A Used Celestron Omni XLT 150 -- Early Impressions & Cleaning

Back in early July, I found a hell of a deal on a telescope tube on craigslist. The Omni XLT 150 optical tube, for $60 bucks. It sells new for $290. (OTA only) I picked it up and brought it home, with the plan being that I would embark in my first journey into Amateur Telescope Making and build a Dobsonian Mount for it, based upon this blog post from 10 Minute Astronomy.

The XLT150 on a photo tripod (for display only)

Perhaps this was ill-advised anyway; I already have a 6" Dobsonian, though since both only have 1.25" focusers, the 6 incher can get to much lower powers and wider fields of view than the 6 incher can, which I figured would allow me to see deep sky objects and star fields better. And it would be more portable than my full-size 6" f/8 dob.

Then I started writing telescope reviews for AstronomySource, and after my first paycheck I had a lot more money to play with, and so I bought the CG-4 mount new for $290, figuring I'd be able to use it with the Omni XLT 150 tube to make an accurate review of the telescope in question for AstronomySource. I was able to afford the mount, and I'd be able to make back about half of that by writing the review. Unfortunately, it took too long for me to do that (as we'll see, there's things I had to get around to fixing first), and AstronomySource was merging with TelescopicWatch. Since TelescopicWatch already had a review of the XLT150, I no longer had a way to get any of the money I spent on the mount back.

I'm not upset that I got the CG-4 mount, because it did end up performing very well indeed. Very stable, sturdy, and smooth, especially when compared with the CG-2 (EQ-1) mount I was used to with the AstroMaster 114EQ I used to own. But the mount is heavy, bulky, and takes much longer to set up than the Dobsonian, especially if I go to the trouble of getting polar aligned. Even though I'll probably mostly use this scope by star-hopping, I'd like to give the setting circle a go. The RA circle has a vernier to increase accuracy, which makes me think it might be genuinely functional rather than just for show on this mount. I did at one point try my 6" f/8 dob's optical tube on the mount, but the XLT150's tube rings are slightly too large; I'd need to shim the rings with a ring of cardboard or something to give it a go. The payload of the mount is 20 lbs, and the 6" dob optical tube is 20 lbs.

After manufacturing a crude collimation cap from the dust cap for the FirstScope I had lying around, I collimated the telescope for the first time in years. It was pretty much the first time I'd actually collimated a telescope. (At least, the first time I'd succeeded in doing so; I'd tried and failed to collimate my Bushnell Voyager a while ago)

My first observing session was done without notes (I was too frustrated), but I did write down my impressions after I got home. They're reproduced below.

Set up on the first night.

Observing: first impressions

My very first time observing with this telescope was full of frustration and discomfort, and I was truly exhausted, having done little actual observing. I don’t, however, blame the telescope for most of this. A large part of the problem was the abysmal weather.

This telescope is heavy. It takes three trips to move its pieces: the mount & tripod, the counterweights, and the optical tube.

Having already been fairly familiar with how to set up an equatorial mount, I spent about 5-10 minutes doing the basic setup, getting the telescope polar-aligned. But I kept running into a problem. The finderscope refused to stay aligned. More on that later. When I eventually did get it aligned and got Polaris in the center, with the declination on the mount set to 90, the mount did not quite track around Polaris. I tried a lot of things to get it to work, all to no avail. I think the problem is that the mount was not level, but I’m not sure if the geometry works out.

Another issue from the comfort standpoint is the eyepiece position. It is possible in some cases to get a comfortable eyepiece position sitting down, other times it’s possible to get a good position both sitting down & standing up. But much of the time the eyepiece was placed too high to observe sitting down in a normal chair. I think I’ll want to get a special observing chair at some point.

For the most part the motions of the mount were perfect. Despite the weird axis compared to altaz, I rarely found it difficult to simply point where I wanted, and the slow motion knobs were routinely helpful in subtle movements which otherwise would have had a small amount of slop/backlash and vibration.

The most frustrating thing to deal with was the finderscope. I’ve dealt with this kind of finder mount before on many occasions, but it was somehow worse here than ever before. It’s the type where the finderscope fits into a ring that has two screws and a tension spring. The finderscope’s screws occasionally wouldn’t have enough travel to align with the view in the eyepiece, and fixing it manually and then fine-tuning with the screws was impossible. I only ever managed to get it “good enough” (in the field of view of the eyepiece but not centered.)

The 6x30 finder is kinda dim, but usable. Being optically identical to the one in my dob, I knew what to expect, but having to stand and bend down to peer through the finder made it much less comfortable to use.

Weather:

Partly cloudy. Perhaps 50% of the sky was cloudy. The southern sky was very dim, covered in haze. Antares was extincted from 1st magnitude to perhaps 2nd. Jupiter and Saturn were also very dim and surrounded by a hazy halo. It was hot and muggy and I was absolutely drenched in sweat. I could not see Sagittarius at all--despite no visible clouds in the area. The sky was very bright, much brighter than a few nights ago when I observed the Sagitarius star cloud and some other nice objects in beautiful detail--not this time.

Clouds often obscured or partially obscured constellations I was looking for.

