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Posts Made By: Mark Holden

May 13, 2005 03:41 AM Forum: Refractors

3 Apo vs. 4 Acro

Posted By Mark Holden
Jeff Thomas said:

Just a quick poll. Given equal quality and prices and assuming a mix of deep sky and planetary observing, would you prefer a 3 inch APO or a 4 inch acromat?

I think I'd go for a 4" apo. grin

Seriously, the 4" will gather 77% more light than a 3" - that's significant.

But to get a very nice image from an achromat, you need a relatively long tube, and then portability starts to become a factor.

If I remove the dew shield, I can fit my TMB 115 into carry on luggage. My astronomical society's Unitron 102mm f15 is a little awkward to transport in a typical car, especially when you factor in a suitable mount. (The mount may need to be scaled up to handle the length and weight of a longer tube.)

How you factor image quality, light gathering, and convenience is up to you - but if it was me, I think I'd see if I could find a star party with a good selection of scopes similar to ones I was considering so I could look them over, and compare them head to head to make my best decision.

Good luck.

Mark

May 20, 2005 04:14 PM Forum: Birding Optics and Photos

Nikon D70 or D100 purchase

Posted By Mark Holden
Mike Dedon said:

I searched and did not find a good suggestion. I'm looking for a DSLR primarily for bird and wildlife. (I guess it's called imaging now). Nikon is my first choice but the Canons look good too. I also want to use it to supplement my dedicated CCD astro gear.

The Nikon D100 and D70 look good to me as well as the Canon EOS. Any quick opinions on these.

I know this is a very subjective question and I'm leaning toward the Nikon D100. Nikon maintains "most" of the AF series lenses are compatible and there are a large number of used lenses in the market. Telephoto and medium range are most important.

Anyone wanting to offer opinions are welcome.

I have a D70, and it's a nice camera.

For normal use, it has somewhat faster response time than the D100.

I use it with the "kit" lens, plus I have an Angenieux 28-70 f2.6 and a 70-200 f2.8 Nikon lens.

The camera only works in manual mode (without the meter) for non AF lenses.

Get a large memory card - you'll probably be taking lots more pictures.

For astro work, you'll want a spare battery or an ac adapter, as the battery runs down quickly during time exposures.


May 20, 2005 10:42 PM Forum: Birding Optics and Photos

DSLR question.

Posted By Mark Holden
Ron Streetenberger said:

I would like to know if a DSLR with a 300mm lens will produce the same image magnification as the same focal length on a film SLR. My Panasonic FZ20 has a max zoom of 72mm and is comparable to a film SLR with a 432mm lens. Thanks in advance to all you gurus out there.

It depends on the DSLR.

Most of the "popularly priced" DSLR's use a CCD that's smaller than a 35mm negative. On a Nikon D70, a 300mm lens gives the same field of view as a 450mm lens on a 35mm camera.

May 23, 2005 12:50 AM Forum: Refractors

Very old 4" Unitron

Posted By Mark Holden
Charles Peterson said:

The Tube and optics could use a good restore job, the paint is shot, as well there is a chip on the 60mm objective! The 4" Objective needs taking apart and cleaning.



Chas

I wouldn't take apart the lens cell unless you're very sure of what you're doing.

My astronomical society recently did some basic restoration on a similar era 102mm F15 Unitron, and we found all the objective needed was to have the lens cell removed from the tube so we could clean the rear element.

It was amazing how much junk found it's way up the tube to land on the back of the lens over the last 50 years. It came out beautiful, and it performs like new.

I've heard too many stories of people taking apart lens cells and when they're done, they end up with a damaged lens because an element cracked or chipped while they were trying to put it back in, or the reassembled lens didn't perform quite right because they couldn't get the spacing or alignment right.

The Unitron 4 is a very nice scope. They were well made, and give beautiful images.

Good luck with it.



May 30, 2005 08:51 PM Forum: Refractors

recoating service

Posted By Mark Holden
Dave Hamrick said:

Hello!
Where on Gods Green Earth can a person get AR coating applied to an old achromat set?
Google turns up very annoying results with no useful info.
Can anyone help?

Not sure what will be involved in preparing, but Majestic does anti reflection coatings, and based on working with him to get the mirrors for a 16" classical cassegrain re coated last summer, I think he'd be pretty easy to work with.

< http://www.majestic-coatings.com/ >

May 31, 2005 11:14 PM Forum: Equipment Talk

Help with Painting Aluminium

Posted By Mark Holden
Some thoughts.

Anodizing is probably the longest lasting finish, but you or whoever does it will need to strip off all the old finish. It won't cover blemishes.

Powder coating is nice - I had a custom shortwave radio cabinet done and it seemed pretty reasonable. I suspect this is a little more tolerant of minor blemishes or old finish remaining, but you can also end up with moving parts that bind.

Epoxy paint - the two part stuff is quite durable - but it's not cheap, (~$100 for 2 gallons) and it's likely to show brush marks unless you can thin it down for a sprayer. Again, you'll need to be careful to make sure moving parts won't bind. Also nice for painting garage floors.

Last but not least - if you use something like Krylon you could touch it up whenever it starts getting banged up for a few bucks a can.

June 24, 2005 11:33 PM Forum: Politics

Here is John Kerry's response to Karl Rove

Posted By Mark Holden
Perhaps he (and you) should read the remarks President Clinton made at Georgetown on November 7, 2001.

Here's a link:

< http://ecumene.org/clinton.htm >


June 24, 2005 11:55 PM Forum: Politics

Bush's Upcoming Speech

Posted By Mark Holden
I think your criticism of his speech is a bit premature since he won't be giving it for another four days.

Is there a reason you feel he shouldn't show or call for support of our troops?




June 28, 2005 07:07 PM Forum: Equipment Talk

Does anyone know anything about Eclipse Telescopes

Posted By Mark Holden
Burgess sells a white version of the eclipse 70 as a "burgess special".

I got one at NEAF, for possible use as a guide scope.

For the price ($60 without eyepieces) it was a good deal, but I'm not sure it's sturdy enough to hold my ST4 on a long term basis.

The larger ones are probably better made, but I think I'd want to see one in person before buying it.




July 6, 2005 02:45 AM Forum: Equipment Talk

Astronomik filters

Posted By Mark Holden
Larry Seguin said:

These have been out for a couple of years now, I understand, but there's relatively little that's been said about them. Have any of the guys out there A/B'd them against Lumicon, and you're willing to render a judgement?
I'm wondering if they're worth pursuing, or is one premium brand pretty much as good as another? Thanks in advance for any input!
Larry Seguin
Taos, New Mexico

I prefer the Astronomiks.

Here's a review someone did of them:

< http://www.astromart.com/articles/article.asp?article_id=227 >

As an update, I see there was a thread about an article by Phil Harrington comparing filters in the August issue of Astronomy - and the DGM Optics NPB was considered "the best", and it's less expensive than the Lumicon or the Astronomik.



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