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Posts Made By: Ed Zarenski

March 3, 2003 09:13 AM Forum: Solar System Observing

First Jupiter Sketch

Posted By Ed Zarenski

Very nice sketch Rich, Did you take art lessons somewhere along the way. Mine don't ever look like that. edz

March 28, 2003 12:20 PM Forum: Equipment Talk

Seeing width in the Cassini Division

Posted By Ed Zarenski

Conclusions (edited to fit)
Utilizing point source limiting criteria for determination of potential to see an extended source is a difficult application, but I believe it is a valid approach.

Although the rings easily fit the extended object criteria of images larger than the detecting element, the width of the Cassini division does not. The width is small, even smaller than the limit of resolution of many small scopes. With exception to its linear dimension, Cassini is a small, very high contrast image requiring fine resolution that fits the definition of a point source.

Extended objects are easier seen if high contrast is present.

The Cassini division separates the brightest portion of the B ring from the brightest portion of the A ring. The dark Cassini division is bounded on either side by the brightest features within the ring system. The Cassini division may just be the highest contrast extended planetary feature in the entire Solar system.

The extent that diffraction causes the light sources to infringe on the dark space is the question left unanswered. A larger aperture will provide a smaller diffraction disk. Greater magnification will reduce the extent of the diffraction fringe. High contrast will improve the ability to see the division. And finally, an added linear dimension to the feature helps make the image easier to see.

Any feature that does not have angular dimension greater than the limit of the telescope’s ability to resolve cannot be observed as anything larger than the Airy disk. Since this is the smallest image spot size that can be achieved by a given telescope, no matter how large that image is magnified, it cannot be construed as having width. You would not be magnifying a resolved feature. You would simply be magnifying the smallest image spot size.

A telescope with an aperture that will produce an Airy disk size smaller than the observed feature is required to see the feature as having dimension. The smallest telescope that significantly exceeds the necessary criteria is a 9” scope. I suspect it takes at least a 9” scope to achieve true dimensional effect.

The telescope will realize potential resolution with a magnification of 1300D. For a 9” scope 1300D produces a magnification of 297x. As the aperture of the telescope increases, and the limit of resolution increases, the magnification needed may be even less than 1300D to achieve dimensional resolution of the feature.

Some observers may achieve eye resolution of 3 arc minutes or 180”, in which case they could perceive dimensional width at a magnification of 180” / 0.7” = 257x. It is more likely that most observers have eye resolution in a middle of a range needed and would require 4 arc minutes or 240”, resulting in a necessary magnification of 240” / 0.7” or 342x to see dimension in this feature.

April 8, 2003 11:18 AM Forum: Astro Binoculars

Broadband coatings

Posted By Ed Zarenski

I keep coming back here looking for any response to this question. I'm very interested to read any responses!

I think the Oberwerk (not sure about current models) claim to use the broadband multicoatings. I haven't had any problems with softness, But I sure am curious.


April 10, 2003 07:48 AM Forum: Equipment Talk

DS-60 focuser

Posted By Ed Zarenski

My orion Sirius 0.965" plossls are exactly the same eyepieces as the 1.25" plossls. the only difference is the screw on sleeve they put on the base to insert into a 0.965" focuser.

The 20mm eyepiece has a 17mm field stop. About the maximum width of field stop you could possible put into a 0.965" eyepiece would be 20mm dia. You would need to jump up over the 20mm eyepiece to show any gain in the fov.

My 1.25" 25mm and 26mm kellners and plossls have field stops of 22.0 to 22.3mm. So field of view for 0.965" eps max out around 22-23mm, that would be the biggest eyepiece you could fit a 20mm field stop into. By the time you make a 25mm eyepiece with a 0.965" barrel, you would need to squeeze the field stop down by 2mm to fit it into the barrel.

So you wouldn't get any gain in field of view by switching to 1.25" eyepieces unless you could go to a 25mm eyepiece. That would be pushing the magnification of that little scope pretty low, maybe too low to be of much use. Is it worth it???


April 11, 2003 03:33 PM Forum: Eyepieces

Field Stop Diameter Measurement

Posted By Ed Zarenski

You might also check that old Dietzgen or Alvin drafting set for a pair of dividers. The screw adjust type work excellent. Use extreme caution when near lenses! Follow the formulas that Mike provided. You need measurements to fractions of a mm.


April 12, 2003 05:17 PM Forum: Astro Binoculars

Exit Pupil Size and Binocular Performance

Posted By Ed Zarenski

Hi John,

I'm not positive what my pupils measure. I'd like to find out.

Each of these issues you raised needs to be addressed.
magnification plays a role in this
what about exit pupil size?
smaller entrance pupil is functioning like an aperture stop
user entrance pupil/binocular exit pupil analysis should be taken into consideration when trying to numerically estimate how well one binocular should perform in comparison to another

In addition to the notes above from your post, any index should take into consideration the qualities of the binocular. A binocular index needs to include a factor for other quality attributes in order to accurately give an indication of relative performance rank. it is my opinion that resolution and contrast are the two qualities that need to be addressed.

I will not be able to address all the issues here, so I will refer you to two articles, a binocular comparison article and a binocular performance article.

In the binocular comparison, note the sections on masking the binocular objectives and the effects on comparitive performance. It provides a clear indication of the benefits of magnification and other info on the change in exit pupil.

In both the comparison and the performance articles note the sections on attributes affecting the overall performance of binoculars.

