Sunday, January 24, 2016

 

The Refractor Way IV: The Further Adventures…


She's big and she is strong...
If you’re a Face-Front-True-Believer, if you are a regular reader here or you are a Facebook friend of mine, you know I’ve been thinning the herd. Selling telescopes or in some cases just giving them away. There are several reasons for that, but a big one is that I am tired of having scopes that could make someone happy sitting around here rotting unused. Thus far, I’ve got rid of three C8s, a 12.5-inch truss tube Dobsonian, a Criterion RV-6 Dynascope (a six-inch Newtonian, kids), and I am hopeful that I’ve found a home for my StarBlast RFT.

With all those scopes going out, I didn’t feel guilty about letting two new ones come in, a SkyWatcher Pro 120 ED refractor, and a Zhumell 6-inch achromatic refractor. Why refractors? While I still have my Edge 800 C8 and my carbon-fiber-tube C11 (and plan to keep both), at this stage I just find refractors easier. No cool-down to speak of. Easy to image with given their moderate focal lengths and lack of floppy primary mirrors. Super visual images (in good refractors). It’s also true that a hallmark of the last year for me has been change. Needed change. Including in amateur astronomy. So, this week we continue with the further adventures of Ethel (the 6-inch achromat), and Hermione (the 120mm ED APO).

Big Ethel

First up was Ms. Ethel Muggs. Two things were on our agenda, a check of her collimation, which I had noticed was out during first light, and which I had adjusted. I also wanted to see just how bad the chromatic aberration, the color purple that devils the images of achromatic refractors, was. Luckily for me, there was a nice crescent Moon in the sky, and, after Venus maybe, there is nothing in the sky that reveals color correction faux pas in a telescope more quickly.

Once I had the VX mount goto aligned, our first destination was bright Deneb, which was still high enough in the west (barely) to be a usable collimation tool. In went my 4.7mm Explore Scientific eyepiece. “Still out, darnit.” Despite so-so-ish seeing, I could tell the in-focus diffraction rings of the star did not make an unbroken ring around its Airy disk. In truth, I was not surprised. When I’d done the initial collimation I’d been in a hurry and hadn’t gone back and re-checked everything like I should have when I made my final adjustment. Sometimes, tweaking one thing will untweak something else. While the collimation of an f/8 refractor is not overly critical, I wanted to give Ethel her best shot at surviving the Moon Test.

Out came my laser and my Cheshire, which are normally all you need for refractor collimation. When you need to deal with things like objective lens element centering, you’ve got a job for an expert, but basic collimation of a refractor isn’t much more difficult than collimating an SCT. No more difficult at all if Check One is good, actually.

The first thing you check is whether the focuser tailpiece is square-on with regard to the objective lens. The simplest way to do that is by putting a collimation laser (a Newtonian collimation laser) in the focuser (no diagonal) and turning it on. Observe the objective: is the bright red spot of the laser beam centered in the lens? If so, you are golden and can go on to the actual lens collimation. If not, you need to center it before collimating the objective.

How you square the focuser depends on the focuser. Expensive ones like Moonlites will have collimation adjustments. The stock focusers on inexpensive Chinese refractors won’t. But that doesn’t mean they can’t be adjusted. What you do is loosen the screws holding the focuser to the tube. Usually the screw holes are large enough or are slightly elongated enough by design or accident, to allow you to adjust the tilt of the focuser sufficiently to center the laser spot. It doesn’t take much. I did some tip-tapping with a hammer and dowel and soon had that pesky spot back in the center of the objective.

If your objective lens is in an adjustable cell, the actual collimation is easy enough. Insert your (Newtonian) Cheshire sight in the focuser in place of the laser. Shine a flashlight into the side port of the Cheshire, and have a look through the peep-hole. You will see one or two (or three depending on your lens) circles of light, reflections in the objective. If you only see one, your collimation is good. If you see more than one? Uh-uh…you’ve got work to do.

