Posts Made By: Greg Nowell

From 2002

I'm surprised at the prevalence of sugar. There have been a lot of studies that indicate that the best food for staying awake is protein based.

I also worry about grease on the equipment. Accordingly I have biased my snacking to various jerkies. There is even salmon jerky which is pretty good stuff, omega three and all. The best snack is when my friend brought a ton of Lox out into the field.

Carbohydrates and sugar are foods that promote drowsiness, something to remember on the road. If you're sleepy stop for coffee and a burger--but throw out the bun & just eat the protein. What I would like is a source of sugar-free jerky. The stuff in the stores typically has some sugar, not a lot, but I'd prefer none.

Protein for staying awake works pretty well actually.


regards
Greg N

The forks don't tend to get much tech support from the companies and somehow most SCTs end up on a GEM. Where they belong, in my view. In any case a c8 on a G11 would be an excellent combination. I'd prefer the 9.25 but t'aint nothin' wrong with the c8. Rock solid.

The one thing you may run into is that on older SCT optical tubes particularly Celestron the Losmandy dovetail doesn't fit on both sides. Only one. Has to do with the way the holes are drilled. You may find yourself faced with using the scope "upside down" (focuser on the left as the OTA is pointed north, counterweight down, instead of the usual and customary right) or else having a machinist or someone else handy drill and tap the correct holes.

regards
Greg N

CI700: solid RA axis; G11 is hollow (hollow is actually stronger)
CI700: you have to loosen clutches in order to move the mount/scope; G11 has "move it when you want to" clutch system. In other words CI700 is like a big version Vixen and clutches that have to be loosened before you move the mount become a HUGE annoyance.
CI700: no longer made, support from Celestron problematic; G11, active user community with extensive aftermarket options including heavy duty aluminum tripod, Berlebach wood tripod; conversion to Gemini, etc.
CI700: tripod not as good as later CGE; inferior to G11
CI700: you're stuck with the worm it comes with unless you're a machinist type or can pay someone else to do the work for you; G11--the new ones (like Tom's) appear to be selling regularly with periodic error in the 5 to 10 arc second range, even less than 5 arc seconds, and the Ovision retrofit offers even better performance for imagers, and can be put on all post 1992 mounts.

In sum, you'll have a more sustainable system, a better performing system, and more aftermarket options if you go with a G11. But CI700s sell at steep discounts and can be a reasonable value.

Greg N

AP Barcon

University Optics has some nice widefield eyepieces. Start with those. You don't need to jump into Pans right off the bat. Get there slowly. And then get Pentax.

regards
Greg N

It means you expressed some sort of view about some sort of technical aspect regarding optics, Roland saw the post, and explained why you didn't really know as much as you thought you did.

regards
Greg N

It's not really the contrast. It's that it is impossible to find a position where you see the entire f.o.v. and avoid kidney bean. I see it at night and in daytime tests; daytime tests showed that what I thought was a comfortable position still would have very slight kidney bean that would darken the center. If you pull back to avoid kidney bean you lose the 82 degrees.

The 16 T5 you get less of that problem but pay for it with very little eye relief. There are no free lunches.

If you tighten up on the fov you can work around some of these issues. Since Pentax has nothing between 14 and 20mm I have settled in recently with the Zeiss 16.7mm from APM.

This is a really nice eyepiece, but it's pricey (around $400) and only available direct from APM in Germany. For Dob guys it offers an extremely light weight. It does however show more field curvature than the Vixen LVW 17mm, but is noticeably sharper on axis. (Don't ask why, I tells it likes I sees it). The reflection control is not that great on planets, it is similar to the the 17mm NT4. However, there is no kidney bean and the whole field of view (69 degrees) is "up front" as with a Pentax. On my f/6.5 refractor and f/11 SCT it is fantastic. I haven't tried it on the f/8 FS128.

Once you have gotten used to an eyepiece collection that has virtually no kidney bean it really "hits you in the face" every time you use a T.V. wide field. The design philosophy banks on the ability of people to train themselves to hold their heads just so and also to ignore the occasional optical imperfections. What is less well known is that you can have a very slight darkening near the center of the f.o.v. that won't show as a "crescent" or "black kidney bean" and you'll think everything is fine and dandy. It's too slight to be easily discernible at night and if you go forward "in" or back out (each of which has other positioning problems) you can swap the center darkening for other edge effects. The darkening effect on this eyepiece works from the center out as you adjust your head; on the Pentax and Zeiss such effects, more difficult to produce, occur from the outside in.