User entrance pupil compared to binocular system exit pupil produces a PERSONAL index, as you refered to when you stated "by a particular observer". But remember this, aperture is probably the largest single component of the binocular system that could vary for an individual observer with the least perceivable change in performance. Aperture is the most under-utilized component of the whole system.

Also, the larger the aperture the better the resolution. Your entrance pupil will reduce the overall light entering the eye, but the resolution will still be dependant on the larger aperture.

Excuse me if you have already been to these articles and you are continuing your discussion as a result. If not, I hope you find reading them worthwhile.

"How To Understand Binocular Performance"

"Comprehensive Comparison of 4 Binoculars"


April 14, 2003 01:53 PM Forum: Deep Sky Observing

Viewing the Red Spot

Posted By Ed Zarenski


When the GRS is just coming around to our viewing side, it is nearly impossible to see until it comes well around closer to the meridian. Your probability is at least reduced to ~25% chance, not 50%. You haven't got as much chance (time) to see it as you thought. Keep trying.


April 15, 2003 10:22 AM Forum: Astro Binoculars

Maximum Star Counts for various binoculars

Posted By Ed Zarenski


Are these counts suposed to represent someone's actual data?
Or are these counts simply fov area / celestial sphere area multiplied times theoretical limit of stars for a given mag.

1st the mag limits. Aren't these the mag limits determined for various aperture sizes? If so, then they are only achieveable at optimum magnification needed to obtain maximum resolution. Maximum resolution usually occurs for any particular objective lens at around e.p. 1mm to 0.75mm, or in other words for a lens of 100mm at a mag of about 100x to 120x, or for an 80mm lens at about 80x to 100x. Magnifications at less than optimum will not achieve these magnitude limits and in fact at typical binocular magnifications will only achieve limits about 1.0 to 1.5 magnitudes less. Right off the bat, doesn't each magnitude double the potential number of stars. Take for instance the mag limit of 13.2 for a 125mm lens. My excellent 5" SCT is just barely capable of seeing the 13.2 mag star at the edge of the Ring nebula under the best of conditions and then only at high magnification. Anybody out there with a 125mm binocular at 25x ever see the 13.2 mag star on the edge of M57?

This table fails to account for the increase in the number of stars seen in a constant aperture while increasing magnification. The difference between 12x and 16x can be anywhere from a 15% to 30% increase while keeping aperture constant. Limiting mag at 16x60 is fainter than limiting mag at 12x60.

Not related to the difference in the counts between the scopes above vs the binocs above as those differences are clearly just related to fov. BUT, two things the assumptions for the scopes are also inccorrect because they are not at optimum magnification and Star counts for binoculars will never equal maximum star counts for telescopes as they can never achieve optimum limits due to the low magnifications in use. This goes right back to the under-utilization of aperture in binoculars. Except for brightness, every other criteria by which we judge performance limits of objective lenses is under-utilized in binoculars due to the limitation of low magnification.

Seems to me a lot of inaccuracies in this data.


April 17, 2003 01:07 PM Forum: Equipment Talk

equipment for school star parties

Posted By Ed Zarenski

I do eight or ten of these school or scout events a year. Crowds are sometimes only 20 people but the main events are generally a crowd of 50 kids pluis 50 parents.

For a small crowd I set up three scopes all on EQ2/CG3 equatoral mounts with RA motor drive.
Celestron G5 5" SCT, Stellarvue 78mm AT1010 and Orion ST90. I have a wide assortment of eyepieces, from Orions to ultimas to UO orthos to TV plossls and SWA's, Radians and Konigs, they all get used. I also mount 2 or 3 pair of binoculars on Parallelograms or just straight up tripods, everthing from 10x50's, 15x70's to Fujinon 16x70's and giant 20x80's.

For the larger crowds of 100 people, in addition to all the above I also bring my 6" Skyview Deluxe Newt and my 6" celestron 150 refractor and another 2 or 3 tripods with binoculars. For a crowd of 100 people I'll have 5 main scopes all driven and 6 mounted binoculars. I also bring two old small "baby scopes on spindle legs" the kids can play around with. Good for finding the moon if it's up.

You want to try and have enough equipment so that no more than 4 or 5 people are standing around at any piece idle. Once you get people moving around in groups, rather than standing in one big long line at the big scope, everybody will soon realize they will get to use all the equipment and see all the sights. It works really well for me and the crowd.

Also, don't forget you need a folding table to lay out all your charts and eyepiece cases and misc. stuff. And you need 2 or 3 step stools for the little kids. When I leave my house to go set up for a night show, my van is pack as solid as a sardine can.

Takes me about an hour to set up before the crowd gets there and about an hour to knock down and pack up at the end of the night.

The ST90 setup can be had for real cheap. Mounted binoculars are a must, people find them so easy to use.

No matter how big or how small you end up doing an event, go for it. Kids will never forget it.


April 24, 2003 04:53 AM Forum: Astro Binoculars

binocular color

Posted By Ed Zarenski

The color thru the binocular is probably mostly associated with the composition of the glass used in the manufacture of the lenses. Sould not have anything to do with contrast, brightness or resolution. Although a lesser quality glass will have an impact on those attributes.

The difference between Fujinons (clear white) and Oberwerks (yellow/beige) is readily apparent but a yellowed tinge to the overall image is not necessarily an indication of poor quality glass. It's the so-called temperature of the image it gives.

The color of the coatings will not result in that color imparted to the image, although it may be blocking certain wavelengths of light that will impart some difference in the image.