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Posts Made By: Dick Jacobson

January 29, 2012 07:27 AM Forum: Eyepieces

A logical approach to eyepieces?

Posted By Dick Jacobson

There are four "magic" focal lengths that you should cover in your collection. I call them Maximum Field, Maximum Starlight, Maximum Definition, and Maximum Resolution. These are not specific millimeter values but depend on your telescope and your eyes.

The Maximum Field eyepiece shows the widest actual field of view possible in your scope. If you have a 2" focuser you should look for eyepieces with a Field Stop of around 40 millimeters, or 27 millimeters for a 1.25" focuser. You can estimate the Field Stop by multiplying the focal length by the apparent field and dividing by 60. The exit pupil may be too large for your eyes, but the whole point is to get as large a field as possible although some star brightness might be sacrificed.

The Maximum Starlight eyepiece shows as many stars as possible at full brightness. The exit pupil should exactly match your eyes (or maybe slightly smaller to allow for misalignment). The exit pupil of the eyepiece equals the focal length divided by the telescope's focal ratio.

The Maximum Definition eyepiece shows faint planetary detail as well as possible. Many observers agree that definition is best when the exit pupil is about 1 millimeter (25x per inch of aperture). The focal length of this eyepiece should equal your telescope's focal ratio.

The Maximum Resolution eyepiece gives the highest usable magnification, usually 50x per inch but this may be too high for telescopes larger than 10".

I like to have ratio of about 1.6 to 1 between magnifications. This meshes perfectly with a 2x Barlow if you want really close spacing.

February 23, 2012 06:03 AM Forum: Deep Sky Observing

Do you see colour in any non-stellar object?

Posted By Dick Jacobson

Except for a faint bluish or greenish tint in a few bright planetary nebulae, I don't see any color in nebulous objects with my 14" and 20" scopes.

March 8, 2012 06:33 AM Forum: Deep Sky Observing

Coloured stars in globular clusters?

Posted By Dick Jacobson

I've never noticed star colors in globulars but haven't really looked for it.

Often when I watch a well-resolved globular drift across the field from right to left, I have the impression that the entire cluster is rotating. The direction of rotation also seems to be right to left. This is obviously nonsense but it's an interesting visual illusion. The illusion could be caused by field curvature but I doubt that there is enough to cause it.

March 21, 2012 01:29 PM Forum: Eyepieces

Large apparent fields

Posted By Dick Jacobson

I purchased an Explore Scientific 14mm 100° eyepiece last summer and enjoyed it a lot more than expected. There is a definite "wow" factor when you can just barely see the edge of the field. There is a very tiny amount of false color on bright objects near the edge of the field, but not a problem in my opinion. Now I've also purchased the ES 9mm 100°.

March 28, 2012 05:47 AM Forum: Deep Sky Observing

Shapes of Open Clusters

Posted By Dick Jacobson

Open clusters are batches of new stars that have just formed out of molecular clouds. They don't have enough mass concentration to stay together and get torn apart by tidal forces in the galaxy. My guess is that the shapes we see are the result of the shapes of the gas clouds and maybe one star's formation triggering another one. I'm sure if you looked through some astronomy textbooks you'd find a better explanation.

April 27, 2012 08:14 AM Forum: Eyepieces

Panning With ES100 14mm

Posted By Dick Jacobson

I agree! Last night I used my ES 14mm 100° eyepiece on the Virgo cluster for the first time. It was great for zipping up and down Markarian's Chain without getting lost. I was skeptical about the need for a 100° field at first, but am enjoying it a lot more than expected.

July 11, 2012 06:26 AM Forum: Antique/Classic/Vintage Optical Instruments

15 Eerie Abandoned Observatories

Posted By Dick Jacobson

Interesting, and rather sad. Anyone who plans to build a personal dome should keep in mind that it is likely to become a liability when the property is sold to someone who has no interest in using or maintaining it.

When Dave Kriege, owner of Obsession Telescopes, build his huge 26x27 foot rolloff, he had in the back of his mind that it could eventually be used as a garage.

August 28, 2012 05:54 AM Forum: Eyepieces

ES 25mm 2" 100 AFOV

Posted By Dick Jacobson

Like you, I've been eagerly awaiting this eyepiece. In theory, it looks like it would have a unique ability to get almost all of the light from a 2" drawtube into a 5mm pupil. One contributor in the Cloudy Nights Eyepieces forum claims that it is impossible to build this eyepiece without some fall-off of light at the edge. It will be interesting to see for myself, if ever...

September 13, 2012 06:28 AM Forum: Telescope Making

Folded Newtonian

Posted By Dick Jacobson

If you want to play with some numbers, an excellent tool is Mel Bartels' diagonal calculator (see http://www.bbastrodesigns.com/diagonal.htm). This doesn't specifically address tilted diagonals but you should be able to adapt the numbers to your design.

Personally, I wouldn't shorten the scope just to improve truss stability. If the poles vibrate, I've heard that you can reduce vibration by wrapping a strap around the entire truss at the middle (haven't tried it myself). Shortening the tube by a couple of inches, pushing the focus outward, might help if you plan to use a binoviewer which requires a lot of inward focus. It also will make it easier to reach the eyepiece at zenith. The cost is a slight loss of image contrast due to a larger diagonal.

September 18, 2012 06:33 AM Forum: ASTRONOMY

Eyes Going Bad -- Need Advice

Posted By Dick Jacobson

Decades ago I switched to contact lenses specifically because they are more compatible with eyepieces. There are some that correct astigmatism. I preferred rigid gas-permeable lenses.

More recently I had cataract surgery (I'm 64) and the implanted lenses fixed my eyesight for good. At least one thing to look forward to in old age!