Posts Made By: Paul Hyndman

Edges or obstructions cause light to diffract or scatter in ways that may or may not be helpful in the creation focusing masks. Ron Wodaski has an excellent section about focusing in his book "The New CCD Astronomy" and has generously made that entire section available online as a sample. I heartily suggest you take a look at it (and consider buying his excellent book!):

http://www.newastro.com/newastro/samples.asp

One of the variants he covers uses two triangular openings whose apexes create diffraction spikes that "point" to the center of each image.

Personally, I get along best with three round openings placed as closely to the perimeter as is possible. The further apart the openings are, the more dramatic the effects of moving in/out of critical focus. The bigger the opening, the more the bulk of its area is shifted towards center (the only direction you CAN go from the edge), diminishing the effects of de-focus.

The trick is to have them be just large enough so as to have each off-axis image be clearly discernable (bright enough), but not larger. Then find a moderately bright star and have at it!

Quite helpful for optical principles and design are: "Telescope Optics Evaluation and Design, by Harrie Rutten and Martin van Venrooij, Published by Willmann-Bell Inc. ISBN : 0-943396-18-2 and "Star Testing Astronomical Telescopes" by Harold Richard (Dick) Suiter also Published by Willmann-Bell Inc. ISBN : 0-943396-44-1

FWIW, the three images below were taken during an equipment (new LCD screen) and focusing technique "shake-down" test (all taken within moments of each other). The first was focused using only the larger display screen, while the second and third images also used a traditional three-holed Hartmann mask in conjunction with the expanded display image.

I've since gone to using *only* point sources (a nearby star) as my target, locking down the focus, then swinging back to my "extended object" target. IMHO, there is much more clarity with point sources rather than trying to use the mask on extended targets. Here's what one of those shots looks like:

http://www.astro-nut.com/moon19oct02.jpg

Paul


More info on the below image can be found at:

http://www.astro-nut.com/interact.html

Uhhh... if you are referring to the triple panel shot:

http://www.astro-nut.com/interact.html

Those were from a 5" Tak' with a Nikon CoolPix995 direct-coupled to a 31mm Nagler Type 5 eyepiece. Since the lens of the Nikon CPs are not removable, afocal is the only method possible. I used a close-coupled Televue adapter that screws into the camera's filter ring:

http://www.astro-nut.com/adapters.jpg

This close arrangement allowed the camera to be used even at 1X (no zoom) without vignetting, though the zoom was used to produce the three different image sizes seen there.

Even though forced to use afocal methods, the Nikon CP was pretty good for Lunar shots:

http://www.astro-nut.com/gibbousmoon.jpg

The shot below was with the same scope, but with a Canon D60. As an SLR, it does not require afocal methods, and was taken at prime focus (no eyepiece, Barlow, Powermate, etc).


Paul

Just my opinion, but...

In that aperture range, and with sky and wildlife shots as a big part of your planned itinerary, I'd be more inclined to put the money into a quality small refractor (preferably an APO ~f/6 and 85mm to 130mm in aperture).

Your coupling method will be afocal (as the CoolPix lens is not removable) and the optical zoom will easily allow you to increase the image scale sufficiently for targets reasonably within your grasp, but a higher f-ratio (and central obstruction) will limit the lower end of your range. 5" of low f-ratio aperture can provide woundrous views of wildlife and celestial targets as well as stunning shots of solar system targets with the appropriate eyepieces.

You might want to visit the Digital Astro egroup for more info:

http://groups.yahoo.com/group/digital_astro/

Paul

Gorgeous Andrew!!!

Thanks too for your info on the Tak' spacers... three of 'em alright (I checked last night). Now the mystery of my diffraction spikes is solved!

Paul

Fantastic image, Andrew... the color was worth waiting for!!!

Paul

As an alternative to precut foam holes or cube-foam, you may want to consider divider boards. You may also want to consider going to Lowes and checking out this item for $20:

http://makeashorterlink.com/?U2C325583

(this case includes BOTH pluck-foam and divider boards!)

Here's what the Lowes case looks like (upper shot) and divider board layout in an eyepiece case (lower shot).

Paul

A Christmas tree ornament will work... set it up so as to catch the glint of the sun, taking care not to aim the scope to the sun! =8^O

Paul

Wal-Mart's, Lowes, etc...

see earlier posting here at:

http://www.astromart.com/messages.asp?message_id=46682&page=2

Tremendous shots... really awesome!!!

Thanks for sharing!

Paul

Wow... gorgeous image Mark!!!

Paul