Posts Made By: Greg Nowell

Actually you would be better off if you just made a circular mask for the last 1/4 inch and put it all the way around the mirror. One of the chief sources of "noise" in the Newtonian mirror is turned down edge which is nigh on impossible to eradicate from the parabolizing process. This is actually a more important source of light scatter than your scratches.

But I agree that the mirror should have been packed in such a way as to avoid this.

regards
Greg N

Yes it will but you're going to have a huge honking leveraged mass on the back of your SCT. Personally I'd prefer getting a 15 or 16mm to such a proposition, but if you really want to barlow that N31, I would consider getting the Astrophysics visual back and diagonal for your 9.25 to reduce the possibility of rotation. At a minimum epoxy the barrel of your diagonal so it won't rotate under the load and dump your eyepiece on the ground.

regards
Greg N

Hey John, 'sup?

I would recommend consideration of the UO HD orthos, and the 15 and 21mm Edmund RKEs. If it were my binos I'd go for the RKE 15 and 21mms for reasons of price and overall good views.

The radians are really a "different statement" than your other choices because they are wider field.

The RKEs can be bought here on the mart at $40 a pop or bought new from Edmund Optical for around $65. They can also be readily sold. To my mind they're a good choice because you get a fair expanse of glass and they're not going to massively increase your weight on the rear of the scope.

I have never found a Radian I really liked. It's like coffee ice cream. I just don't get it.

My UO 18mm is pretty good and shows no reflections that I recall on Jupiter etc. I don't use it much on planets however as the fl is wrong, too low on the Tak, OK on the C14.

An extremely unorthodox suggestion is to get the Pentax XLs and "make them skinny" (I'd suggest the XL 14 or 21mm). Not to be confused with the XWs. The XWs don't come apart. But the XLs do. That's high quality glass for planets, and a nice 65 degree afov. Most people, including "the market" as reflected in prices, seem to put a higher value on the Pentax XLs than the equivalent radians.

Somewhere I have posted a photo essay on how to disassemble an XL. It might be on Yahoo! refractors. I suppose I should find the pictures on my pc and upload it as an astromart article.

good luck,

Greg N

Funny, I don't have a dream scope no mo'.

But there is no one scope that would "do it" for me. I think what I would like most is a place where I could use this stuff more than 20 or 30 nights a year. I'm not sure that's my dream either. Don't know what life would be like if I picked up and moved.

But I'm in a good spot where I have more optics than I have time to use them. I guess one thing that *WOULD* be nice is a $10k solar setup.

Another way of looking at this is I have my dream scopes already. If you look at the mounts and scopes that were available back in the 60s when my "formative fantasies" about scopes were made, not to mention the eyepieces, I have far exceeded my catalog dreams of yore. My eyepiece box kinda scares me when I think what's in it. In a world where folks are starving, etc., it seems a bit obscene to have such a pile of glass. (But I let that feeling pass too.) I also feel a bit self-conscious (may not always show) about the folks that are struggling to put together their first rig and trying to make $1k go in every direction.

So I done got me my dream scopes, bottom line. Gimme some cash and I can figure out ways to get mo' betta', but I'm surprised I have been able to procure and use this many toys. So I try to cultivate an attitude of gratitude for what I have already, which is also an exceedingly cost effective approach. Cost effective, that is, after high initial outlays.

regards
Greg N

Some combinations are greater than the sum of their parts. The C14 + Vixen 102 f/6.5 on an AP900QMD shows me the sky in many different ways. It's main downside is the setup time and the "heave ho" to load the thing into the saddle.

My runner up is the FS128. I've had lovely times with it. The planet performance is excellent but side-by-side with the C14, you can see more in the c14. It's got a lovely wide field but not as wide as the Vixen.

I tend to save the FS128 for short nights when the moon is coming up or the weather is uncertain. The C14 + 102mm is such a versatile sky tool that it's hard to settle for a five inch aperture when I can use a four inch and fourteen inch in tandem. The addition of the four inch to the fourteen inch changed my appreciation of the sky.

regards
Greg N

Kevin Brennan said:

Gents, I am trying to collimate an Astro-Tech 6" RC for a friend and clubmember. This type has a factory fixed primary and they intend you to collimate with secondary alone which seems a bit different that your standard RC according to what I read.