Objects:

Coat Hanger does not quite fit in the full field of view with the 25mm Plossl.

Lots of nice starry regions in Cygnus and Aquila. Nice to scan around in, even with a bright background.

Messier 27 (The Dumbbell Nebula) was quite noticeable. If it had a better surface brightness than the dob, it was hard to tell--because of the low transparency in the sky and the fact I didn’t take my dob out for a direct comparison.

Messier 13 (globular cluster) was a bright ball with no granularity. With the 6mm eyepiece I could just barely make out granularity but it wasn’t very bright. Bad transparency + 6mm is too high power. (For comparison, most of the time I've seen M13 with my 6" f/8 Dob it has been a spectacular sparkly ball)

Saturn. 25mm EP, I could just barely split the rings. 6mm EP, I could easily split the rings and make out the Cassini division, and I saw Titan. Still a bit uncomfortably small.

Jupiter. 25mm EP shows the moons and just barely shows the banding. 6mm EP shows a small but detailed view of the cloud bands. Northern eq band was noticeably distorted, wobbly. All moons visible too. 6mm plus cheap singlet barlow: view was too dim to really see due to bad sky transparency.

How to solve problems:

• Polar alignment scope and/or finding a more level ground. Or spending more time leveling the tripod legs?

• Telrad reflex finder to replace/supplement the finderscope--use 32mm plossl for fine-finding.

• Get an observing chair or deal with the fact that I’ll just have to stand most of the time with this scope.

• Use it on a night with good, clear weather--at least a week of waiting! 

So far it’s a mixed bag. But I expect that the more I use the telescope, the more I’ll get used to setup and operation, and the less frustrating it will be. The smoothness of the mount makes all the difference in the world compared to the last GEM-newt I had (AstroMaster 114EQ)

 

Front of the sun filter cap.

 

Sun Observing - WARNING! SERIOUS RISK OF INJURY AND BLINDNESS! DO YOUR RESEARCH AND PROCEED AT YOUR OWN RISK.

The Omni XLT 150’s dust cap includes an aperture stop intended for lunar observing. Instead, I taped in the back of the dust cap a sun filter card that I had left over from August 2017. The view is kind of dirty due to the filter card not being very clean, and for some reason ther 15mm Goldline eyepiece shows dirt and spots much worse than the 25mm and 32mm Plossls. I was able to observe a sunspot--with the 10mm Kellner at first, then on two different days with the 25mm and 32mm. It works as a stop-gap solution until I can acquire a proper full-aperture solar filter for it and/or the dob. It is presently the only solar observing solution I have.

Solar Observations Log.

It ought to be safe--it’s taped in there real sturdy so that it won’t pop out, and I tested how it holds even after the sun heats it up. It’s ridiculously easy to find the Sun without a finder (the finderscope must be removed for safety), you simply line up the shadow of the front tube ring onto the back tube ring, and you’re easily close enough to see it with the 32mm or 25mm eyepiece.

 

Second Observing Night

The second observing night was a little better, and I saw quite a few things. It’s a lot of fun to scan through the milky way with this telescope.

I had set up the telescope on more even ground this time, and it was easy to get a pretty darn close polar alignment this time. I didn’t fret getting it exact, because for visual use it’s not that necessary.

My 32mm eyepiece had arrived and I was able to use it with the telescope. The views were fairly nice and bright, but the weather was never great, and I always felt a little underwhelmed. 

Milky mirror.

 When I took  a picture with flash on I realized why: the telescope was white with dust and/or dew. I’m not entirely sure what happened or how dew formed on the primary of a newt. I had initially thought it was just caked in dust that I’d somehow never noticed before, but having taken it out the next day, it looked a lot more transparent/reflective than before, but still with a kind of glittery look to it.

I really hope the telescope hasn’t been improperly cleaned by a previous owner, as that would mean the telescope is not a valid instrument for a review. I think what had happened is that the telescope’s mirror is dusty, and that rougher texture allowed for dew to more readily fog up the mirror. I intend to carefully clean it myself.

The telescope works fine for planets. In principle it’s capable of the same maximum magnification as the f/8 dob, but I’d need a higher power eyepiece to unlock that capability. Still, I was easily able to see the cloud banding on Jupiter, including some interesting white blobs in the North Equatorial Band, and the pinprick black shadow of a transiting satellite.

 

Another Bad Observing Night

I took the scope out on another occasion and I looked at Jupiter & Saturn. They seemed… fuzzy and indistinct, again just due to bad hazy weather, which turned out to be smoke. I confirmed that the mirror lacked transparency and appeared very dusty even before it had a chance to dew up, I watched another transit of Jupiter for a bit, and packed it up. The view was not better in the dob, and I went home not long after.

The counterweights don’t seem to like to stay in place sometimes. Really need to make sure their locking screw is tightened absolutely as much as possible.

 

The dusty mirror.

Cleaning the Mirror

 

Yesterday (September 23rd) I finally got around to getting a jug of distilled water (I didn't dare clean it with well water alone) and taking the telescope apart.

