Posts Made By: Rod Kaufman

Dale,
Adding an ED barlow won't remove the blue halo but adding an 81B photographic filter will do the trick.

Although I have a celestron power tank, I decided to buy a back-up unit from the sportsman's guide. The unit has a 110 volt ac power outlet with up to 140 watts and a 12 volt power outlet with a 10 amp max. It also has a USB power outlet plus a AM/FM radio with decent fidelity and a removable/rechargeable green/red-blinky flashlight. The entire unit can be recharged at home from its AC adapter or in the car from its 12 volt adapter. The unit is yellow in color and is rubber-armor coated. It's a lightweight at 8.3 lbs and the dimensions are 7 x 6.75 x 11.25 inches, making it very compact. I think it meets or exceeds "Floyd Blue specs" and it's priced right: $39.00.
If you google sportsmans guide and click on amazing deals at the top of the page, it will appear on the last page (#9) of the website.

According to the text, "Telescopes for Skygazing": increasing the central obstruction above a tenth of the diameter (only 1% of the area) in a reflector noticeably changes the perfect stellar diffraction pattern, and image contrast of extended areas is reduced with startling rapidity with increased obstruction. A 1 1/2" diameter diagonal used with a six-inch mirror (1/4 the aperture) obstructs only 6% of the incoming light, yet there is considerable loss of image contrast. In the attached figure we see (left) the effect of increasing the central obstruction going from zero obstruction at the 12 o'clock position clockwise through 1/6, 1/4, 1/3, 1/2 up to 5/8 of the aperture obstructed at the 10 o'clock position. The central pattern is also from an unobstructed aperture, but from a star one magnitude fainter. If we refer to the light distribution pattern, more and more light is going to the outer defraction rings as the obstruction gets larger, thus reducing surface contrast of any object viewed. Also note that as obstruction increases the central disc actually gets smaller; this being the reason obstructed apertures can under favorable conditions separate closer stars, yet can fail on faint surface detail differences, as the second photo of the figure illustrates with equal magnitude double stars at Dawes' limit separation. Double having low-intensity companions falling on an enhanced diffraction ring may be missed entirely, as illustrated in the third photograph with unequal double stars at 1.2 times Dawes' limit separation--companion 2.5 magnitudes fainter.

I see Oneil photo is reporting Antares won't be selling their 10" dobs anymore and only the 8" and 12" sizes will be available. Oneil says the 10" models were slow sellers and the 8" and 12" sizes are easier to move around and offer more bang for the buck. I have to disagree with that, I like the 10" size and I find it to be portable with good results on many DSOs. In fact, I've got two mirrors for my 10" asian dob and I swamp them back and forth with relative ease.
Let's hear it for the 10" dobs!

I had the opportunity to view through a 16" F4.5 Meade Lightbridge which was set-up next to my own 16" F4.5 Astrosystems dob at Mt Pinos on Saturday night.
Conditions were generally good with the sky being darker than the previous Saturday, with a reading of 21.66 on the dark sky meter after 2:00am. I could see the difference visually through my scope wherein the extensions on the Saturn nebula were more easily seen than in the previous week. The crescent nebula was a real blazer as well.
Overall, the lightbridge is shorter than my own 16" even though the F ratio is the same. The base, however, is quite wide and I suspect it would be a bit of a chore taking it in and out of a vehicle. The mirror box reportedly weighed in at a hefty 70 lbs and the owner said he needs a second person to help him move it.
The motions were generally smooth as I panned widefield views of the milky way in Cygnus. Gone are the annoying bearings of the hardin GSO dobs, a fore-runner of the lightbridge.
I enjoyed the widefield views of the milky way and coma was reasonable (and not objectionable) given the fast f-ratio of the scope. I didn't star test the scope but I did enjoy the contrast of the images, although Jupiter was too low for views of its belts and bands.
I did not like the typical GSO secondary attachment to the spider with glue. It's a fairly big secondary, though appropriate in size, but I had my secondary fall off and chip my mirror in my hardin 10" dob.
All in all, I think the 16" lightbridge is a whole lot of scope for the buck...

Nova optical systems is back as an astromart sponsor. That's good to see since they offer high quality coating services at reasonable fees with fast turn around and something I couldn't find stated on other coating websites: a one year guarantee on their work.
I've used their services and I've been pleased with their work.
Glad to see they're back as a sponsor

David,
I have a book entitled "Telescopes For Skygazing" by the late Henry E Paul who lists Tinsely as a supplier of "cassegrain telescopes". It also pictures a Tinsely 24" in a dome and it appears to be a classical design but I can't be sure of it. I remember Tinsely used to advertize in Sky and Tel many years ago. Paul's book had an address for them in Berkley,California although I haven't heard about them in years. From what I've read on the net, the current Tinsely lab redid the Hubble and fixed its faulty optics with a corrector, implanted by an astronaut? They're still listed near San Francisco.

Assuming the scope has cooled for awhile to adjust to the ambient temperature, perhaps an optic is pinched causing astigmatism or a problem with the collimation of the scope. If it's really bad, I'm wondering if you've got a flipped element or one that's not rotated in alignment with the other. perhaps a missing spacer in the objective? The objective might be pinched in its cell or perhaps you've been the unfortunate recipient of a bad objective with one or more defects. I assume you've tried it with and without the diagonal and the scope is collimated. Have you tried a star test by racking the eyepiece inside and outside of focus to see if the rings formed by the objective appear concentric and relatively similar in both positions?
If you're not sure what's going on, try Orion's customer service. You may need another unit.

While it appears Obsession has the market with their new ultracompact scope, it appears to have compromises like any other scope. While I think the strengths of the 18" ultracompact appear to significantly negate its weaknesses, here's what I would be concerned about:
1. 3 vane spider: I thought I read somewhere that the three vane units produce 6 spikes on the star image or is this not correct?
2. Use of a 3.1" secondary: I can't see how this size would be large enough to manage an 18" short f-ratio mirror without reducing the effective aperture of the primary. I note my 16" F4.5 mirror utilizes a 3.1" secondary which just encompasses the entire primary so I can't understand how it could possibly do the same with a shorter F ratio 18" mirror. I note both night sky scopes and starmaster use 3.5" secondaries with their 18" models which operate at F4.5.
3. Cost: While expensive, I certainly like the design and the latter, in my opinion, makes it worth the price.
Still, I've looked for an alternative and I found a website offering a lightweight design:
http://www.liteboxtelescopes.com/
Anybody have any experience with this design?

Do you think this gizmo works?
I hope it works better than its website:
http://supernorthstars.tripod.com/id2.html