An adjustable objective cell will almost always be collimated with three pairs of push-pull bolts spaced 120-degrees apart. Observe the reflections. One should be toward the center of the objective, and one off to the side. Pick the pair (or pairs) of bolts opposite the reflection that is off to the side, loosen the push screw (the longer one) slightly, and tighten the pull screw. Go back to the Cheshire and look. Did you move in the correct direction? If so, continue till the circles merge. If not, tighten push and loosen pull. When you are done, do a check with the laser again to make sure the spot is still hitting the center and adjust the focuser again if necessary (and double check with the Cheshire following that). I did all those things and was shortly ready to star test again.

Collimation verified on bright Rigel, it was time for some fun with Luna. In truth, I didn’t know what to expect. I began at low power with an old 82-degree 30mm eyepiece I got from Herb York many a Moon ago. “Dang! Awful sharp!” But it wasn’t just sharp; the pre-first-quarter Moon was also amazingly color free. Sure, there was a little purple, including a thin line along the limb, but when I held my eye on axis, it was minor, as you can see in the iPhone shot I snapped by the simple expedient of holding my cell phone up to the eyepiece. This picture does a good job of showing exactly what I saw.

While the Moon was bright—my collimation exercise had allowed time for astronomical twilight to come on—one thing other than brightness of the subject can make chromatic aberration appear worse: higher magnification. I retrieved the 4.7mm ocular I’d used during collimation and had a look. At 255x there was no doubt there was “excess color.” Not just along the limb, but in the shadows, which now had a distinct violet hue. However, it was surprisingly unobtrusive and the view was still sharp even in average seeing. I had a #11 yellow-green filter on hand, and using it immediately banished that moldy oldie “Purple Haze” from my soundtrack, but I didn’t think it was necessary and preferred the view without it.

One of the beauties of a big refractor is or should be that its unobstructed nature and contrast characteristics allow you to use stupid high powers considerably in excess of the mythical “60x per inch of aperture” limitation. OK. What would Ethel do with the 4.7 in a 2x TeleVue Big Barlow to the tune of 510x? The seeing was not quite good enough for that much power, but Ethel accepted it willingly nevertheless. The image was still sharp enough considering the state of the atmosphere, and the color was still decently controlled. 510x on this inexpensive 6-inch? Kinda amazing if you ask me.

What else was amazing? That silly little Celestron VX mount. With the Moon high in the sky, I was using the GEM with its tripod legs fully extended. Even then, the eyepiece was low to the ground—but bearable. With the legs all the way out, I was still able to focus easily at 510x, and what shakes there were didn’t hinder my enjoyment of the stark lunar landscape.

What else did I look at on this evening? I did a tour of the prominent deep sky objects, more as a comparison of image quality before and after fine-tuning the collimation than anything else (maybe some better, not like night and day), but the main course was the Moon. I just couldn’t get enough of her in the mighty refractor, and the photo above is a mere suggestion of the beautiful images that appeared before my eyes. I probably cruised the Moon for a good two hours before my feet got cold, the signal that it is time to end an observing run.

Hermione

Hermione Granger
As I mentioned in my initial report on my new SkyWatcher Pro 120 ED refractor, Hermione Granger, her primary task would be deep sky imaging with my DSLRs. I’d actually already done some of that beyond a quick snap of M15 through a layer of clouds. I’d spent a couple of evenings with a far more challenging target, the Bubble Nebula. The telescope acquitted herself well riding on the CGEM mount.  While I could no doubt have done pretty well imaging with the 120 on the smaller Celestron VX due to the telescope’s relatively light weight of 11.5-pounds, I thought the CGEM was a must on these windy winter nights.

