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Posts Made By: Mike Hosea

April 11, 2003 08:36 PM Forum: Eyepieces

Field Stop Diameter Measurement

Posted By Mike Hosea
If the field stop is exposed in the barrel, you can measure it directly. If it is hard to get to with the calipers, you can cut off the cotton ends from a Qtip and insert the stick or something similar with a pair of needle nose pliers (careful!) and then mark the stick with a pen, then remove and measure on the stick.

If the field stop is behind a lens element, the field stop diameter is irrelevant without knowing how to account for the lens element in front of it. Your only option is to measure the true field of view and "back into" the field stop diameter:

field stop diameter ~ true_field_degrees * telescope_focal_length / 57.3

Finally, if you only want something quick and dirty, you can assume that TFoV = AFoV / Magnification, which yields the crude approximation:

field stop diameter ~ apparent_field_degrees * eyepiece_focal_length / 57.3

I often use this last approximation in spreadsheets where I prefer to use field stop data when I have it but am willing to use the crude approximation when I don't.
--
Mike

April 13, 2003 08:04 PM Forum: Equipment Talk

Prism vs Mirror Diagonals

Posted By Mike Hosea
Well, even if they are perfectly made, prisms add chromatic and spherical aberration. The faster the scope, the more aberration added. Few recommend 1.25" prism diagonals on anything faster than about f/6 and most would probably draw the line more conservatively at about f/8 or even slower. In any case, it's essential to get a high quality diagonal whatever its design.

FYI, nobody seems to recommend 2" prism diagonals under virtually any circumstances.
--
Mike

April 15, 2003 06:27 AM Forum: Eyepieces

Parracor+2" EP=Naglers-Panoptics?

Posted By Mike Hosea
The Paracorr is a field flattener as well as a coma corrector and a 1.15x "Barlow", so most eyepieces will benefit from more than just the reduction of coma with its use. Now as to what compares "favorably" to using a Nagler or Panoptic alone, that's a complicated question that depends a lot on how the Nagler or Panoptic performs alone, as opposed just to how the 4 and 5-element eyepieces perform with the Paracorr. For example, neither the 16mm Nagler Type 5 nor the 17mm Nagler Type 4 were very impressive at all in my 10" f/5 Newt and no Paracorr. It's a different story with the 5mm and 7mm Nagler Type 6's.
--
Mike

April 16, 2003 03:34 AM Forum: Equipment Talk

Help! I think I royally screwed up my mirror

Posted By Mike Hosea
I have a hard time believing it wasn't there before, but in any case it sounds like you have a cleaning problem, not a screwed up mirror. It's time for a conventional cleaning instead of just a rinse.

It may be most comforting for you to note that you can't possibly have screwed up anything that recoating couldn't fix.
--
Mike

April 17, 2003 11:46 PM Forum: Eyepieces

New TMB Super Mono

Posted By Mike Hosea
Markus, when will we be able to order these TMB eyepieces? Is the price still around $200?
--
Mike

April 18, 2003 07:06 AM Forum: APM

New TMB SuperMonocentric

Posted By Mike Hosea
You know, Markus, while some people have asked for a conical top because it would allow air to circulate better, it just occurred to me that a conical top will cause the eyepiece to need more in-travel of the focuser. I think I prefer the current top to that. I have low-profile, helical focusers on my Newts, and I'm always having to think about how to parfocalize new eyepieces with my existing set. Generally needing out-travel is easier to accommodate than needing in-travel.
--
Mike

April 25, 2003 06:47 PM Forum: Equipment Talk

Barlow effect on focus

Posted By Mike Hosea
Normally, shorty Barlows require some in-travel. However, when inserted to the hilt in a 2" diagonal (be careful!) or a Newtonian, some are nearly parfocal (like the Tele Vue 2x). However, the exact amount of travel required will depend somewhat on the eyepiece used.
--
Mike

April 28, 2003 01:57 AM Forum: Eyepieces

Pentex E.P.s

Posted By Mike Hosea
I've owned all the Pentax XLs except for the 28mm, and I've used them in a 10" f/5 Newt and 6" f/12 MCT.

My first reaction to your report is surprise at seeing a RED fringe around Jupiter and Saturn in the 14mm. That is bizarre. I've never seen red fringing in any Pentax XL, or any other eyepiece that I can recall, for that matter, and the 14mm Pentax XL was as sharp as anything in the MCT. My problem with the 14mm Pentax XL in my 10" f/5 Newt was the field curvature--the edge was defocused when the center was focused and vice-versa.

Each Pentax XL shares the same comfort and feel, but there are performance differences. I personally think the 5.2mm, 7mm, and 10.5mm are head and shoulders above the 14mm and 21mm. The 40mm is, of course, an entirely different design. It isn't perfect, but it does represent a reasonable tradeoff between size, weight, cost, and edge correction for scopes down to f/5 or so.
--
Mike

April 28, 2003 07:37 PM Forum: Telescope Making

Flatest, Blackest Black Paint?

Posted By Mike Hosea
I think black chalkboard paint is available as brush-on. It's not bad.
--
Mike

April 29, 2003 03:30 AM Forum: Eyepieces

Celestron X-Cel

Posted By Mike Hosea
I noticed and also noticed the similarities to the Orion Epic ED-2 series. Since Orion doesn't make anything, and since there is a precedent for Orion-specific modifications (Vixen LVs), my best guess is that these eyepieces are related. It's just a guess, though.
--
Mike

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