I posted a request in early 2007 for help in collimating my Celestron short tube Powerseeker 127 EQ and got information regarding removing the inbuilt Barlow lens from the focuser when collimating. This was a great help but I still want to try to get better results with the scope especially as I now have a Philips 900 webcam that I want to start using.
So far, I have made the following "improvements". (I have been criticised for spending too much time on a cheap telescope but I'd like to make it as good as it can reasonably be and I have done the work over a year of spare time.) All this work and this post is in spite of the fact that I have a GSO 200mm Dob on the way.
1. Replaced all possible mount and scope screws with stronger hex cap-head bolts.
2. Disassembled and re-greased and adjusted everything.
3. Replaced the focuser bearing strips with ice-cream container strips. Much less slop.
4. Installed an ice-cream container washer between tripod and mount.
5. Made a much more useful and rigid accessory tray out of MDF and square aluminium tube with in built spirit levels and clock and will take 4 eyepieces in their protective tubes. I can post a photo if anyone is interested
6. Filled the 9 aluminium tripod leg sections with steel rods and mortar. Much more rigid and damped. (Messed up the the compass I originally had in my new tray though)
7. Filled tripod top bracket with mortar with aluminium tubing between the leg mounting holes.
8. Green laser pointer to replace finder.
9. Replaced the RA and Dec slow motion cables with simple 1 inch dia knobs. They don't get in the way and work well if scope is balanced and mount is greased/adjusted. You have to have this to use an RA motor anyway.
10. The reason for this post
I am thinking of replacing the OTA tube so I can dispense with the inbuilt focuser lens. The result will be an appreciably longer OTA but a simpler, cleaner optical path and much easier collimation as I won't have to disassemble the focuser every time. I have documentation on how to position everything on a tube when building from scratch but I have the following questions. Note that I am planning on using a 1 metre length of 150 mm PVC storm water pipe from Bunnings for the tube which I have found is an absolutely perfect fit for the Celestron tube end pieces (better than the original in fact). The scope has figures of Primary mirror dia 127mm, Focal length 1000 mm (I will check this) and this gives a focal ratio of 7.87.
1. Does anyone know of any problems in using PVC tubing? It is white but I am planning to line the inside with something black.
2. Will my documentation work given that the original scope was designed around using the inbuilt lens? ie are the focuser, primary mirror, etc going to work in this role?
3. Even if the focuser will work would I need to remove the inbuilt lens mount assembly? It effectively reduces the diameter of the focuser tube but I figure I will have a narrower light cone when it gets to the focuser anyway. Leaving it in will give me the option of using the the scope with both tubes perhaps?
4. Any other problems that anyone can foresee?
I have found further information on my collimation problem and a reply on an overseas forum seems to sum up things. It isn't good news for anyone with a short tube 'scope
Re: Will it work?
What you have is a Jones-Bird design reflector telescope. It uses a spherical mirror rather than the more expensive parabolic mirrors used in a regular Newtonian reflector.
The Spherical mirror in combination with a Barlow/Corrector lens inserted into the focuser's draw tube is what gives you the total focal length of the telescope.. You "do not" want to remove that barlow/corrector. If you do, you will have all kind of spherical aberration problems. You wouldn't even want to use that mirror in a longer OTA. It won't work without the correction of that barlow/corrector lens, or some kind of corrector plate on the end of the OTA. Then you would essentially have yourself something similar to a SNT if you did that.
Jones-Birds are a cheap design, made cheap to reduce the cost of manufacturing in order to increase profit margin.. They are notorious for collimation problems.
My roller coaster ride didn't end there though. I then found a Mead 5" 1000mm fl mirror on e-bay yesterday and thought about a full rebuild with that and a long tube. A-Ha, I thought, I can beat it!
I investigated still further but it turns out that Meade uses the Bird-Jones approach as well on a number of problematic models and replacing a spherical mirror with another spherical mirror isn't going to do much. So....it turns out that I can't beat it.
At the moment, the thing is sitting over in the corner mocking me. I bought it as a compact solution for use on holidays in the country and it is fine for looking at the moon but will never give me even a half decent look at the planets by the look of it.
I think they say a man needs to be able to know when he is beaten.
At the moment, the thing is sitting over in the corner mocking me. I bought it as a compact solution for use on holidays in the country and [/COLOR][/COLOR]it is fine for looking at the moon but will never give me even a half decent look at the planets by the look of it.
I think they say a man needs to be able to know when he is beaten.