I was pleased with the Bubble from my less than perfect backyard. No, the images were not comparable to what I could have obtained from even our club’s semi-dark observing site, and the need to image through a mild LPR filter gave bright stars a rather odd look and applied a hard to banish color cast to the images, but not bad, not bad at all. Heck, if somebody had told me 20 years ago that I’d be getting recognizable pictures of the Bubble from my suburban backyard someday, I’d have laughed in their face.

Given my good results with the Bubble, I was eager to do more and was anxious to get started when the next front passage brought clear (if somewhat unsteady) skies. The target this time would be even more challenging, I thought:  NGC 281, the Pacman Nebula. Not only would I be going after a harder subject, a new piece of gear would be thrown into the mix, a new guide camera, a QHY 5L-II.

I’d been using the same camera for auto-guiding for ages, an Orion StarShoot Autoguider, which is actually the original QHY guide cam with an Orion nameplate slapped on it. It worked. I’d never been unable to guide with it. It had a drawback, though. Fairly low sensitivity. Nevertheless, using it in concert with a 50mm finder/guide-scope, I never landed on a field where I couldn’t find a guide star. Admittedly, some of them could be a little low in the signal-to-noise ratio department, but I was always able to guide OK.

There was a more important reason for buying a new camera than just having a larger selection of good guide stars to choose from, however. Before you know it, another terrific Mars apparition will be upon us. I wanted to be prepared for that, and since I didn’t have a monochrome planetary imaging camera, it seemed like a good time to invest in one, one that could also be used for guiding (the StarShoot is monochrome, but cannot normally be used for imaging).

Which new guide camera? I gave some consideration to the Starlight Xpress Lodestar, which gets rave reviews, but I didn’t want to spend 600 dollars. Instead, I narrowed it down to two much less expensive cams, one of which was the ZWO ASI120MM. I have the color version of that camera (which was made before ZWO began added ST-4 guide outputs to their cameras) and it seemed like a natural. I knew the mono ZWO was sensitive, and I’d had a good buying experience with ZWO. However, my Chiefland friend Paul Lavoie told me he had never had any luck using the ZWO with a USB extension cable. In some cases I would need a longer run of USB to the camera, so that was a strike against it.

What Paul was using when I was at CAV in November was the QHY-5LII, a little guide/planetary cam that contains the same 1/3-inch sensor as the ZWO. I watched it in action and was impressed. It was obviously very sensitive and was light in weight and amazingly small, about the size of as a 1.25-inch eyepiece. I was impressed enough to eventually turn over my credit card number to a QHY vendor, the primary QHY vendor in the U.S., Astrofactors, which has an excellent reputation for service and support.

One caveat? If you want this camera, get the QHY badged version. Orion has their own version of the QHY 5L-II, which they call the StarShoot Autoguider Pro.  The Orion Pro and the QHY 5L-II are quite obviously the same camera save for one difference: the price.  The Orion version is $379.00 compared to $249.00 for the QHY. You do get some imaging software with the Orion version, but it certainly is no better than the freeware program I use, Firecapture.

The QHY arrived quickly, was nicely packaged in a little metal tin, and included a parfocal ring, an extension barrel, a (fairly short) USB cable, and an ST-4 guide cable. There were no instructions included, but they were easy to find on the QHY website. Same with the drivers; there was no disk in the package, but there was a card with a url for downloading them. That is maybe a good thing, since it ensures everybody gets the most recent version. I was impressed by both the packaging and the cute little camera. It comes in various metallic colors; I got the attractive chrome-barrel version.

Getting it up and running was simplicity itself and would not be a challenge for anyone who’s used a similar camera. Download the driver package, click on it (it’s a .zip file) in the downloads folder, and install it. Plug in your camera. That’s it. When I brought up PHD2, I was able to select the QHY without a hitch.  Clicking “loop exposures” in PHD2 and removing the camera’s lens cap resulted in an immediate whitening of the display, so it was obvious it was working from the get-go. “Simple. Neat. No trouble at all,” as Mr. Poe would say.

