Posts Made By: Greg Nowell

Are you sure you need a dioptrx at 13mm? I use one at 40mm, but I do OK without at higher magnifications. This is very common.

Otherwise you may have to take the plunge to find out. You won't have a hard time re-selling the ethos. regards Greg N

If the XLs haven't impressed you as better than the TV eyepieces in your line up then you are impervious to the Pentax aesthetic. Regrettably, this is probably a hopeless condition, but the upside is a box o' Naglers ain't bad, they just ain't Pentax.

The more logical question is why you aren't ditching the TV eyepieces and getting the entire Pentax line. But that's just the way a Pentaxian would see it. Greg N

Guy Earle said:

I'm kind of befuddled on this, and would like some opinions. First, a question regarding Jupiter. In all my eyepieces, I'm noticing that Jupiter just isn't sharp. Seeing aside, even on mediocre nights I can still get a sharp low power view, but it mushes in and out, and when it seems just at that sweet spot, there is almost a double image around part of the limb. Now, when I look at the Moon or deep sky stuff, I don't see it, but ever since my rebuild I just can't get it good enough.

I rechecked and adjusted (thinking that was it) the spider, centering it. I'm using the Collimation Handbook, btw. My focuser is square and the secondary centered in each axis. I use a laser collimator to center the secondary, then a Cheshire for the primary. Even so, I'm not quite happy. Does anyone have any ideas?

Also, I have started using a pair of 16 t5 nags I just picked up off astromart. I'm noticing on bright objects, when racked out of focus, there are dark marks like some ink blot test. The images rotate with the eyepieces, so it's not the Denk. When I focus in, they disappear, and all is well. But I'm disturbed by them, b/c I don't remember seeing them in my old pair of 16's. Is this normal?

Thanks,

Guy

If you're using binoviewers your binos could be out of collimation. Your testing of the SCOPE should be done with a single eyepiece. When that is spiffy if there are other problems that would be the binoviewer. Binoviewers and binoculars can go out of collimation and the results lead to noticeable deterioration of performance.

The black stuff sounds like your eyepieces need to be cleaned. When you are out of focus you see it and it is blurred out when you are in-focus. Try Purosol and kim wipes or unscented high quality kleenex.

A blower is nice.

regards
greg n

The 40mm TMB Paragon is good at its price point. regards Greg N

I have found that with time I have come to have strong preferences and reactions to eyepieces. This is in part due to optical performance, but also due to ergonomics and feel. Some of the dob users here have noted their preference for better off-axis performance in an eyepiece because they don't have tracking and need to let go of the scope to let it hold still for the "best view." But the object drifts. So someone with that consideration may actually "see more" out of a wide field eyepiece on planets than with a specialized planetary eyepiece. The small fov of the planetary eyepiece would mean that a higher % of time must be spent repositioning the scope, and that higher % of time means that a higher % of the best moments of seeing are lost, and so, any theoretical gain from the specialized eyepiece will be reduced by considerations that have nothing to do with eyepiece design, except of course in the sense of fov as it impacts one's observing capabilities.

If you have tracking you might come to different conclusions. Ergonomics also matters even if you do have tracking. If an eyepiece is exerting a subtle level of discomfort you will likely tire of using it and again miss the moments of good seeing because you spend less time chasing that last detail.

It seems to me more rational to invest in wide field eyepieces first, starting at longer fls and then gradually narrowing down, and save the simple designs for the very last things on one's acquisition list. Here's why:

1. On any given night, the sky is the biggest optical enemy. The differences between widefield eyepieces and specialized "small and simple" won't show "more often than not."
2. There aren't that many planets to play with, but there are many dsos. With the possible exception of splitting doubles, wide gets you more.
3. Not only are there not that many planets to play with, but six months a year they are not optimally positioned; and they take hemispheric jaunts which further compromise their performance, as Jupiter these past two years. And of course they are typically up only part of the night. Saturn has the same issues AND the effing rings turn edge on.
4. Then there's Mars which gets reallllll tiny when not in opposition.