I've owned my NT4 for years and always loved it but now that I have been paying attention to this issue I feel that I've been "had" by the "fad" of 82 degree eyepieces. I also had some kidney bean problems with the ethos. I think this is just part of the ultra wide field design. As you shrink down to 70 degrees you get better control of this effect at the price of f.o.v.

Anyhow what most people call "contrast" or "light throughput" on these eyepieces is IMHO the very slight center kidney bean/darkening effect that occurs when you try to position your head to get the whole f.o.v. and at the same time avoid the side-crescents 1/3 of the way out. In the 17mm range I think the Zeiss 16.7 is the next-best solution to the non-existent Pentax in this fl and the Vixen LVW 17mm is probably no. 2 and more cost effective. But I was surprised in my friend's 15" Obsession to see repeatedly that I got sharper on-axis focus but had more field curvature at the edges (in the Zeiss) and a slight lack of focus tightness in the Vixen that you would never notice if you weren't swapping, but better control of the edges. I'm not sure which I would pick if I had an f/4.5 instrument but I'd probably go with the razor sharp center.

Obviously daytime tests have the problem of a small pupil vs. a large night time pupil but if you think of it as a "stress test" that will show exaggerated effects, and then look more carefully at what is going on at night, I think you'll learn a great deal.

Currently I lean toward bridging the "Pentax hole" between 14 and 20mm with a UO HD 18mm for Jupiter and Saturn and the Zeiss 16.7 for most other applications. I will be keeping the NT4 for awhile to convince myself that I really don't want to keep it, and to do some additional look-sees on my FS128 which I haven't tested it out with yet. But in essence I am leaning away from Nagler designs in my personal tastes. The one eyepiece I really like of all the Nagler wide fields is the Pan 41, which I think is underestimated by the astronomy community. To me it seems equivalent to the XW40. It is a great deal heavier.

regards
Greg N

1. The used market is soft. You have to be prepared to wait longer for items to sell than in the past and be prepared to take a lower price.

2. When the price goes too low you sometimes get people shying away because they think something is wrong with the item. But if you price too high you can't sell it. This is called "you can't win for losing."

3. I don't think there's a lot of confidence in used go-to mounts of this type. The owner of the Nexstar 6 may have gotten lucky.

4. The competition from other mounts that have better reputations with regard to carrying capacity will work against you. The ASGT go-to for example sells used in the $500 range.

5. I find it somewhat amazing that Super Polaris mounts are down around $200 to $300 now. I'd rather have one of those than a go-to for $500.

6. An OTA configured for one mount only is less valuable than OTAs that can easily be swapped around on different mounts. I don't know how easy it is to swap out your C5.

regards
Greg N

George Golitzin said:

Does Greg Nowell's dioptrx trick work on the shorter focal length XWs? I'm looking hard at the 14mm, but I will need astigmatism correction on it, unfortunately. At f/5, anything above my 11mm Nagler seems to need some help. It looks like the smaller focal lengths have pretty much the same mechanical assembly at the eyelens level as the longer ones...

Maybe Greg could comment...

Thanks!
George

I'm glad s.o. has read my article! Surely I must be famous. Like Roland???

1. Short answer: It won't work. The XW30 and XW40 have different diameters from the rest of the series.

2. Longer answer: If you disassemble the dioptrx a bit, "possibilities arise." The lens assembly comes out of the lettered ring. Some caution is advised as the inner most ring can unscrew and release the lens completely. But in the end there is a ring in side the ring which can hold the lens, and if you play with it you'll see that this asssembly can actually hang readily (but not pop on) on the inner metal lip of the Pentax. It works well enough that you could use the eyepieces that way, but you'd have to grab the lens as you switch positions on the scope or, if other users are around, if you walk away from it. Most folks wouldn't expect something like that to fall off, so letting it hang on the eyepiece would be a solo practice. It would work like a monocle that you pop on as needed. You would be sacrificing the rubber eyecup which is a big part of the satisfying ergonomics of these eyepieces. It might be possible to put a soft spacer on the inside of the TV ring to make it actually hold with some pressure. It certainly requires thought in the presence of the eyepiece and the Dx so you can try to figure it out.

3. You mention f/5 and all I will say about that is that the XW is most-frequently-sold by dob guys because they don't like the field curvature this eyepiece gives at short scope focal lengths. The XW20 is the other eyepiece that dob guys typically don't like, although my friends think they do OK, so maybe it's a regional cultural thing. I use all the Pentaxes at fls of 6.5, 8.1, and 11 and think they're the cat's meow. But the curvature thing is something to consider before spending a lot of dough on a 14mm. I'd like to say Pentax can do no wrong but a lot of fast Dob owners have rebelled at the 14 and 20mm. I assume this would apply to fast refractors though mine is dandy at f/6.5.

regards
Greg N