The new Stellarium...
Out in the back forty with Hermione and the CGEM, things went in similarly smooth fashion. First up was focusing the new guide camera with my guide scope, an Orion Mini. I’d screwed the extension nosepiece onto the camera and positioned the parfocal ring a similar distance from the chip as on my StarShoot. Pointed to Caph, which I figgered would show up even way out of focus, and began looping frames with PHD2. But not a thing did I see. Just some noise. What the—?   “Rod, you dummy, it helps to remove the lens caps from the guide scope.”

That minor detail seen to, I set the exposure to 1-second and tried again. My first tip-off that this is one sensitive little camera was that I didn’t just see one large out of focus star-disk (Caph), I saw many. I repositioned the parfocal ring a bit to get focus nearly in, and then tightened the camera down with the mini-guider’s set-screws. Final step was refining focus by screwing in/out the guide scope’s objective assembly while observing the s/n ratio on one of the stars on PHD2. When focus was good, I locked the mini-guider’s objective down and we were ready to roll.

With the guide scope in focus, I was simply gobsmacked at the difference between the 5L-II and the StarShoot at my usual 1.5 – 2-second guide exposures. With the StarShoot, I’d have seen stars, but some would have been on the dim side. With the new one, the screen was full of bright stars. Only drawback? At 2-seconds, PHD2 said more than a few of them were “saturated,” (a no-no). I had to back the exposure off to 1.5-seconds to de-saturate the star near the center which I fancied as a good candidate. 1.5-seconds is maybe not quite as good in poor seeing as 2-seconds, but it worked fine. I also tried 1-second exposures, and, as with 1.5-seconds, there were many more stars visible—many more—than there would have been with the StarShoot.

Backyard Bubble...
Actually, the guide camera was not the only change on this evening. The other was the software I used to send the telescope on gotos. What I’ve been using for quite a while is Bisque’s TheSky 6 Professional. Works perfectly. Well, but for one thing. This no longer supported software (it’s long since been superseded by TheSky X) can no longer automatically download comet elements, and I had the wild idea I was going to stay up late and shoot Comet Catalina.

What to do? Normally, I’d have switched to Cartes du Ciel, but I’d been having problems with that program on my laptop, getting crashes and weird error messages. I’d tried several things to exorcise these demons, but nothing had worked. What then? How about Stellarium? I’d used this program now and then and had been somewhat impressed. And, I’d recently been told it was in a new version and better than ever. So, I downloaded and installed the new Stellarium and found it easy to automatically update its comet elements including those for Catalina.

How was the new Stellarium otherwise? Version 0.14.2 is impressive. So impressive that I may devote a blog entry to it some Sunday soon. Suffice to say that it is more than powerful enough to do my admittedly rather simple tasks. Honestly, I never needed the power of TheSky 6 Pro and never used even a fraction of its many features. Stellarium looks good and works good. Not just inside on the desktop, but outdoors in concert with the telescope.

Let me also give a big shout-out to the developers of the Stellarium add-on program, Stellariumscope. While Stellarium now has built-in drivers for several telescopes/mounts, I preferred to use the excellent new Celestron ASCOM driver. To do that, I needed to make Stellarium ASCOM compatible, and you do that with Stellariumscope. The combination of Stellarium/Stellariumscope works well and simply. Search for your object. When it’s found, center it with the space bar. Hold down CTRL and press “1,” and the scope goes to it. That is all there is to it, and is really all I need astro-software to do at the moment.

Before I began acquiring images, I wanted to do one other thing. There was that bright Moon hovering overhead, and I couldn’t resist sending Hermione to it. How would she compare to Ethel there? While Ethel did a fine job, there was no denying Hermione was better. As I’d expected given her amazing color-free performance on Sirius, nary a hint of false color did I see. And so sharp! This is not to take anything away from the achromat, but the 120 is just flat out amazing on everything.