So I'm not saying don't get "planetary glass". I'm just saying widefields and the aperture you prefer should come first. As for the rest: I'm deep into XWs, and if XWs did not exist, I would have XLs, and indeed had XLs until I got my XWs. I like wide fields but I'm simply happiest with an XW due to my sense of sharpness, clarity, comfort, etc. This is very difficult to describe. I flit from telescope to telescope like anyone else at a star party but there is this feeling of "ahhhhh" when I chance to alight on a Pentax. Like everyone else 90% of what I encounter among those springing for "upper tier" glass is Televue and I do fine with it, but the direction of my personal eyepiece box is moving away from TV.

I have recently acquired some glass which I haven't had a chance to use yet, but suffice it to say, I followed my own advice.

regards
Greg N

We'd like to see that at one of the club events, Alan! R U intending to heave it on to that bigfoot dingus? Some of us keep waiting for the day when the AP1200 will see service for the 14.5 and now this scope too!

regards


Greg N

Somewhere in this world is a criterion group, maybe it's on yahoo, where you can get help. I'm not as convinced as once I was that refurbishing the old mount is the best choice; there are so many nice alternatives, the GEMs of the past twenty years blow away those of yesteryear, with the possible exception of the cheap china stuff.

But I can help you in another way. There's a guy named Don Rothman who has a page on a nearly moribund astromart competitor called astronomy-mall (back in the late 90s early 00s astronomy-mall had some umpf). He used to be called "teletrader" but ran into a name conflict with a big powerful corporation and now he's got some other name I don't recall. Don has oodles of parts and could probably help you get your criterion running again. He also has a lot of stuff So from yesteryear, orange tube SCTs, fork mounts, Optical Craftsman, Criterion, etc. Don is not the cheapest source of parts around but so far as I know he's the only one in the country to maintain such a large inventory of older equipment. So when you pay the higher price you're paying for the fact that he had to keep it for five years before he found a buyer.

It's always helpful in these kinds of discussions to post pictures of what your problem is, if possible. There are gifted people around who can tell you what to do even though they've never owned a criterion.

regards
Greg N

Maralee Knowlen said:

I'm glad you enjoy your telescope. Do you like Brandons?

Has the original post in this thread disappeared?

Anyhow, no, I'm not of the Brandon tribe. I'm into Pentax XWs.


Greg N

The collimators that are available for scts really don't work the way they do for Newtonians. You're really best off with a star.

It shouldn't take you 3 hours to collimate once you have a better idea of what you're doing.

One problem people have is deciding which screw(s) to adjust. One way to find out is to take a half inch stick, or your straight hand, and run it from the adjustment screw to the edge of the corrector (don't touch the optics). On the out of focus star you will see a shadow which indicates this radius.

Now look at how the black hole in the out of focus star is off center. One of the three possible shadow positions will describe a diagonal that lies closest to the out of center hole. That is the screw you adjust.

Typically you work with two screws and leave the third alone. If you're spending three hours you may be going back and forth undoing adjustments that were otherwise good. The other problem is that the seeing may not give you a good "doughnut". The answer to this dilemma is that you can collimate as well as the seeing permits. Estimate the approximate shape of the deforming out of focus star and center the secondary shadow (black hole) as best you can. On a steadier night, touch it up; on a bad night where you can't collimate, you won't be able to see much fine detail on planets anyhow. The other thing to do is check it out again after a few hours as a hot mirror simulates the problems of bad seeing.

If I could figure out how to draw perfect circles with a black hole inside I would put an article up on this.

I'll have to think about it. Right now my regular desktop is down for repairs and I don't have all my usual resources.

keep the faith,
Greg N

The ZAOIIs are very cheap, and in the short focal lengths you get all the satisfaction of peering through a tiny peep of a hole. The parameters are important. in cold climes you're better off with bigger lenses that can retain some heat while observing. That way you don't fog them over with our eye heat or breath. regards Greg N