Right away, I noticed pretty severe tube currents which is what I believe he is seeing rather than an out of collimation scope. These lasted for a looong time. Is this typical of this design? The scope was brought out from a 73 degree house, to a 65 degree nite and was set up at 9:45. By 11:20, it was just beginning to settle down enough to see round fresnel rings. I've had SCTs that cool faster than that. Do the baffles affect this?

Thanks,
Kevin

Well Roland just posted that on refractors you want the aluminum tube shrinkage to match the change in the curvature of the lens.

However, on Yahoo C14--totally different design of course--the opinion is that the carbon fiber does indeed help one stay with one focus point. People make the switch and they report good results.

Getting to the thermodynamics of the SCT, I have studied it in my rudimentary way. The aluminum OTA is extremely efficient at radiating heat, and the heavy rear casting, while also a "heat sink" that might on the one hand prolong cooldown, is also pretty a pretty good radiator and you get conduction through the baffle and the thick part of the mirror.

Certainly the major tube current in my C14, I was surprised to find out there with my infrared zapper, is not from the mirror to the top but from the ground side to the sky side. The temperature delta is huge and swamps almost any other thermodynamic in the tube, nearly as I can see.

Running the *internal* dew heater at 22 watts sounds like a thermal nightmare. In fact it barely provides enough heat to keep the corrector clear. (A larger external dew heater would fail to heat the corrector enough). But it seems to be that even though the internal dew heater is not in direct contact with the aluminum, the sky/ground dynamic is the main source of currents. If you zap the corrector (the IR gizmo reads the glass temperature) and the aluminum nearest to the dew heater, there is very little delta from ambient. Total variation over the corrector is no more than a degree and it too is very close to ambient.

This behavior is completely at odds with what I registered in garage experiments when, with the same rig, I easily jacked the corrector plate up to 5 or 6 degrees above garage ambient. The difference is in the radiative cooling, for me this was a lesson in physics. The heated garage walls are warm so there is very little radiative loss on inside experiments.

Your baffles, if made of aluminum, are still inside carbon fiber, so even if they are aluminum, their largest exposed area is not to the sky but to the carbon fiber which doesn't want to radiate to the exterior or for that matter absorb heat losses from the aluminum baffles. So yeah it seems to me this could slow things down relative to a 100% aluminum OTA. Basically the only way you have of losing heat is through convection from the mirror out the top end, and it seems like that isn't working too well.

For reasons I consider mysterious, my C14 cools down more quickly than my friend's 15" Obsession, whose mirror is surrounded by wood and sometimes is in a shroud and sometimes is not. But maybe the conical design of the SCT primary helps.

Given all this it is surprising, but not a total surprise, that the carbon fiber open tube may be on the slow side to cool down, but jeeze, six inches just aint much, and if it has a hole in it and is conical, the largest optical real estate on the edges should cool fairly readily.

But if you're looking for a solution I'd say the standard dob procedure of having a fan blow across the thermal boundary would probably be your best bet. I think that 90% of the optical cruddiness is in the first inch over the mirror. Another solution would be, when you take the thing outside, to put a fan up the behind of the OTA to improve the efficiency of the convection cooling.

regards
Greg N

If you're gong to have a database the usual de minimis requirement, in my view, is the NGC and IC, which is 12,000 objects. If you are using in alt-az mode you will have a devil of a time finding objects by RA and Dec, even if there is a readout option, unless there is a special algorithm which converts RA and Dec into a go-to coordinate. Really. Even if you know what you are doing, moving an alt-az mounted scope with RA/Dec coordinates and an RA/Dec readout is as much trouble as star hopping.

So my general thought would be, this is going too far to save a few bucks.