The Pac-meister...
Next, I CTRL–1ed my way to the Bubble Nebula, set my fave DSLR control program, Nebulosity 3, to take 35 180-second exposures with the Canon 400D, made one last check of everything, pushed the “Capture Series” button and headed inside to watch TV in the warm den while Hermione did all the work. Before decamping, I did take a look at PHD2. It was guiding with an RMS error of between 1 and 1.5-arc seconds. That is quite good enough to ensure smallish round stars with the refractor so I didn’t feel the need to mess with it.

Over the course of the next several hours, the scope and computer did their thing. I’d wander out on to the deck (where the computer is; the scope is on the ground) to check on the progress once in a while, but I really didn’t have to. The telescope and CGEM continued to guide well with PHD2, and Neb wasn’t missing a beat.

At the end of the evening I was rewarded with the nice picture of silly old Pacman you see here. Oh, maybe it wasn’t quite that easy. Next day I stacked subframes, did some preliminary processing in Nebulosity, and some further tweaking in Adobe Photoshop and Adobe Lightroom, but it wasn’t hard. Yeah, my picture would have been better from a dark site, but at a darksite I wouldn’t have had a warm den and a big-screen TV to enjoy while the sequence was acquired. At any rate, I was at least as pleased with Hermione’s Pacman as I had been with her Bubble. And, no, I didn’t stay up for the comet. The temperature was dropping steadily, and so were my eyelids. I pulled the Big Switch as soon as the last Pacman subframe came in.

What’s next for my refractor girls? Not only is there a waxing Moon in the sky, there’s more rain on the way, so I suspect all will be quiet here till the next dark of the Moon cycle. When that comes, I hope to get Ethel out to the club site for some visual fun. If I get really sanguine, I may even cart out a load of gear and do some real deep sky imaging there with sister Hermione.

That’s for later, though. For now, I am just so pleased about both my new scopes. Both will, I think, add a lot to my (modest) observing programs. If nothing else, they’ve served to do one very important thing:  get me excited about astronomy again after a few months of the semi-doldrums. Can’t ask for more than that. 

Excelsior, y'all.

Comments:
Hi Rod,

Until your entry last week I'd never given serious thought to refractor collimation, believing that that was a need for only my Newtonians. So, I plopped my Tectron Cheshire into the focuser of my mid-80's 6" f/8 Astro-Physics, and I found that indeed, I had two non-concentric rings. My old AP does not have collimation screws on the cell, but there is some limited adjustment in the original R-P focuser in the form of three recessed hex set screws in the body opposite the rack & pinion mechanism. I made some adjustments and more or less was able to get concentric rings but have not yet had a chance to check things under the stars. I’m not sure if focuser only adjustment will be sufficient, and the tension needed to retain collimation has made the mechanism a little stiff.

Thanks for sharing your experiences with your 6”. I’m amazed at just how far larger affordable refractors have come. In fact, I’m amazed that there are affordable refractors at all!

John O’Hara
Oil City, PA

 
Hello Rod,

Re laser collimation shining dot centred in the objective ... you can also put the scope objective end on top of a mirror which reflects the laser back to the eyepiece end thus acts like it does in a reflector and you collimate the same way.

But your tip on using the cheshire was new and a valuable tip - thanks heaps for that one.

Cheers
 
Enjoyable write up on Ethel and Hermione Rod.
More than once I've heard (and used myself) the expression "There is just something about using a refractor!"

I have a Skywatcher 102mm short tube f5.0 that provides gorgeous views of M45 and the like and more lately an F8.3 120mm Celestron Omni. I can't really explain why I enjoy the views so much through my refractors (both humble achros) other than to say "There definitely IS something". Pleasing? Contrasty? Absorbing? I don't know, but I'm so glad to see you write "get me excited about astronomy again".

Alec (UK)



 
Where did you find the Explorer Scientific for $299? I found it for $399

 
Looks like the price has gone up a bit again. Still a good value...
 
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