Collimating Newt short tube Advise

[rodroger]

Hi,
Can any one advise me on a problem I have with the collimation of a 6"/f9 (150mmx1400mm) Newtonian.
It is a short tube and uses a corrector Lens (Glass) at the bottom of the focus tube to achieve the focal length.
I have tried to collimate this quite a few times now but still having problems.
I used a Celestron Cheshire/sight tube to allign the soundary, and a Saxon Laser as well, The only way I could align the main mirror was by removing the corrector lens (Jim Fly recommended to collimate without it in place). Every thing looks good but when tested on a star, I still do not get any leeway on the focus using a 25mm eyepiece. The image is either triangle in shape or ghosted - see two objects instead of one. (Mirror isd not pinched or not any more, as this was checked and fixed.) I noticed that if I rack the focus out the star colimation looks good but if I rack the focus in I get a elongated object. I removed the corrector and reversed the lens in the holder and tried again but still have the same problem but in reverse????
I also have noticed if I have it in focus (well what I think is in focus) I see a double of the image but if I avert my eyes sometimes or if I look down through bottom edge of the eyepice the image will come good. ?????

This is really sending me crazy:screwy:, does any body have any suggestions besides getting rid of the telescope.????
Is the corrector lens flawed????

I can get this scope replaced by the maufacturer, as it is still under warranty, So far I have been under the assumtion that the initial problem when I recieved the telescope by courier was the optical train was out of alignment but if it is flawed then it has to go back.
I want to make sure before it goes back that it was not just the alignment or I will be charged a fee if not a warranty issue.
.
I have purchased one of Jim Fly's collimating Kits for a 1.25" and he has also put me on to a supplier of a Autocollimator for a 1.25" focuser which I will be buying at the end of the week
I am going to have so many collimating devices on hand soon that I will become an expert at it.

I just quckly tweeked my sons 6"/f5 and it is brilliant , even with a 2 x barlows and high mag lens (4mm) - amazing:eyepop:. .................Yet this scope is from the same maufacturer as mine (next model down - cheaper price too) except the focal length dose not use a corrector lens

[Glenhuon]

I had those same problems with one of those 'scopes (Just sold it). You are correct that the lens needs to be removed from the focuser but you will also find that the focus tube is very sloppy in the barrel. I had to shim mine with .5 mm brass shim before I could do any good. When you start the alignment bring the focus tube out about 5 or 6 mm from the barrel, I found if I left it all the way in it moved and threw everything out when I tried to focus an eyepeice later.Start with the cheshire and rack the focus tube out untill the end of the tube is out of view. Adjust the secondary mirror until you can see all 3 Primary mirror clips at the edge of the field. With the laser in place adjust the secondary until the dot is dead centre of the main mirror, check to make sure you can still see all 3 mirror clips before continuing. If all is well you can now adjust the primary so the laser dot disappears back up the hole from whence it came. Once adjusted the secondary should not need touching for a long time and tweaking the primary is pretty easy after a bit of practice.
When you have finished screw the lens back in and give it a try. I had a about 3 goes before it was reasonably good.

Best of Luck
Bill

[Blue Skies]

A double image problem usually indicates that the secondary mirror is not in the right place. I'd concentrate on getting that right (although I know all too well what a pig of an experience that can be!) but if you get that right the primary mirror adjustment is a doddle.

[rodroger]

Hi,
Thanks "Blue Skies" & "Glenhuon" for the advice, I will concentrate of the secondary first before doing the primary.

How did you go about shimming the focuser tube Glenhuon? did you place the shimms in between the focus tube and the holder? Or where you shimming the focuser mount screws to centre tube properly?

I have a smaller scope, 4.5" that had a lot of play in the focuser tube but I rectified this by wrapping thread tape around near top and bottom (above and below rack) and placed adesive tape between to each side of the rack and has lasted quite awhile, good quick fix.

