Posts Made By: Mike Hosea

Jason, I had a 5mm Tak LE a few years back and liked it. I'm sure you would, too, but I don't think it would be any better than an 11mm Tele Vue Plossl with the 2x Tele Vue Barlow. In fact, I suspect that combo will be tough to beat in terms of sharpness and contrast. Not everybody agrees, of course, but if 9-10mm of eye relief is okay, I think the 3-6mm Nagler zoom would be worth considering. It will take you from an exit pupil of 1mm down to 0.5mm, which is ideal. I'm not a zoom kinda guy, myself, but you might be.
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Mike

There is the (in)famous Dickinson/Dyer experiment in which an APO was fitted with paper obstructions of differing sizes. The aperture was held fixed in that case. Is this in the Backyard Astronomer's Guide? I read about it while browsing at a bookstore years ago--haven't ever owned a copy. IIRC, they came up with the 20% figure that gets tossed around nowadays as being the threshold for noticeable CO effects (but needless to say, some people think the threshold is lower).

I don't know that anybody has done a similar experiment allowing the obstructed configuration to have greater aperture.
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Mike

Floyd,

I think in a sense you are correct, although there are various ways of looking at it. The Airy disk for a 1-hole mask would be larger, though, and I think there are very short spikes created within that general (and obviously tiny) area. It would take fairly good seeing and very high magnification to get a good look at this. Certainly there are no long spikes emanating from bright objects to detract from the view.
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Mike

Tony,

Don't do the sling. In fact, Tom himself is dumping the sling in the next batch of Teleports. I have a 7" f/6.7 Mark harry mirror, also 1" thick. You can hardly go wrong in this case, but push-pull cells aren't hard. I used Nylon spacer/sleeves on long 1/4"x20 bolts. The bolts extend through the rear ring of the push-pull cell to be "pulled". My lower posts are 120 degrees apart because the push pull cell was a replacement of a built-in sling cell. I'd have gone 90 degrees if I could have, or better yet gone with a rocker bar on each side and a 4-point lower support. However, with the 7" mirror that is 1" thick, I really don't think it matters much.
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Mike

That is how Nagler described it in an Ask Al response years ago, i.e. before they actually bothered to make a 2" 2x Powermate.
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Mike

It corresponds to field stop size, not focal length, but the only thing I noticed that you got wrong was the bit about the 17mm eyepiece having a 94% illumination. To figure that we'll need the field stop diameter of the particular 17mm eyepiece in question.

The 1/2" protrusion is what would bother me. Too bad you can't find a 2.3" secondary.

Your 31mm Nagler will work either way. People get too uptight about off-axis illumination levels for visual use. Heck, my 7" drops to less than 20% illumination at the edge of a 40mm Pentax XL or UO MK-70 Konig. Does that stop me from using the 40mm eyepieces with it sometimes. Heck no. The view's fine.
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Mike

David,

It can be difficult to install a secondary before you are proficient in other aspects of collimation. In placing the secondary, one normally proceeds in steps.

Step 1 is to fix how far the secondary is from the primary. looking through the Cheshire it should appear centered top to bottom (I am speaking as if the scope were pointing straight up).

Step 2 is to fix it's rotational position so that it faces the focuser. This is tricky because it depends on the tilt which you haven't adjusted yet. There is a way to get it right very quickly, but rather than overload you with shortcuts, let me just say that you should adjust the rotational position, then adjust the tilt. If it no longer appears centered left-to-right after you adjust the tilt with the laser, rotate it slightly in the direction that it is cocked and readjust the tilt with the laser. Eventually it should appear centered left-to-right and top-to-bottom with the laser being directed to the primary center. Do not bother to proceed to adjust the primary until you have achieved this! And after you have achieved it, leave the secondary alone!

Once the secondary is properly adjusted, you should NOT try to make the return beam of the laser agree with the Cheshire. Use the laser for a ROUGH alignment of the primary because that's all it's good for as is. Don't even worry about "tweaking it in" so that the beam returns exactly to its source. Instead, once you are close, switch to the Cheshire and TRUST it. The laser will magnify a number of small sources of error that are not relevant in adjusting the primary, so don't worry if the Cheshire says the primary is aligned and the laser says "not quite". The Cheshire is correct.
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Mike

I have regularly taken my 10" f/5 Teleport to 500x for the moon and Saturn, although I generally prefer the 363x that I get with my 3.5mm Nagler Type 6. I have observed Saturn with good detail at 948x (6.7mm Meade Ultrawide + 2x Tele Vue Barlow + 2.5x Powermate), but in my opinion this is not a desirable combination or magnification for a 10" scope. The exit pupil is too small for my eyes, and getting the object centered with the point of hand contact just a few feet from the center of alt-azimuth motion requires a very delicate touch. I used to use my 2.5mm Nagler Type 6 for some lunar viewing, but ultimately I felt better about letting it go than keeping it for that one night in a thousand where I would actually need it, as opposed to just wasting time trying it only to see the same amount of detail as in the 3.5mm Nagler Type 6.
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Mike

The narrower field is obviously not necessary in general when it comes to long eye relief and short focal lengths (witness the Radians, Pentax XWs, and Pentax XLs, not to mention the Vixen LVWs), but it may have enabled them to make the LV eyepieces a little bit less expensive by allowing them to retain a simpler field group design. It may also have helped them to keep the eyepieces relatively short in size. Possibly what you would see if you removed the field stop is a slight increase in usable field and vignetting such as is often observed with long focal length Plossls and short Barlows.
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Mike

The Kineoptics HC-1.

http://www.kineoptics.com/HC-1.html