Steve
It really is a shame that so many people get bitten by these rubbish scopes, scopes that will never perform to a reasonable standard. Some cheap scopes can be improved with a bit of work, esp the mount, but these short tube newtonians are almost universally condemned as a waste of money by anyone who knows what they are talking about.
Yep one needs to know when to cut ones losses and not throw good money at a lemon.
If you are looking for a compact, lightweight, and pleasant to use scope for travelling i can recommend this one with the caveat that you might want to upgrade the focuser.
I used a short tube 114mm reflector from K-mart for over a decade. I saw a lot with this scope including detail on Jupiter, Saturn and Mars. I found both Uranus and Neptune with it and frequently observed nebula (orion, eta and tarantula) and open and globular clusters with it.
I also did prime focus and eyepiece projection of the Moon and did piggyback photography of constellations.
On day I tried removing the built-in barlow/corrector in the hope it would give a cleaner light path. The spherical aberation was so shocking I could barely see a thing.
You can see a lot with these scopes and they are very portable. It's just a shame that that cheap corrector must remain in place
ive got the same scope, and i was also thinking about replacing the tube with something longer to get rid of that barlow (before i knew the problems this would cause). but then i considered the size of the secondary mirror in a longer tube. being so much further above the primary, the secondary would be way too large and would need to be replaced with something smaller to get the most out of the scope.
is there any way of making use of the spherical mirror? is there something i could do with it, maybe not even astronomy related? its useless in this telescope but there would have to be a use for it somewhere else. even if its just for a bit of fun.
I said that I'd gotten to the stage of knowing that I've been beaten but apparently I hadn't. Further investigations uncovered the availability of a 5.2 inch (132 mm) 650 mm parabolic mirror and I have just ordered one from the States. I will also go back to Bunnings and get a length of 150mm storm-water PVC pipe which you may remember is the perfect outside diameter and begin my rebuild. It will be longer and heavier than the original tube but at about 650 mm hopefully not too heavy for my strengthened tripod.
I have also looked into the secondary size issue but I'd prefer not to go to the even further expense of a new secondary and holder. In any case, the new primary will have a focal length of 650mm and my calculations give a suitable secondary size of 36.5 mm. The similarly spec'd Skywatcher SKP13065EQ2 lists a secondary size of 34.5 mm which may or may not be the perfect size but is a similar result. I measure the Powerseeker 127 secondary size at 40 mm inside the beveled edges and given the fact that the primary mirror is slightly bigger (which I figure negates the effect of a slightly large secondary obstruction) and also the fact that I can only find 41mm or 32mm secondaries available on the web anyway I won't worry about trying to get things exactly right.
A second factor in my decision is that I've just got my new GSO 8" Dob and a 10:1 Crayford to go on it so my Powerseeker will be relegated to 2nd / holiday scope status. Somehow the GSO and my design and build of a Equatorial Platform has assumed a higher priority for me.
Jarrod, if you are interested in pursuing an upgrade to your scope, I will be happy to let you know how I go but maybe don't do anything drastic until I know my results as I guess there is the possibility that this may all end in (more) tears.
Concerning your last question, I have no idea what to do with the redundant spherical mirror. If you can come up with something, let me know.
Some people have tried flexing spherical mirrors to mimic a parabolic figure. I have seen articles on this in Sky and Telescope (USA). Do a search (I will do one as well) and see if you can come up with something similar. They use epoxy of something to mount a long bolt onto the back of the mirror (in the middle) and then thread this through a metal back with a central hole bored into it. This piece probably needs to be strong so when you start to tighten the wingnut, the mirror will flex instead of the metal back.
Thanks for this. I will have a look around for some more info on this because I just received my 5 inch 650 mm mirror except when I opened it up they've actually sent a 6 inch ???mm mirror which even if I do some serious cell surgery might fit but my guess is that the focal length is either 750 or 1000 mm which means an even longer, heavier tube than I was planning. I think I may be pushing things considering the mount was too weak even for the original configuration. I'll look into your suggestion but my first thought was OMG because the I thought the first rule was "Make sure you have no stresses on the primary" and I have to think that pulling the centre out will put pressure on the 4 clamp areas so I think I will end up ordering another correct mirror and sell the (now) 2 useless ones I have.
This astronomy thing is getting toooooo stressful. I am so impressed with the views from my new GS-680 Dob but I am having so many dramas getting a reasonably priced EQ5 (or better) mount and tripod for it (plus the problems with this Powerseeker 127) that I'm feeling very jaded. Are all the ups and downs part of the deal because I'm not sure I like it.
