Jason 335 Comet Chaser 76mm F 7.9


I have one of these little tabletop newts, cute as a speckled puppy. It was a gift from some friends down the road, they are aware of my astronomy problem, and they had this little scope that wasn’t being used, so they donated it to my telescope addiction. Now the thing is, I have been trying to evaluate this little scope recently, partly because it looks kinda fun, and partly because I was thinking of offering it up for sale (too many toys, not enough time to play with them all). Anyway, I have done multiple internet searches looking for specs, and assembly instructions, and not finding anything that answered any questions I have, (common problem for me) I decided to post my ordeal here, just in case someone picks one of these up at a yard sale and has questions, maybe this will show up in a search engine…

Note: They do not come with the eyepiece pictured. This scope had some sort of accesory in the box that affixes to the focuser on one end and the other end just happens to be the exact thread on my 1.25″ plossl eyepieces. You have to remove the barrel from the eyepiece to reveal these threads. The plossls are a huge advantage over the original eyepieces and there is enough focuser movement to bring them to focus.

I took this thing apart, cleaned up the optics, and put it back together a while back, but never got around to trying it out till just recently. I took it out a couple nights ago, with a gibbous moon above, that was my first target. The image was ok, a little soft, but no doubt it was magnifying the moon fairly well. Saturn was tiny, but identifiable as a planetary orb surrounded by the ring structure, no ring detail visible. With this aperture, and magnification I wasn’t expecting much, but the image just wasn’t sharp as I thought it should be, it was a little fuzzy. The image issues had me concerned so I took the scope apart to investigate.

A quick rundown on this scope, and others like it. They are Newtonian telescopes with a plate glass covering (imitating but not functioning as a Schmidt Cassegrain) This glass plate is really just a holder for the secondary mirror, and an effective dust cover. This particular model sports a 76mm mirror with a 600mm focal length. The primary mirror is the light gathering mirror, the secondary mirror reflects this gathered light back up through the focuser tube, so the eyepiece can magnify the image. Also worth mentioning there is a barlow lens in the focuser tube on these little rascals, usually a design you want to avoid, but hey did I mention how cute it is?

The primary mirror in these scopes is fixed, no adjustments here. The secondary mirror has collimation screws, but I didn’t know if they really worked like they should, or if they were just for show. Anyway I looked down the focuser tube (with no eyepiece in place) and noticed the secondary mirror was twisted around such that 20-25% of the primary mirror reflection wasn’t even visible. Problem identified. If you can’t see a round image reflection of the primary mirror, looking down the focuser tube, the secondary mirror is misaligned and you are not getting full delivery of the light gathered by the primary. It is effectively reducing aperture, and causing a misalignment that will cause a fuzzy image at the eyepiece. So I had to disassemble the front end of the scope,  by slowly taking apart the secondary assembly. I discovered in the process, that indeed the collimation screws do function as they should.

Note: the next 3 pics will be known as pics 1, 2, and 3.

The secondary assembly (pic 1, right smack in the middle of the tube) consists of  the main body that has the collimation screws on the outer side and a retaining ring with a set screw on the backside that tightens up  to hold it in place against the glass plate (pic 1 and 2, front/back side view).  This mirror anchor so to speak is held in place by the aforementioned retaining ring. There is a  phillips head screw that screws in from the front side of the main body, which holds the secondary mirror adjusting mechanism in place (pic 1).  The secondary mirror itself has a small phillips screw to hold it on to the collimation adjustment mechanism, the mirror is housed in a plastic mount that is removable by that small phillips screw (pic 3).  I hope that all made sense. Try looking at the pics for clarity.

