achieve a concentric ring pattern (i.e. centering the secondary shadow inside the diffraction rings) while *tightening* each of the screws a little at a time until they are just snug.

Be aware that those screws, in addition to setting collimation, also hold the secondary in place. Turn them all too far in one direction (too loose) and the secondary can detach from the corrector plate and fall into tube and the primary mirror! Perfoming this with the tube parallel to the ground is a good idea for beginners. (Hang the ornament at the height of the scope and you'll avoid any serious problem!)

Don't be afraid. Develop an understanding the proper procedure and it soon will be second nature to you. As suggested if you can find and experienced individual to walk you through it once you'll see it is an easy thing to master.

Replacing the stock collimation screws with Bob's Knobs will simplify the whole collimation process. It eliminates having to manipulate a tool (no screwdriver or allen wrench required). Installation of Bob's Knobs requires you to remove the secondary screws, *one at a time*, and replacing them with Bob's Knobs. They make it a breeze to touch up collimation under the stars in the dark (Note 3).

Notes:

1) The green filter (or better yet, a Baader Solar Continuum filter) will pass a narrow band of light through making the diffraction pattern easier to discern.

2) If you are too far out of collimation and cannot see diffraction rings then strongly defocus to see the off-center shadow of the secondary and rough-center the shadow inside the white ring. then return to a slight out-of-focus setting to see the diffraction rings.

3) Let the tube reach thermal equalibrium at night to fine tune collimation while pointing at a bright star.


Refer to http://legault.perso.sfr.fr/collim.html for more collimation details and reference images

The boundary layer directly in front of the primary and/or secondary mirror surface is the thermal effect you are battling. There are multiple variables that will determine the best solution for your setup (mirror thickness, telescope structure, and temperature variations at your locale).

Dual boundary layer cooling is employed through an array of holes on one side of the rocker box and fans pulling air across the mirror in Teeter designed dobsonian scopes. (see photo). Alan Adler recomends blowing air across the mirror in his January 2002 S&T article "Thermal management in Newtonian Reflectors". Bryan Greer mentions dew prevention as a side benefit of blowing air directly onto the primary, but that photograph showing a fan suspended in the shadow of the secondary directly above the primary with wire... 8O ... Scares me!

REFERENCES:
http://www.fpi-protostar.com/bgreer/fanselect.htm
http://www.pha.jhu.edu/~atolea/WAS/thermal_management_newtonians.pdf
http://www.teeterstelescopes.com/catalog.html

Alex,

I reprocessed your raw image to bring out some detail on the spacestation and surprise... it isn't what you thought it was! 8O

I have the 31 and 26 Nagler and use them quite extensively with my C11 as well as with my other scopes. I love the wide views they provide. They both beat my Tele Vue 40mm Widefield by quite a margin in all respects.

However I recently read an article where a C14 owner compared the performance of the 41 Panoptic against the 31 Nagler and he picked the 41 Panoptic over the 31 Nagler stating it did a better job of countering the coma present in the C14 making for a sharper image across the entire FOV. I have not used the 41 Panoptic myself.

You might see if some folks in your local club will let you do a hands-on comparison before you buy.

Here it is....
http://www.astromart.com/classifieds/details.asp?classified_id=663743

Simply beeeeauteeeful!!!!!! 8)

This is a well covered topic. A quick search from 2005 forward on "corrector plate cleaning" yields the following:

Home > Forums > Beginning Astronomy? > Message #683925
Home > Forums > Reflectors > Message #625653
Home > Forums > Other Forums List > Celestron > Message #486271
Home > Forums > Equipment Talk > Message #476385
Home > Forums > Other Forums List > Schmidt Cassegrains > Message #440277
Home > Forums > Beginning Astronomy? > Message #432672