My first reflector was one of those short tube thingy's and I had similar problems. Finally just cut my losses and bought better one. Sold the short tube to someone who occasionally uses it for 1/2 what it cost. Don't let the lemons put you off, this is a great hobby
Meridian Telescopes http://www.meridiantelescopes.com/index.htm were fantastic and sent me a correct 5.2 inch f5 mirror very quickly when I explained what happened so as soon it arrived, I got the 150mm PVC pipe from Bunnings and got to work. The mirror was US$59.95 plus US$21.?? postage which came to about AU$93 at the time. USPS shipping only takes 7 days (both times) . The other thing is that they were the only company out of 4 that I tried in the States and Canada who responded to my queries. I can recommend them completely.
I worked out the new tube length I needed by simply sitting the mirror in a low table outside and sort of standing to one side, getting some stars in focus looking through a 20mm eyepiece then measuring the overall height and subtracting and adding the appropriate lengths to get the actual tube length. For example, the front of the tube to the focuser centre line is 60mm and needs to be added (I just measured this from the original tube as it is a fairly non-adjustable distance) and the focuser length from the tube centre line to the originally measured point on the eyepiece was 230mm (80mm + 150mm) which needs to be subtracted.
Anyway, my final result was 535mm which seemed a bit short so I cut it at 550mm as I figured I could cut a bit off if it was too long. I should have trusted my calculations. I'll take 15mm off the back end of the tube soon but with the primary mirror as far in as possible (while still allowing collimation) it works although I'd prefer to shorten it so the focuser tube is further out of the main tube and doesn't obstruct the main tube at all at normal focus settings.
Notes:
- I marked, drilled and carefully tapped all the mounting screw holes in the tube so neither nuts nor longer screws are needed.
- The new primary mirror fitted the mounting cell perfectly. The original 127 mirror is a bit bigger than 127mm and the 5.2 inch (132mm) is a bit smaller than 132mm.
- The new total OTA weight is 6.6 Kg against an original 6.2 Kg so it isn't much heavier although it is appreciably longer (see photos).
- I removed both the focuser internal lens and it's holder. The holder was a bit of work. Remove the rack mounting screw (I forgot) and soak the end of the tube in petrol to soften the glue to make it easier.
- My 'new' scope is appreciably better optically than the original (although I have now been spoilt by my GS-680) and I can actually collimate it! Not only that but I don't have to take the lens out to do so.
- Keep in mind that the focal length is now 650mm (or so) instead of 1000mm.
- It now looks like a "real" telescope (but still isn't so long that I can't take it away on holidays with the family).
- I still have no idea what to do with the original spherical mirror.
- I still have to flock the inside of the tube.
- I don't think I can call it a Celestron Powerseeker 127 any more.
If anyone else wants to do this and has any questions I am happy to help. I had some dramas but it should be fairly easy with the info I've given and I believe is definitely worth the effort. I've gone from being disgusted with it to being very happy with the final result.
While in quite antipodal time zones, I am also here enjoying southern skys from Uruguay.
I was presented with a Celestron Powerseeker 127 by my son some weeks ago, being this my first telescope. For several years now I have been interested in astronomy, following different venues of information from Scientific American to Astronomy to Sky & Telescope magazines, but had not have direct observing experience untill now.
Introduction over, now the bussines of putting this scope to good use.
First even for a novice, the scope out of the box looked grossly out of collimation, comet like stars and Jupiter a featureless disc casting a long light smear to a side.
I begun to work out a homemade laser collimator out of a scrap lenght of brass, but before procuring a laser pointer to put into it, I noticed the primary collimation screws appart from not very practical to work with at night, did not provide good enough range given the very basic stacked rubber rings included as spring force for backward cell movement. I replaced 2 of the collimation screws and rubber rings with 2 new longer screws fitted with knurled knobs, and inserted springs between the mirror cell and back plate.
Improvement was readily apparent, being able to easilly collimate just looking to stars and adjusting the now much more friendly controls while looking through the eyepiece.
Next upgrade was filling the tripod legs with mortar, and building a wooden spreader with a tension screw conveniently attached to the central hole of the eyepiece holder hole, which turned the tripod itself in a much more stable platform, leaving only the EQ mount itself as major contributor to tube vibration.
I can see right now Jupiter's cloud bands and (very pale) red spot, moon details like Huygen's sword stand out obvious, though stars are not pinpoints either.