To rotate this entire secondary body as a unit, I backed out the tiny set screw in the retaining ring, (pic 2 underneath the secondary mirror close to the glass plate is the small standard screw head) but could not get a grip on the ring from the back to loosen it, or turn the body from the front. It was pretty snug.  So I backed out the collimation screws (allen heads, front side, pic one, the 3 screws surrounding the phillips head) so that I could take a flat edge and use the screws as leverage with one hand, and using the other hand to hold the retaining ring as I turned the flat edge against the protruding collimation screws so it could pop loose. That worked. Leaving it still pretty snug, but loose enough to turn, I put the whole unit, secondary, glass plate and metal frame over the end of the scope tube. Then aligned the secondary by eye,  slowly turning the secondary assembly with my straight edge leveraged against the collimation screws, while looking down the focuser tube, until a circular view of the primary mirror was visible and centered.  (I don’t have or even want to look for specific tools for this thing, and I have a pretty good eye, this ought to be good enough)  Then I removed the entire assembly from the tube and snugged down the retaining ring as best I could, and tightened up the set screw.

Note: to do this as described, this is basically a rehash of the last paragraph, maybe this one will make more sense. You have to place the entire front assembly (the secondary mirror and the glass plate are both attached to the round metal frame that is held to the tube by 3 small screws) over the end of the tube.  Then line up the the screw holes, where they attach to the tube (see close to bottom of pic 2 below for one such hole). Then rotate the secondary mirror with the flat edge so the primary mirror reflection is perfectly round and centered in the focuser tube. Make sure that outer metal edge didn’t rotate away from the screw points on the tube. Once your alignment is good, then pop the entire assembly back out, and tighten up the retaining ring to snug  the secondary back in place. Take care not to let the secondary rotate when you do this or you will have to align the mirror again.

Now after all of this, the collimation screws were completely out of whack and still protruding out into space. So I slowly screwed them back in and snugged them up by eye, getting it as straight as possible (see pic 2 above, note the equal distance between the bottom of the secondary mirror holder and the retaining ring) before attaching it all back to the tube. I have a laser that is made for collimating telescopes, but it is useless in this situation, this small scope has .965 eyepieces and modern eyepieces and tools are 1.25″ or 2.00″  Lasers are mostly good for just getting “pretty darn close” anyway, the best way to collimate is with a star. So I waited for nightfall.

First look was the moon. Not bad, but off. So I picked out Vega* (bright star in the constellation Lyra) and defocused the image in the eyepiece until it resembled a bullseye with a dark spot in the center. The bullseye rings (only a couple of them with this scope and this magnification, with the supplied 20mm .965 eyepiece) were a little lopsided. It was a matter of trial and error here slowly adjusting the collimation screws until the bullseye was as close to uniform as possible. The result? Well, that fuzzy Saturn is sharp now. Stars are pinpoints, and this little scope is ready for some fun.

For serious telescope nuts, the intrafocal and extrafocal bullseyes varied by a large amount. So I just adjusted this thing by essentially splitting the differences. The end result is darn well good enough for this thing. It is not exactly a piece of optical perfection to start with, but the images it produces now are decent considering its aperture and design limitations.

The biggest drawback to this scope now is the finder. It is absolute junk. If I could rig up a better finder I might actually use this thing more than I do. Having an 80mm apo refractor on an easy to move mount, makes it all too simple to grab it, instead of this little guy when I’m out for a quick look. But when I am feeling more whimsical than serious about observing, the Jason gets the nod.

If I had a tripod, with an alt az head, and a decent finder scope on this thing, it might make a good little rig for kids. My kids are already quite proficient with my other telescopes though, so it’s really a toy for me… If you happen to run across one of these little rascals at a yard sale or on Craigslist, and if you can get it cheap enough, they are fun to play with, just don’t expect premium optics or a usable finder scope. The build quality isn’t terribly bad, and the tabletop mount takes some getting used to, but the worst thing about this scope design is the barlow in the light path, and of course the finder scope. I could maybe forget the barlow if the finder scope was usable.

Newtonian: or Newt for short: A telescope design using a primary mirror at the back, a secondary mirror up at the front. Usually housed in a solid tube.

Schmidt Cassegrain: is a telescope design using a refractive corrector at the front of the tube, and a primary/secondary mirror as in a Newtonian design.

