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Posts Made By: Alex McConahay

November 20, 2006 05:13 PM Forum: Beginning Astronomy?

Correction?

Posted By Alex McConahay
Correction in this context means adjusting the shape of the mirror so that all the rays of light focus in the same place. If one does not do this, the light from the outer regions of the mirror (further from the center) will focus shorter or longer than the light from the inner regions (towards the center).

With an under corrected mirror, the marginal rays cross nearer to the objective than the central rays. With an over corrected mirror, the marginal rays cross further away from the objective.

In either case, the light is not as tightly focused as it should be. The practical implications:

Object does not come to a crisp focus, but rather slides through focus.

Object is not as bright as it otherwise would be because whatever light it has is spread over a slightly wider area.

Alex



November 21, 2006 04:45 AM Forum: CCD Imaging and Processing/Deep Sky

Interesting article on Megapixels..

Posted By Alex McConahay
I think the article is very accurate for the average person. It is quite possible to have too many pixels--witness how many times somebody posts to this site with a picture that has to be scrolled to be seen.

(I still want more for my purposes, though. Having more megapixels allows for cropping or blowing up bnigger.)

Alex

November 22, 2006 01:29 AM Forum: Equipment Talk

Mounting Question

Posted By Alex McConahay
I don't see how the 11 could be happy, and have my doubts about the 9.25.

Celestron sold the 9.25 on a CG-5 for a few years (maybe it still does). I found mine inadequate for anything but the least demanding visual.

The weakest point is obviously the tripod. But there are other weak points. One is where the head mates to the base. It just is not sturdy enough. You can see the head itself flex, completely setting aside where it mates to the tripod.

In short--you might get by by replacing the tripod with a pier/pedestal, but I doubt you will be happy for long. ANd at least in the version of CG-5 I had, there was no real provision for autoguiding, so the mount was really not good for astrophotography.

Alex

November 22, 2006 03:35 PM Forum: Beginning Astronomy?

Another observing chair question

Posted By Alex McConahay
Sitting on one of those SBIG chairs can be uncomfortably cold unless you turn off the cooling chip.

Alex

November 24, 2006 02:53 PM Forum: Equipment Talk

Can't decide which SCT

Posted By Alex McConahay
You seem to be having difficulty with the decision in part based on not wanting to lose the larger aperture of the Dob. That should not be a concern.

Dobs and SCT's have different jobs in life. You cannot compare them only on aperture. Yes, you get more aperture with your Dob, and will be able to see wider fields with more resolution, probably just a bit more contrasty. However that does not mean you cannot step down in aperture when you buy an SCT. Your SCT will get more inherent magnification, and could be used for astrophotography.

As for portability: I don't think you will be gaining all that much in portability overall. Yes, the Dob is probably larger. But when you throw in a Tripod and perhaps an equatorial wedge--you aren't saving all that much space in the truck. And setup is an important part of portability. It will take longer to set up the SCT.

In short--we cannot answer your question for you. Go to some star parties and help people set up, and do some viewing, and all that--and then make your choice.

Alex


Alex

November 26, 2006 05:12 AM Forum: CCD Imaging and Processing/Deep Sky

Same old dilemma: AVG or SUM?

Posted By Alex McConahay
It also depends on what your raw data is.

If your raw date is bright enough, then when you add it all together in a straight sum, you may oversaturate the highlights, and lose detail.

On the other hand, if you simply avarage, and you don't have enough range in your individual exposures, you will have to do more stretching than you would with a sum.

I think in general your rule about the more exposures you have the better averaging becomes holds true only if you have enough signal in the exposures that they don't need to be stretched as much later.

Alex

November 28, 2006 01:54 AM Forum: Beginning Astronomy?

Astronomy Software

Posted By Alex McConahay
Do a search for "Cartes du Ciel"

DOwnload it, and see how you like it. It is a pretty powerful program, and free. It will give you some taste of what a planetarium program can do. If you need more, then you can continue looking for other software.

Alex

December 4, 2006 12:16 AM Forum: Equipment Talk

All I want to do is astro-image...what do I get ne

Posted By Alex McConahay
In my opinion, get or make an equatorial wedge.

Get something to mount the refractor to the LX200 piggyback style.

Get whatever adapters you need to use the Digital Rebel on the refractor.

And start taking pictures.

When you have all the bugs worked out, then think of where you want to go next.

Alex

December 4, 2006 12:19 AM Forum: TeleVue

Focusmate on TV-85-wonderful or merely great?

Posted By Alex McConahay
I had some money burining a hole in my pocket, and bought one. It is nice, but not at all necessary. There would have been better uses for the money.

The slight improvement in focusing is nice, but not as nice as having the TV 85 fit easily into its case.

Alex

December 4, 2006 06:39 PM Forum: AstroMart FAQ

Need Help about size and Focal Length.

Posted By Alex McConahay
Jim,

To start with, I think this is just the kind of question that the people up in the "equipment" or "Beginner's" Forums would just love to discuss. These people in this Forum mostly talk about how Astromart itself works (or, rarely, does not work). So, try reposting in the Equipment or Beginner's Forums. You will get a better response.

But, to answer your question. SCT's are famous for having long focal lengths folded into a short tube. A newt, on the other hand, has a longer tube with a relatively short focal length. The length of the tube is related to the length of the optical path in Newt's and Refractors (not exactly, because of dew shields, maybe diagonals, and so forth) but not in an SCT, which folds the light path.

So, don't be surpised.

Alex

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