The provided eyepieces and barlow are next in my list. The 20 mm piece is probably as per some references I found a Kellner, and more or less decent but definitely on the cheap side. The 5 mm piece by the same token is probably a Ramsden, quite poor in resolution, field of view and eye relief. The 3xbarlow works good in my completely unexperienced eyes, but as I learned is quite far also from being a decent part.
I am considering a 5 mm X-Cell series from Celestron and same brand Barlow, and may be I will go also to a 20 mm one. As far as I know any eyepiece investment is sound both in seeing improvement and as a reusable asset if I later upgrade to a new scope.
Last, and the more serious part where I am unsure about the best strategy.
I learned from previous posts in this forum an other resources, the optical design for this particular scope - Jones Bird - corrects a spherical relatively fast mirror with a combination spherical corrector - focal extender (barlow) built in the focuser drawtube end.
I can either work out a solution like Bucky1379 did, scrapping the original mirror, or I may explore a mirror flexing scheme as detailed by Alan Adler in the Nov/2000 issue of Sky&Telescope. This particular scheme calls for both an appropriately dimensioned pusher ring and puller plate which in theory provides a better shaping for a given mirror size and f/ratio.
The rub is, will this work in the Jones Bird design, given that the corrector lens also compensates for spherical aberrations?
So much for a single post, thanks for any advise and good seeing for everyone.
Rodolfo
PS I may post images of the hacks, a camera support for afocal imaging I also built, and some sky images I shoot though I may probably wait for better performance later.
Last edited by ingrast; 17-08-2008 at 02:35 AM.
Reason: Somehow extra space was inadvertently included at the bottom
I bought myself the Celestron Powerseeker 127 EQ for Xmas. Only got it because Dickies sold them off for $178 a few days prior to Xmas !
A hour of reading web review feedback suggested it was value for money, but would need tinkering with. I agree with both now, from my short experience.
The telescope was terribly out of collimation when I set it up, but having read the WWW stuff I knew it was to be expected. I made my own combination collimation tube from plans.
My own experience varies from one of the guys here though in that I can collimate mine correctly, as in I can indeed see the primary mirror edge and it's three mounts though the focuser tube just fine. I don't seem to need to remove anything from the focuser tube ?
Collimation improved the image fantastically :-)
I have already started some improvements too (will follow a similar list to Steve) but am not going to change the mirror.
I have a very old 3" Tasco refractor too. I bought it with saved up pocket money in about 1976. It had three Japanese made 25mm size lenses with it. 20mm, 12.5mm, 6mm. I've machined up (have lathe) an aluminium eye peice adaptor to fit them to the Powerseeker 127. However I'm disappointed to find they are nasty to view with compared to the two lenses supplied with the Powerseeker 127 EQ. Harder to focus, and blury on the edges by comparison.
I read on other forums that the standard supplied Celestron lenses are themselves rated as very poor quality, so the old Tasco ones must be awful !
Thinking of two first up improvements. Red dot rifle type finder or green laser (Ebay), and a 'nicer' lens. Budget is about $100 for both together, with some DIY involved too.
On lenses, what's a better brand than a stock Celestron for around $40-60 ?
(I think I'd be best with about a 6mm on my scope...)
I eventually bought a 9mm GSO plossel eyepeice for the Celestron. While this is a noticably better eyepeice I'm not real happy with the scope now. I've become quite good at collimation, but find that the resolving power of this scope is not any better than my 30 year old Tasco 60mm refractor. In fact looking at the surface of the moon the old refractor looks sharper.
Question, is there any benift to removing the correction lens inside the focuser to collimate the scope, will I get a sharper resulting image even though collimation proccess seems to work ok with it in place ?
I'm late in this post, but because it is a good reference for every person that buy a Celestron 127eq, I want to show my personal solution for the collimation problem of our telescope. The whole reference is in the following url: http://www.taringa.net/posts/ebooks-...barlow%29.html
it is in spanish, but the images are better than words. Basically you only need to get a bottle cap of 40mm of diameter and you need to cut a 5mm in the side and make a hole in the top center of 1.5mm, put the modified cap in the bottom of the focuser, then you will be able to collimate it with a laser tool without the necessity of remove the internal barlow.
Last edited by niuno15; 06-04-2010 at 09:48 AM.
Reason: more information
I'll look into your suggestion but my first thought was OMG because the I thought the first rule was "Make sure you have no stresses on the primary" and I have to think that pulling the centre out will put pressure on the 4 clamp areas so I think I will end up ordering another correct mirror and sell the (now) 2 useless ones I have.
. but maybe theres a 'proper' way of making such a device...?
Complete.
Its a lovely feeling when a plan comes together.
