Posts Made By: David Wasch

Greetings,

I will be taking a trip to the Pacific coast of Costa Rica next week with my newly acquired 80mm short refractor (Nighthawk ala Astromart). Although it seems I will not be able to see below about -72 degrees declination, I have never had the opportunity to see below what Maryland offers.

Does anyone have a recommended sight I should not miss? Will the seeing be ok on the coast, or should I try to get to a higher elevation? Should I stick with wide views or push the magnification? Thanks for all your help. I'll come back with a report (but no photos).

-David

Although I know there are always trade-offs, I decided to build a 12.5" dobsonian that will be used for visual use and CCD photography. I plan on getting a Tom O. platform for tracking the sky. I will probably go with a ST-2000XM for a camera (if I can scrape together the funds).

I've ordered a very good mirror and plan on investing in an excellent cell and focuser. I expect to design this dob a bit heavier and more stable than most.

The question is-- Is there any general design guidance any of you can offer? For instance, I'm not sure if I should go with a very rigid truss or minimal secondary support (to reduce wind resistance). Would it be wise for me to build in a way to 'lock' the position of the dob? etc, etc...

Thanks for you time.

-David

Howdy,

I don't know how many people saw the ad for a 3.3 Megapixel CCD for $450(#332235), but it definately piqued my curiosity. The builder, Tom Gyun, lives in China and seems to be building very inexpensive cameras. It doesn't appear he has sold anything on Astromart as of yet.

I know, this ad has "WARNING" all over it. The fellow is in a distant foreign country, the ad is nearly too good to be true, etc. However, taking a look at the seller's web site gave me the impression that he is sincere in his efforts.

http://qhy.lamost.org/3megaccd/index.htm

I have communicated with him a bit, and it seems he will be willing to build some cameras to spec, if asked. Buying a camera through this vendor is uncharted territory, and risky, but for the price involved, I think it might be worth the risk.

I'd appreciate some input from the group on this.

-David

I'm building a 12.5" f/5.3 dob, which will be used for CCD photography. Since I'm looking to have a clean field over 26mm, I plan on using a Paracorr.

I have read about the 3/8" in-travel that the Paracorr requires for focus, but on the Al's web site he also states that "You must adjust distance from last lens surface to CCD surface to 55mm+4mm for best performance."

Does this mean that in order to use the Paracorr and CCD camera, I need to have total backfocus of 68mm (9+55+4)!? That seems huge. It would push my focal plane to at least 13" from the center of the scope and push me towards a 3.1" secondary.

This seems like a bit much, and I'm hoping that I'm misunderstanding something. Someone, please enlighten me. Thanks.

-David

I am pretty embarrassed about this...

I was entering a rating and comments for Ralph Ford, who just sent me a very nice Pegasus Mirror, and I referred to him as 'Tom' in my comments, twice. I am working on a sale of an ota to a Tom and having skipped my coffee this morning, made the blunder.

Understandably, the system here does not allow one to change ratings once they are entered; there is only the option to delete a rating. However, since this is basically a typo, I would like to make the change, for Ralph's sake. He was great to do business with, and deserves a rating folks will take seriously.

If anyone knows how I can go about fixing this, please let me know.

-David

From what I can tell, combining a 35mm Panoptic with a x4 Powermate, you can get great relief and a wider field of view than is often available from shorter focal length eyepieces. Adding a 17mm Nagler to the mix would seem to get me four premium eyepieces for the price of three.

Is the Powermate that good? Am I missing something?

Thanks for your input.

-David

Ok, I’m starting to come to the end of my rope with a mirror vendor.

I purchased a 12.5” mirror one year ago (!) from a vendor (who shall remain anonymous) with a reputation of taking a little long to get out the final product. The mirror was initially promised in three months, but I knew it would take longer. I recognized the risk, and plunked down the money because I had some time before I was to begin the project.

Flash forward 10 months, and I start to become concerned because Mars is coming and I want to have the scope finished. The vendor and I decide to pull the plug on the order, and he says he will get a refund out to me ‘shortly.’

Since that time (two months ago), the vendor has refused to answer three e-mails over the past six weeks asking for the status of the refund. I know the vendor is actively receiving e-mail, because he posts on these forums.

I have run out of the friendly options for the refund and I need my money back to finish building my telescope. I figure I can go to the Better Business Bureau, but don’t know what other steps will shake a few coins out of this guy’s pockets. Any advice would be appreciated (yes, I know I should not have gone with a vendor with a bad rep).

-David

Greetings,

I have read in this forum that an eyepiece 1.5-3mm exit pupil hits the 'sweet spot' for the viewer. I have looked for (but may have missed) an explanation of why this is so. Why not a 4mm or 5?

Thanks

-David

In June I put up a post on the exit pupil ‘sweet spot.’ I really enjoyed the variety of views on the topic, and recognizing that there was no broad agreement on the topic, filed it under ‘unresolved’ in my mind.

The latest Sky and Telescope (September 2005) shed some further light on the topic, suggesting that Maksutov might have been onto something when he claimed that 2-3mm provides the best resolution for the human eye. According to S&T, the eye’s aberrations (including spherical and chromatic) overwhelm aperture gains with a pupil of 3-4mm. This makes sense from an evolutionary perspective: night time pupils don't need to have such high resolution as day time pupils for survival.

Does this mean long focal ratio eyepieces aren’t worth it? Of course not. And yes, subjective experience at the eyepiece is more important than theory. But, it does seem that the pupil ‘sweet spot’ is not mere bunk, and may have its feet in science.

-David

Mike,

What are you interested in imaging, planets, nebulae? If you are interested in long focal lengths for high magnification, you should consider a high quality dob like Starmaster on a Tom O. platform. You could also buy great optics and build your own. I'm pulling together a great imaging scope and mount for under $4,000 (thanks to Astromart).

-David