Collimation: is the aligning of telescope optics, so that they perform as they should and at their best. There are many articles on collimation on the web, Google is your friend.

Aperture: is the amount of light gathering possible, by measuring the primary mirror in reflecting telescopes, or the objective in a refracting telescope. More is better.

Barlow: is a magnifying lens that increases magnification.

* Vega, usually when collimating a telescope you want to use Polaris, the north star. This star remains stationary in the sky, and is by far easier to collimate with. That said, the finder scope on these little scopes is barely usable, Vega is nice and bright, easy to find, and positioned so I could see it from the front porch, which required no extra hassles moving around and in situations like this I am a lazy S.O.B.

9 thoughts on “Jason 335 Comet Chaser 76mm F 7.9

  1. ** Copied and pasted conversation with Fred Garvey on the “About Me” page. It is relevant to this article and has technical details on this particular telescope as well as some interesting mods done by Mr. Garvey.

    Fred Garvey
    June 22, 2020
    8:30 pm

    Excellent article on the Jason Comet 600 Model 335 Newtonian reflector telescope. While not my best telescope, I did pick it up for $ 10.00 at a yard sale.

    A scope like this was probably meant to be used as a rich field reflector at low magnifications and a wide field of view. It was never meant for planetary observing or high power views. Overall, it’s just a small scope with decent optics, but 3-inch reflector telescopes seem to suffer from just being too small in practice.

    As with this, as well as my other reflector telescope, the actual primary mirror size is not quite as advertised:

    2.91″ primary mirror = 6.65 square inches
    – 1.19″ secondary mirror = 1.11 square inches
    = 5.54″ primary mirror actual/usable square inches

    I’ve found that inexpensive/budget telescope primary mirrors, with the included set of lenses top out at about 15x per square inch. So in practice, this 2.91-inch reflector works, at best, up to 83x. But it is rare indeed that instruments this size would benefit at all from two times that telescope magnification, as such, the OEM 2x Barlow lens had to go.

    I truly believe an engineer who initially designed the Jason Comet 600 Model 335 Newtonian reflector telescope did not include a 2x Barlow lens, but I suspect the marketing people took over, added the 2x Barlow lens, and targeted the beginner, who is usually easily wowed by magnification.

    So… I sawed off 1/4″ from the cast aluminum rack-and-pinion lens housing, and removed the 2x Barlow lens and sawed off the bottom end of the rack-and-pinion tube flush with the end of the rack-and-pinion track. I then carefully counter-sunk and finished my modification to give it that clean, machine shop look.

    Without the 2x Barlow lens, the focal length is now shortened by about 15mm, so I wrapped a couple turns of black plastic tape around the chromed part of the 20mm Kellner lens and inserted it directly into the rack-and-pinion tube for a snug fit, followed by tightly wrapping black plastic tape around the lens and the rack-and-pinion tube for an overall sound joint. A simple if not elegant solution, but clean looking.

    So, at 30x magnification the focus and clarity is perfect, and the modification prevents the next owner from frustrating himself, wasting their time with higher magnifications with this particular instrument.

    Just saying.


    • shelldigger
      June 23, 2020
      12:37 am

      Well hello Fred!

      Yes once these little rascals are cleared of optical issues, mine as you know had a rotated secondary, they do a fine job at low mags. They will never excell at planetary observations. Too many obstacles to overcome to get there with the Bird Jones design, the wobbles of the mount, and the completely inadequate finder. Not to mention the relatively small aperture.

      I know better designs can do some planetary observations, my ED 80 for example has no problem on a good night hitting a sharp 200x on planets. But we are talking about a pretty decent piece of glass vs. a novice instrument.

      The Jason for all of its quibbles, does have limited potential for low power wide field views, and it can do that if one has the patience and determination to make a go of it.

      It was good to hear from you. Nice to know another astro enthusiast has one of these little scopes to play with 🙂

      And your mods are intriguing. I left mine stock as it does work as is, and the ability to use plossls with it is kinda neat.

