Posts Made By: John Biretta

October 10, 2012 01:09 AM Forum: Telescope Making

question about dob construction // secondary mirro

Posted By John Biretta

I think you just made a oversight somewhere. The distance from mirror surface to focuser does not depend on the diagonal size, at least for this program. It will depend on stuff like the focuser height, extra focus distance, tube thickness and tube diameter.

As you change the diagonal minor axis from 3.1 to 3.5, you should notice this value "0.xx" in inches right below the "Calc" button changing:
"100% Illuminated Diameter: 0.xx inch"
It should be about 0.4 inches larger for the 3.5 inch diagonal (assuming both values are larger than zero).

October 21, 2012 07:42 PM Forum: CCD Imaging and Processing/Deep Sky

Images for October

Posted By John Biretta

Very nice! Great details ans subtle yellow-brown colors in dusty regions.

November 24, 2012 06:33 AM Forum: Equipment Talk

ICX 618 vs ICX 098 comparison measurements

Posted By John Biretta

It would be interesting to compare the signal-to-noise ratios produced by the two cameras for some identical exposure. For example, you could expose both cameras with the same illumination and exposure time, and then compute the average counts in some region near the center of the detector divided by the standard deviation for the same region. This has the advantage that it is independent of the gains (counts per detected photon) in the two cameras. You could also do it for different light levels or exposure times. In the limit of bright targets, the answer will depend only on the quantum efficiency of the detectors. But in the short exposure / faint target limit it will also depend on the so-called "read noise" of the camera.

The number of counts produced by the cameras is not a great indicator of the performance, since the counts per detected photon is somewhat arbitrary. One manufacturer might set the gain to produce 2 counts per detected photon, while another might set it at one count per detected photon -- so just because one camera produces twice as many counts as another does not necessarily mean it is twice as sensitive.

January 25, 2013 10:07 AM Forum: Equipment Talk

Pronto Vs Televue 60

Posted By John Biretta

I have used a Pronto and cannot recommend it. Your money would be better spent on a modern ED
refractor like this one at a lower price point. I am very happy with mine:

http://www.telescope.com/Telescopes/Refractor-Telescopes/Refractor-Telescopes-with-Premium-Optics/Orion-ED80-80mm-f75-Apochromatic-Refractor-Telescope/pc/1/c/10/sc/332/p/9895.uts

If you are physically able to move a larger telescope, and are only interested in astronomy (no birding),
then some sort of 6" to 8" Dobsonian telescope might be a better choice. The increased aperture (light gathering) would let you see more objects:

http://www.telescope.com/Telescopes/Dobsonian-Telescopes/IntelliScope-Dobsonians/Orion-StarBlast-6i-IntelliScope-Reflector-Telescope/pc/1/c/12/sc/27/p/102026.uts?refineByCategoryId=27

http://www.telescope.com/Telescopes/Dobsonian-Telescopes/IntelliScope-Dobsonians/Orion-SkyQuest-XT8i-IntelliScope-Dobsonian-Telescope/pc/1/c/12/sc/27/p/102012.uts?refineByCategoryId=27

February 26, 2013 09:42 PM Forum: Refractors

Orion Vixen 102FL Fluorite

Posted By John Biretta

The focuser on mine got upgraded to Moonlight while the scope was briefly owned by another chap. I don't think it was money well spent. The original focuser on these was pretty good already. I used it mostly for visual planetary observing. Maybe for imaging or something like that you'd want a finer focus control. Or maybe higher weight capacity if you plan to routinely use a 31 Nagler on it. But for simple visual observing I don't think the upgrade added much. At least thats my opinion.

March 5, 2013 09:03 PM Forum: Refractors

Star testing oddity

Posted By John Biretta

I wonder if these could be floaters within the eye? Do they change appearance if you use the other eye? Outside focus, the star will come to focus somewhere in the vitreous humor inside the eye. Any microscopic imperfections there would be greatly enhanced and might be perceived as "bubbles." Inside focus there is no focal point within the eye, so these floaters would be greatly reduced in appearance.

March 28, 2013 02:21 AM Forum: Equipment Talk

Need Help Celestron 9.25

Posted By John Biretta

Is the problem at the front of the scope -- where the corrector plate / lens is located, along with the secondary mirror and secondary baffle? Or at the rear of the scope where the primary mirror is carried by the baffle tube? The former case you have some hope of fixing yourself. The latter case will need Celestron as the alignment is very precise.

April 2, 2013 12:43 AM Forum: Telescope Making

April 9, 2013 01:34 PM Forum: ASTRONOMY

SCAM

Posted By John Biretta

There have been a few cases where long-delayed item is not scam, but instead is waiting in customs for processing or payment of import duty by buyer. Sometimes a month or more. Someone by the same name seems to have had good astromart transactions and postings to astro discussion boards up to 2010. I suppose a good person could turn scammer, or have their account hijacked by hacker. Good luck.

April 11, 2013 03:19 AM Forum: Equipment Talk

Would a barlow be of use in my setup?

Posted By John Biretta

A Barlow would give you a larger image scale and should reach focus OK. Image will be dimmer. I'd skip the field flattener; it probably won't work well with the Barlow in there -- the optical path will be very different from its design point. YMMV.

What are you trying to image? For faint stuff, the fact the image is dimmer with the Barlow will probably hurt.

For bright stuff like moon and planets, the Barlow will probably help overall. So called "eyepiece projection" is another way to get a large increase in image scale for moon, but it takes a special adapter that can hold both the eyepiece and camera. Another approach is the "afocal"
method where you leave the lens on the camera, and just stick it looking into the eyepeice -- helps to have an adapter for this, otherwise very hard to hold everything steady.