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Nice Telescope with a few Mirror Alignment Issues
on January 9, 2014
I got this one as a Christmas gift. It seemed like a good telescope for basic astronomy, with many nice features. But I was initially quite disappointed by the fact that the images seemed rather blurry. It took me two weeks to correct this problem, to the point where now I am finally starting to like it.
I should add that the telescope had probably traveled a few miles around the globe when it got to me, but the box and packaging were in pretty good shape when it arrived.
When I got it, I initially compared it to another $50 reflector, the Celestron 76 mm Discovery, and the results were very disappointing, the smaller one was much sharper. I spent hours reading on the Celestron and other sites on how to adjust collimation with a simple "hole in the cap" and got nowhere. The image was always relatively blurry. Day or night, polaris or no polaris.
I later bought the Celestron 24mm to 8mm zoom eyepiece, which allows me to zoom in without changing eyepiece, and it works very well on the smaller 76mm telescope, but again blurry images on this one.
After quickly becoming an expert on reflector collimation, I noticed that nothing seemed collimated properly. I guess they don't even try at the factory on this one? I decided to order a Celestron 1.25" collimation eyepiece ($30), which can be useful in aligning the optics (the two mirrors) in reflectors like this one. I tried it out on this one, and got repeatedly confused on what should be seen in what reflection when you adjust this or that. I spent entire afternoons fiddling with secondary versus primary mirror adjustments, achieving virtually nothing as far as improved sharpness is concerned. I did replace the secondary mirror alignment screws with better stainless steel ones that would not strip, they are metric m4.
I then tried to collimate this thing on the North Star (Polaris) and that is, for this one, another pure fantasy. The reason is that Polaris is faint, and every time you move a mirror by a tiny bit (as explained in the Celestron instruction) the star just darts out of view in the eyepiece. More frustration and still no luck in getting this thing in focus.
Lastly, I ditched all the Celestron recommendations on day and night collimation (using either the celestron collimation eyepiece or the "ring pattern" for out of focus point sources) and did instead the "EYE-DOCTOR TEST" :
I wanted to see how sharp I can get this one, when there is no wind, no shake, no atmospherics, no moving planet, no mist etc. So I placed the telescope at one end of a long corridor in my house, and a nice clean printed envelope with some sharp text on it at the other end. With this method (which I seemingly invented, as it is not described anywhere in the instructions nor on the Celestron site) I was finally able to adjust (by very small increments) the three screws on the secondary mirror till I FINALLY got a nice sharp picture of the writing on the letter. Note that this last procedure did NOT require the collimating eyepiece! Just the regular 4mm eyepiece that comes with the telescope. Success!
As a by product, I found that in fact in the end all three eyepieces work rather well, down to the 4mm which is a bit faint, the 20mm with the 3X Barlow is better.
Now I can finally see the main two stripes on Jupiter and the Orion Nebula with some clarity. In conclusion:
Plusses : Potentially sharp optics and large aperture. Reasonable price. Sturdy mounts. Useful eyepieces.
Cons: Imo optics needs to be carefully aligned by the method described here. Mine was definitely NOT aligned and, initially, as a result disappointingly blurry.
EDIT: After a few more weeks of use (February 2014), I spent some time using the Celestron collimation 1.25" eyepiece ($28 here on Amazon). My conclusion is that it is a very useful, if not essential, tool for this telescope. To avoid any further issues due to my previous messing around, I first screwed in the secondary mirror (by loosening up the three alignment screws, and pulling in the secondary mirror all the way in until it barely touches the mount), and later pulled out the primary mirror as well(by pulling out all six screws until the whole unit comes out, then reinserting the mirror after making sure the secondary was pointing the right way, straight to the back). Then, using the Celestron collimation eyepiece with its crosshair, I carefully adjusted the secondary and primary orientations (three screws for each mirror) until all the crosshairs overlapped perfectly. In other words, the crosshair in the eyepiece has to overlap perfectly with its reflection through mirrors 1 and 2, and back to the eyepiece. This takes time and patience. After having done that, the image quality seems pretty good and rather sharp. I went down to about 8mm, I don't recommend getting lower than that. The best setup for this one is the 20mm eyepiece, either by itself or with the included 3x Barlow (which then gives 20/3 = ca. 7mm). I also got some Ploessel eyepieces, but they will do you no good if the mirrors aren't aligned first.
PPS. I found (April 2014) that the best way to collimate this (Bird-Jones or catadioptric design)telescope and get nice sharp images is to remove the focusing lens at the bottom of the focusing tube (takes 10 mins), align the secondary and primary mirrors with an inexpensive LASER collimator (mine is an LK1 $30 from seben dot com, takes another 10 mins to do this part), put the corrector lens back in and reinsert the focusing tube (don't touch the lens with your hands, takes around 5 mins). With this method the results are guaranteed to be reproducible and consistent. The images are then consistently sharp.
PPPS. The other day (June 2014)I talked at length to a very nice and helpful person at Celestron technical support (Will?). He suggested to check the following thing. The secondary (smaller, flat) mirror is oval-shape and mounted right under the focusing tube, held in place by three (outside)-plus-one (center) screws. Now put a focusing cap (just an eyepiece cap with a small 1mm hole in the center) at the (top) end of the focusing tube. Then make absolutely sure (after you take again very carefully the correcting lens out of the focusing tube) that the inside of the focusing tube and the secondary mirror, as viewed through the focuser, are perfectly concentric when you view them through the hole in the cap. That is, the secondary mirror has to be perfectly centered when viewed from the top of the focusing tube. Note that the secondary mirror is oval shaped, but will look like a perfect disc when tilted at about 45 degrees. On mine this required several turns on the (secondary) center screw. After this is done, make also sure that the tilt on the secondary mirror is such that you can see the center of the primary mirror (on mine I put a black pen mark at the dead center). Now re-align the secondary and primary mirrors with a laser (in my case), with the cap with a hole, or a cheshire eyepiece. Then put back the correcting lens in the focuser, and you are done. The end result is that on mine it improved the sharpness a bit (I did the eyedoctor test again). I was also able to see more detail on Saturn with a standard 9mm eyepiece, will try taking a few pictures soon.
PPPPS: This telescope really shines (due to the light gathering abilities of it's fairly large mirror) when you want to look at fainter objects. Recently we had good viewing conditions and I had a chance to look the the Great Cluster in Hercules (M13), the Lagoon Nebula (M8), the Omega Nebula (M17), and two more star clusters in the same general region (M4 and M62). I took some fairly nice pictures of these objects with a Sony HX200 camera (30x zoom) mounted piggyback on the telescope, using the Celestron motor drive for the 127EQ and long 30sec exposures at 800ISO. See the pictures I posted on the right. I was surprised how well the telecope mount, equipped with the Celestron $30 clockdrive, works when taking long exposures.
PPPPPS: It's October, seven months after I did the laser collimation, and everything is still fine and exactly the same. That tells me that the collimation on this one only needs to be done once, maybe if it gets out of whack during shipping. After that there's no need - unless you bump it or drop it badly. At least that's my experience.
PPPPPPS: It is end of February 2015 now, and I had some very good views of the great Nebula in Orion M42. The scope is still perfectly collimated since almost a year ago, last time I did the collimation with a laser. Again, the message here is that if you spend the time to collimate it properly and don't bump it after that, it will stay sharp almost forever ... Btw I love the $32 celestron R/A single axis motor drive on these telescope, and in my opinion it is a very worthwhile investment. Objects stay in view for almost an hour w/o adjustments.