      One thing for sure though, no matter how they function, they will always look cool! I love the looks of them 🙂


  2. Fred Garvey
    June 23, 2020
    4:35 am

    Ever the tinkerer, I further modified my Jason Comet 600 Model 335 telescope again, right after posting my comment.

    Without the 2x Barlow lens, the focal length is now shortened by about 15mm,to a true 335mm focal length, so I carefully measured, marked, and removed the primary mirror and holder, and accurately cut off 24mm from the end of the telescope tube, re-drilled 3 holes, and mounted the primary mirror and holder back on the end of the tube. I also removed the black tape from the 20mm Kellner lens and put back the original 2-screw lens holder. A simple if not elegant solution, and clean looking, except for part of the Jason logo covered by the re-positioned primary mirror holder.

    With the OEM 2x Barlow lens removed, this reflector telescope now enjoys a true 335mm focal length, f/4.5 aperture. I’ll laser collimate this telescope next, heeding your advice on alignment.

    So now the OEM 20mm Kellner lens provides a modest 17x magnification, but the focus and clarity is perfect, and this simple primary mirror modification allows focusing from 20 feet to infinity, and this telescope is ready to accept other lenses for greater magnification and detail… and without that darn 2x Barlow lens in the way.


  3. shelldigger
    June 23, 2020
    2:34 pm

    Well done!

    It takes a true tinkerer to chop a scope tube. I don’t have that much tinkerer in me, yet. The day could come though.

    The collimation turned out well then?


  4. Fred Garvey
    June 23, 2020

    In reply to shelldigger.

    Actually, if one is patient and practices good craftsmanship and double-checks their math and measurements, shortening a telescope tube is just as clean and accurate as it’s done by the original manufacturer.

    I used my bench shears to carefully trim out and double-checked a 24mm wide strip of paper, wrapped it around the telescope tube and taped it, end-to-end, but allowing it to slide freely up and down the tube. The front end (glass + secondary mirror) was left in place to provide strength and rigidity to the tube during the cutting process. Next, standing the telescope tube on a flat, absolutely smooth tabletop (mine was a smooth formica surface) and I slid the paper loop down to the table surface, and marked a black line around the tube with a fine point Sharpie, exactly 24mm from the end.

    I then stuffed some crumpled newspaper into the telescope tube to protect the secondary mirror and rack-and-pinion mechanism from cutting grit and metal shavings. Mounting a #540 Dremel Cut-Off Wheel (1.25″ diameter x 3/32″ thick) to my Dremel (25,000 rpm), I held the telescope tube in my lap and carefully cut (actually a high-speed grinding process) right along at the edge of the thin black line until the 24mm wide piece of tube fell through. You’ll wear down almost two of these grinding disks in the process. I then dressed the metal burrs off the cut end with a large, fine file until clean, carefully rotating the tube during the filing so as to create a near-perfect 90-gegree cut. Note: These Dremel Cut-Off Wheel are quick and effective. I accurately cut out a large hole in the metal roof of my electric pickup truck to install a removable sunroof with these wicked little cutting disks.

    Placing the telescope tube on its newly cut end down, on the tabletop, I carefully rotated the tube and checked it all around with a 90-degree angle gauge. If the cut was somehow off, I would have lightly dressed the uneven (the slightly longer length) again with the fine file, until the cut was 90-degrees true all around.

    I then removed the crumpled newspaper from the tube, carefully cleaned the inside, and then pressed the primary mirror-holder back into place. Standing the telescope vertically on the tabletop, with the primary mirror-holder on the bottom, I carefully drilled three 3/32″ holes into the tube, through and in alignment with the primary mirror-holder. The screw threads in the primary mirror-holder were not damaged by the slightly undersize drill. I then screwed the three screws that hold the black mirror holder in place. I did align the primary mirror-holder to its original 3-hole position, just in case the manufacturer had made off-center corrections.

