I had an impulse buy of a Celestron Powerseeker 127 which is a 5-inch newt on an equ mount for less than $200. I remembered that celestron seemed a respected brand from 25-odd years ago as a teenager when I had a cheap 60mm refractor and read a lot about astronomy.
Despite my best attempts at collimation it is out - images are less clear/focussed when the entire primary mirror is used than when just a section of the primary is used. I have discovered that there is an *extra lens* in the focuser which seems to halve the focal length. I want to remove this lens on the presumption that collimation will be much easier when only two elements need to be collimated compared with three.
The primary is a 127mm/5" F8 so FL=1000mm. I have bought a 1m long PVC pipe from bunnings which fits both end mirror-holder 'brackets' nicely and am planning on reconstructing the OTA by replacing the existing short alum tube with the longer PVC pipe.
Thats where I am now - so I must plan the job ahead:
1) I can figure out via simple maths how long to cut the pipe and where to cut out the focuser hole. I am supposing that the focal point should be in the middle of the focussing 'travel'?
2) The secondary will be slightly oversized when it is placed about twice as far from primary mirror. I don't see this as a problem.
3) The inside of the tube should be painted matt-black or else flocking(?) glued on. Is this ultra-important or can I get the OTA 'working' first? What's the practical effect if it isn't done?
4) As I'm on a tightish budget as a mortgaged family man, I'm planning on knocking out the lens from the existing focuser (and maybe buying a better focuser later)
Is there anything I’m missing here or should it be fairly straightforward?
The problem you will face is that the primary mirror is probably a spherical mirror and the extra lens corrects for its abberations. With it removed, it may not perform well at all
Oh dear, yet another victim of the cheap Jones-Bird telescope.
Im sorry to have to give you the bad news, but theres not much that can be done to improve this scope, other than to align the optics as well as possible with the lens removed, then putting it back in situ.
The lens acts as a barlow which actually doubles the focal length. The primary mirror is a short focal length spherical type, instead of a paraboloid, and the focal length extending effect of the lens is meant to compensate to some degree for this shortcoming. In short, you cant remove the lens and do anything useful with the scope.
If you use this sites search function on the word "Jones" or "Bird", you will read many tales of woe attributed to this cheap scope design.
HI , Maurice Gavin ( in the UK) has modded a very similar Jones Bird into a Lensesless Schmidt for astro -imaging . Have a look here at what he did and several images with the kit :-http://www.astroman.fsnet.co.uk/lscmg.htm . He is using it with the new Starlight CCD Guide camera.
