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Posts Made By: Tom Mengel

April 12, 2002 08:13 AM Forum: Equipment Talk

Barlowed secondary newts...

Posted By Tom Mengel

Hi all,

I know some of the Celestron 6" reflectors (HD-150?) have a small "corrector lens" in front of the secondary that makes the F4 primary into a F7 overall system. I'm assuming this is a small Barlow/corrector that also cuts down on the secondary size as it allows the secondary to be "pushed back" in the optical path.

Doing some preliminary ray-trace work ups with a F5 6" mirror (I have a partially polished mirror I'm debating on finishing) In the ray-trace I come out with a 1.2' dia. "Barlow" in front of a secondary of the same size (minor axis) that yields an unobstructed 1.8" dia. field at the focus. This would end up with about a 20% obstruction and would yield a F ratio of about F8 in a tube about 36" long.

Why is this design not more common? It would seem the maker could do some nifty things off axis correction wise (and maybe even use cheaper spherical mirrors) modifying the small lens and you end up with an OTA about 1/2 the length (and weight) to boot.

So ... would any of you much more knowledgeable forum lurkers care to shoot some holes into this idea before I put any more time in on it??

Thanks!

July 10, 2002 03:02 PM Forum: Equipment Talk

New (?) Celestron NexStars

Posted By Tom Mengel

What's new about the Celestron NexStar 5i and 8i? They both seem to be a few humdred less than the first generation, any idea how they cut the prices?

July 25, 2002 04:08 PM Forum: Off Topic Discussions

Yet another suggestion for Herb...

Posted By Tom Mengel

I was just looking at some of the "other" topic headers here in forum land and it hit me as to why I don't visit more than two more often (I tend to "hang" at the "Equipment" and "Telescope Building" forums). It's just a pain to wade through the headers to view all the entries I might be interested in, even though I like all the alternative forums, too.

Why can't the forums be set up like the AM advert categories so you can list either ALL the new/within-date entries for ALL the forums, or (alterably) pick and choose what categories/headers you want? Maybe just a user profile controlled "mixed headers" group where you can click off what individual forum groups you want to collectively look at under that selection. Of course, you should be able to still look at individual group headers as it's set up now. This might also get folks to "spread out" their posts over all into the "correct" forums instead of lumping them all in the most popular hang-outs just to be sure they get responses.

Just a thought... otherwise Herb is doing an outstanding job here.

Tom Mengel

August 1, 2002 02:55 PM Forum: Equipment Talk

Re: Conrando or Thousand Oaks

Posted By Tom Mengel

I had a $$ (older) 1000 Oaks glass filter for my 8 inch SCT and ended up trading it back in on other astro goodies. It was just TOO dark to be usable! Even when shading the EP with a dark towel it was really hard to see the solar image and it was a dark yellow/brown that was not the most contrasty color to try to work with.

I've not tried it myself but for cheaps you could try making a Baader film filter (APM sells it, you have to mount it into a cap that fits over your OTA objective). Others will correct me here I'm sure but it's supposed to be a natural white view (nutral density only, no color modifications) and much brighter.

Good luck!

Tom Mengel

October 3, 2002 12:31 PM Forum: Equipment Talk

Finder question

Posted By Tom Mengel

I have an old dbl lens reflex camera "viewer" lens that's about 22mm diam. by 75mm FL. Just for kicks I taped it onto the end of an old 90* prism diagonal and slapped a (scrapped) Tasco bino WA EP (about 20 some mm FL at 65* AFOV) into the 1 1/4 barrel and tried it out as a "something between a 1X reflex and a real-finder" finder. In short it work really well! About 15* FOV at about 3.7X that still give an exit pupil of about 6.5mm. It also cuts through the light pollution MUCH better than a red-dot where I live and is still wide angle enough to get onto an object with ease (and is about 1/4 the weight of a "traditional" 7X50 finder to boot).

So - am I missing something here? Seems to me at 3 to 4 power anything larger than a 20-25mm objective is wasted exit pupil size anyway, and the resulting (smaller) finder is not a lot bigger than many 1X reflex sights that can't help cut through the light haze.

Any comments?



October 3, 2002 02:42 PM Forum: Deep Sky Observing

Posted By Tom Mengel

This might be better in the "off topic" list but the military folks are concerned that high exploding meteors could be mistaken for an atomic attack... we have the technology to tell the difference but others may not (just one more reason to "just say no" to Saddam...).

See the following URL (sorry it's so long!):


http://www.newsday.com/news/nationworld/wire/sns-ap-asteroid-dangers1003oct03,0,4216424.story?coll=sns%2Dap%2Dnationworld%2Dheadlines

(And they also seem to confuse "Asteroid Explosions" for meteors...)

October 15, 2002 12:06 PM Forum: Astro Binoculars

Posted By Tom Mengel

I have an old pair of WW-II vintage B&L 7X50's Mk. I's. The lenses are suffering "cementing craze" (The balsam cement between the objective elements is old and dried out - my Dad kept them in a car trunk in CO where is regularly got down to -20F … ) but it also needs cleaning and a general tune up. BTW - with the exception of a smallish 50* AFOV and dirt and the crazed objective lenses, "as is" these will beats the socks off many newer binos...

My problem is it was dropped at some point and the lens housing with the retaining ring that holds the objective in got bent so getting the ring (and the objective) out of one side is going to be "interesting" (read "a real Bear"). The lens itself is fine, just the housing is bent.

Texas Nautical was mentioned a while back as a good repair place but I want to throw this out here one more time, esp. considering the damaged lens mounting. Any feedback as to where I can get these fixed up?

Thanks!

Tom Mengel

January 14, 2003 11:06 AM Forum: Religion

Rabbits and weeds....

Posted By Tom Mengel

Man - I'm gone for 3 days and the first 4 pages in this forum are all marked "new" with 6 new (and LONG) threads... I'm impressed!

Thanks Herb, for putting this on a separate forum. Definably better than clutting up the others.



June 5, 2003 11:37 AM Forum: Equipment Talk

Cheap Binos

Posted By Tom Mengel

Does anyone need a cheap pair of decent tag-along "BigBinos" for your car trunk?

FWIW Surplus Shed has a very good price what looks to be the Bear/Astroview (China) 10X70's for 72 dollars US. Don't know if the EP's are standard or WF or if the prisms are BAK-4 glass, but they sure do look like my old Astroview 12X60's on steroids.

This is about 1/2 the price of comparable units. At about a buck a mm for a pair of 70s (and not a lot more than what a little bubble-wrapped pair of TASCO binos costs) that's not a bad deal. I've ordered a pair and will give some feedback here when I get them.

TM

June 5, 2003 11:39 AM Forum: Astro Binoculars

Cheap Binos

Posted By Tom Mengel

Does anyone need a cheap pair of decent tag-along "BigBinos" for your car trunk?

FWIW Surplus Shed has a very good price what looks to be the Bear/Astroview (China) 10X70's for 72 dollars US. Don't know if the EP's are standard or WF or if the prisms are BAK-4 glass, but they sure do look like my old Astroview 12X60's on steroids.

This is about 1/2 the price of comparable units. At about a buck a mm for a pair of 70s (and not a lot more than what a little bubble-wrapped pair of TASCO binos costs) that's not a bad deal. I've ordered a pair and will give some feedback here when I get them.

TM