    Visually, both mirrors seemed aligned, and a field test out my front door went quite well, with excellent sharpness and clarity. I haven’t used my laser collimator on this particular telescope yet, but the secondary mirror check and adjustment should be quite routine. But adjustments, if needed, for the primary mirror. I’ll probably start with 120-degree rotations of the mirror first. If more is needed, I’ll just continue to laser collimate with the telescope standing up on a tabletop, with the primary mirror-holder on the bottom, with all three screws out. That way I can take laser measurements and fairly quickly open the primary mirror and add shims, etc. until everything is in alignment.

    I swapped out the OEM 0.965″ 20mm Kellner lens (17x magnification) with my 1.25″ 12.5mm Plossl lens (27x magnification) and my 1.25″ 6mm Plossl lens (56x magnification) and again the image was sharp and clear.

    And I forgot to mention that I removed the OEM finder scope early on and replaced it with a far more functional illuminated red dot finder.

    I’ve used your star collimation, years ago, but the quick and easy laser device gets it there, at least to my not-so-exacting standards anyway. But, who knows, I might give it a try again, as maybe the sight of an off-center star may irritate me enough to fine-tune the telescope alignment to your standards.


    • Ha! My standards aren’t that great. I just learned how to collimate at high powers using a star. Mostly because I’m too cheap to go out and spend a bunch of money on collimation tools. 🙂

      Stars are cheap, always available, and all you have to know really is how to tweak the collimation to make as perfect a bullseye you can get with a defocused eyepiece. Then swap eyepieces to a higher power and do it again.

      It does take some practice. I had to collimate my 8″ SCT when I let it accidentally tip over while extending a tripod leg. Stupid, stupid, stupid! I was horrified when it tilted over and hit the dirt with a thud. Fortunately the finderscope broke the fall, and took the hit, it busted up pretty good. The SCT corrector was fine, as well as the primary mirror, but the fall did knock it out of collimation. Well I had no choice then but to learn how the hard way. Turns out it isn’t too difficult, it just takes doing it to get it.

      I am very reluctant to chop a scope tube, I don’t know why really, other than I haven’t had the need or the urge to do so. When that time comes though, I’ll have no problem making the attempt. When I do I might look you up for some tips 🙂

      Good to know the little Jason came out of it with sharp stars. Clear skies my friend.


  5. You have more patience than I do! But you’re making me want to go get my 11″ Celestron schmidt cassegrain back from my son. Alas, light pollution here has gotten so bad here over the last few years that it’s hard to tell if the skies are cloudy or if they’re clear 😦


    • Yeah, I can relate. I used to live in the country. Easily Bortle 4 out my front door. Now with fradycats moving in all over the damn place, leaving their porch lights on, or those damnable “insecurity lights” on all night long. It’s just a damn disaster.

      The skyglow isn’t bad here, but people afraid of the damn dark are pathetic. 😉

      I can load my 12.5″ dob up in my truck in a few minutes, and go find a back country road with little traffic and enjoy an evening. Out my front door, not so much.

      I have an 8″ SCT on an EQ too. But the big dob is just so damn easy to setup. With the aid of a hand truck.

      Liked by 1 person

      • When I first got the Celestron (CPC-1100) conditions here were decent for being in a small town, but then they started installing new street lights, including one right in front of my house and another behind it, that pretty much wiped out everything. I love that scope but moving it is a major operation. It weighs about 70 lbs and it’s huge, plus the massive tripd. Then it needs a good sized battery pack to run it. So it’s at my son’s place now because he can get out to dark sites more often than I can. Love that scope, tho. It’s the first computerized one I had. I’m horrible at trying to find astronomical objects. Before that trying to find something like the ring nebula was an exercise in frustration, and if I did manage to find it by the time I got the scope focused it was out of view and I’d have to hunt for it again. With the Celestron just tell it what I wanted to see and it would swing itself around and there it was, centered in the field of view.


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