On the other hand, with an ETX 90 or 102mm scope your reach isn't going to be as deep so you might get along fine with a small database. Certainly some of these 100k databases I've seen are pretty much overkill.

The Sky Atlas 2000 has about 3,000 objects, but the problem with it is that it misses a lot of fairly bright objects. I'm often surprised to see that something is not listed on the SA2000.

On balance, if you're going to stay in the 3.5 to 4 inch aperture, you'll probably be OK. But there was an NGC "mini-max" and I observed with a friend who had that and he was constantly not finding things in his small database (about 5k objects). But at that time he used a GEM, so flipping into RA Dec wasn't hard. And he had an 11 inch scope.

One of the reasons I'm cautious is that Sue French's work and the refractor revolution generally has shown you can see an awful lot with a four inch aperture. If you're prepared to star hop you can go anywhere anyone else can, and often, you'll find that where you're going is near something else that IS in your database, so as I say, it can work out. "This idea is OK, but is approved without great enthusiasm."

My 2c, anyhow.

regards
Greg N

regards
Greg N

I just can't imagine recommending a 90mm achromat (even if it is f/10) over a 6 inch Newtonian for viewing planets. I have a very nice 102mm ED refractor that I put on my C14 and I routinely get to check the refractor's performance. When you start pushing the 4" refractor into good observing territory, 130x and over, it gets too dim.

I also think that views of planets at 100x don't tell us much. That's what I get with a 40mm in my C14. At that power it shows more than the 102mm at the same power, but "nothin' to write home about."

The view in my 5" refractor of Jupiter etc. is quite good and shows enough detail to maintain interest. It doesn't show the color and festoon detail that the 14" does but it is enough to maintain interest. I have seen here and there people say "get a 90mm refractor" for planet viewing over an 8 inch Newt etc but gosh 'n' golly a 90mm scope on a cheap EQ mount is not what I would nominate for a newbie. I see however that there are some very positive user reviews posted. One person said it was a lot better than his dept store scope. That, I can believe.

regards
Greg N

You've got a very good list. If you want field of view you should forget the TMB Supermonos which at 30 degrees are tight tight tight.

Your orthos are mostly 40 to 45 degrees, mostly towards 45 degrees.

Hard to believe that plossls were once considered wide field eyepieces (at around 50 degrees). The TV plossls have a strong following and a good reputation but Chris Lord didn't have much positive to say about plossls in his History of the Eyepiece. If I remember the argument correctly, the plossls sacrifice a tad of on-axis performance to get their "wide field." Whether anyone but Chris Lord would notice is a hard question.

I see SOME people swearing by their Tak LE's but by and large when people decide money is no object they seem to sell their Tak LEs.

The UO HD orthos are well regarded.

The radian is one of a very limited number of eyepieces with which I have had truly bad experiences, as in replacing a radian with another eyepiece and easily seeing more detail in the other eyepiece. This also happened once with a Burgess Optics "planetary" eyepiece which I would like to revisit again to see if my impressions were accurate. Anyhow with the radians it has happened to me twice, and I have read similar reports from other observers, and they are not "competitive" as planetary choices in my view.

With some of the less expensive options I think you should just buy a few used and try them and sell any you don't like.

I think it is a mistake not to have the Edmund Optics RKEs on your list and certainly the Pentax XWs. The latter qualify as excellent planetary eyepieces and also have the most field of view of any on your list. I've tried a number of the options you list, and on that list, I give the nod to ZAO II and UO, but opinions vary.

good luck,
Greg N

You should be OK with the 10mm or 13mm, but the 17mm is getting to be at the edge of what your c8 will allow through the baffle tube without vignetting.

There are many ways to improve over a celestron set without going to ethos. Options include but not limited to Baader Hyperions, Edmund RKEs (I'm very fond of the 8 and 28mm), and the Pentax XWs (also XLs used). The Edmund RKEs are not wide fields. The Baader Hyperions and Pentax XWs and XLs are. If you've been using a bunch of plossls 68 to 70 degrees is going to be plenty. A 10mm Ethos will be a treat, however.

regards
Greg N