[Glenhuon]

You're welcome Rod, I've had a lot of helpfull advice given by the folks on here and it's nice to return the favour.
I took the tube out of the focuser body and glued the shim inside with contact cement. I used brass shim, but a suitable thickness of plastic from a drinks container or similar would do just as well. It was put in in 2 pieces, the length of the focuser body on either side of the rack and clear of the plastic strip opposite. Turn the tube so the rack is uppermost while doing the mods as the plastic strip will fall out.
Not something I did, but it would be a good idea to check if the focuser is square to the tube and shim it if not. Has to improve the image.

Cheers
Bill

[rodroger]

Well I think I realy and truely fixed it this time:( , I removed the secondary to check the squaring of the focus tube and to also check the secondary position in the mount.
As I was holding the scondary up for a closer look the clip and the mirror fell out, the mirror bounced on the carpet and out the front door (I was standing close to the door with it open to let some light in), it landed on its face on the concrete - broke at least about 5-10mm of the top, two huge chips on both edges and numerous scratches, looks like back to the manufacturer - but don't know whether its worth it now:sadeyes: - the Mounts great - Too bad about the ATO, it must be jinxed or has had a curse put on it.

[Glenhuon]

Oh Oh !
You should be able to get a replacement, either from the supplier or one of the Astro Shops. They are not very big mirrors so cost should be minimal.
If all else fails, have a look at SurplusShed in USA (they sell stuff on Ebay sometimes too) they might have something suitable.

Bill

[Jarrod]

sorry to here about that, but if you decide to replace the secondary mirror, then here are some idea's regarding collimation.

I have a telescope much the same as yours, yes they are a pain in the bum to collimate, but it is possible. I don't own any collimating tools, which makes it all the more challenging, but it can be done. I also had the problem of seeing all stars as doubles, but the problem disappeared after I collimated the damn thing. I find that the best way to collimate my 'scope is to get every thing aligned during the day as you would on any other newt, but follow a different procedure at night: I point my telescope at a bright star, and adjust the main mirror. When I move the mirror the right way (after some trial and error), the two images slowly move together to form one. after that, I just follow normal newtonian procedure for all the fine tuning.

I've only ever seen the 'double star' once, after a very long and bumpy car trip. although in your case, bad collimation may not be the cause. I think the best cure for this issue, at least in my opinion, is to get a new telescope (or at least a new OTA, as your happy with your mount), because having to collimate the bugger has spoiled a lot of fine nights.

Jarrod

[rodroger]

Hi,
Looks like the supplier is sourcing me another OTA but maybe a 150mm x 750mm (no corrector to worry about) Even though these scopes are listed at a lower price than the 150mm x 1400mm, I think they are a better scope, after all the corrector is only a glorified barlows to some extent. You get more metal for a lower price (longer tube:)).
I will have to get another counter weight, this is something I have noticed with my sons scope which is a 150mm x 750mm and uses the same mount as mine, it won't balance out with the supplied weight, jury rigged it by tapping a hammer head to the bottom of the existing weight and balances out ok.

If they end up replacing my scope with the same specs then I guess they will make sure at their end that the optics are O.K before sending it out too me (I would hope).
I went to my first AAQ meeting tonight :) and met someone who is willing to have a look at the existing one to see what I'm doing wrong with the collimation. Also went to the Lunar Feild Night tonight :thumbsup: which was really cool, we had resonable skies for a change, at least for a couple of hours, getting to know my way around the Moon (hogging my sons scope I'm sorry to say) for the first time, I have been more a deep sky / double star person.

Thanks so far for everybodys help on this, and I will let you know what eventuates.
If I get to keep the old OTA I will get another scondary for it and have another go at getting it right.

While I have been waiting I have built my own dew heaters and controller, modified version of the one listed under projects here. Added individual switches, also can control a high bright red LED which I have rigged up for a red light. Working in a way to control individal Amps per heater as this will give me total control over the indivdual Heaters output. I can callibrate with a lab thermometer and a bit of maths:).