The mirror was noticeably filthy (though I needed to use flash to get the dust to show up on camera well). However, if it were just the specs of dust and strings of fur here alone, it probably wouldn't have been that big of a deal.

It took a while before I paid attention to it closely enough to tell, but there was this sparkly granularity and lack of contrast, mostly visible when looking at my black hair thru the mirror--seeing it as a dull dark gray. Still, when I compared my pics of the mirror alone to the milky mirror, I was much less sure the drop in contrast came from dust instead of dew. My hypothesis, that the dew problem was exaggerated by the dust, stood. And I figured the only way to be sure would be to give it a clean anyway, even though the conventional wisdom is that if it's just a bit of dust, its image-forming properties surprisingly resilient. (See the research telescope with a bullet hole in it at McDonald Observatory)

 

I should stress strongly that this is NOT advice, it's a record of what I did. I was more or less following the instructions in this article.

I put the telescope on a folded towel in the clean sink. So anyway I started blasting. I tried to keep the water at a middle warm temperature but the temperature control on the sink is like four temperatures: HOT! hot. cool. COLD! and no true middle warm. After a minute, I inspected the mirror up close. There was no more dust, and I could now see the sparkly granularity better than before. I put a bit of detergent on the mirror and washed it very lightly with clean hands. I didn't have sterile cotton and I figured i'd be able to feel any sleeks forming better than if I used a cloth. That cleaned it up a little better. After washing it some more for quite some time, it seemed that the sparkle wasn't really leaving. It seemed to be gone in regions where the water pooled up, so I figured it might be a different sparkle originating from droplet spray of hard water. So I switched to detergent, spray clean with tap water, rinse with the distilled water. I did this a few times.

While setting it out to dry, I was worried about large droplets sticking to the mirror and leaving residue. So I at first tried using a piece of toilet paper to dab up (without scrubbing, just letting capillary action do its thing). That left paper lint, so I rinsed it off again and tried with a piece of toilet paper. I used a blowdryer to get rid of the rest of the spots of water.

Should I have waited for them to air dry? Maybe? Maybe I just did this because I'm impatient.

 

The final result wasn't perfect. There seemed to be some residue left over, and the sparkling, while reduced, was still visible in parts of the mirror. But the actual contrast looked a fair bit better.

The AFTER image shows the speckling almost better than the BEFORE picture did. No I'm not putting my finger on the mirror.

 

I didn't recollimate the telescope after putting it back together, nor did I clean the secondary mirror (which seemed to have a bit of dust but none of these specks)

 

I'm not 100% sure it's much better than when I started. But I think the only way to tell will be to actually give a go at using it. My current plan is to wait for a very nice night, ideally one where the humidity is low, to pit the Omni XLT 150 and the Apertura DT6 against one another at similar magnifications (And to see how the very low power--both using the 32mm eyepiece--compare, whether image brightness and field of view is a good tradeoff compared to magnification)

 

The DT6 (6" f/8) can't quite put the entire Pleiades into the field of view all at once, with the 32mm Plossl. It can fit in all seven sisters, but not the surrounding cluster. The XLT 150 should easily frame the entire cluster with the 32mm eyepiece. I'm also curious if features in the Andromeda Galaxy, and the fainter companion M110, become easier or if they become harder with the lower power.

 

I also should see, especially on a humid night, how the mirror changes in a flash camera shot from the beginning of the night to when 'm ready to pack it up, to see if dew is really a problem and if it is soley responsible for the murky views.

 

The Plan

 

I am getting close to being able to afford an 8” or 10” dobsonian, which would be an upgrade to my 6” dob. I’d really like to get a 10”, but I’d need to make some money before I can afford that. My original plan was to refurbish this scope and sell it, but that would leave me with a 6” dob and a 10” dob, which seems more redundant than a 6” equatorial mounted scope and a 10” dob.

So my plan now is to compare the views I get in the XLT150 with the DT6 dob. The XLT150’s reflective coatings should be slightly superior to the dob’s, but I doubt this will be noticeable. The two are also diffraction limited mirrors, so unless the seeing is spectacular there will be no difference in planetary performance for a given magnification. But that’s assuming the cleaning was successful I can't sell both because, with telescope shipping the way it is now during the pandemic, I might be waiting for months without a primary instrument.

If the Omni XLT150 performs well, I’ll keep it and sell the 6” dob so I can buy a 10” one. If I end up using the 10” dob so much more often than the XLT150, I’ll either sell it then or donate it to the Cline Observatory.

The Pleiades doesn’t fit in the field of view of the f=1200 dob with a 1.25” diameter focuser. But a 10” dob will have a 2” focuser, and that 2” focuser will accept a wide-field eyepiece which can fit the Pleiades.

However, it may be that the Omni XLT150 can do some basic long exposure astrophotography when fitted with tracking motors, and that might be worth a try if I can afford the motors and a polar alignment scope.

When I've made my decision--keep the Omni XLT 150 or sell it--and if I decide to keep it, I'll give it a nickname and animate pronouns.

Stay tuned to hear more about this project when I actually get a chance to continue it. (Reliant on good weather and, frankly, my own energy) 

 

Clear skies.