Lens in focuser

[Chr]

Greetings all,

Just a small question from me. I have a saxon refractor, that is claimed to have a focal length of 1400mm, with a physical tube length of only 550mm. There is however, a small lens screwed into the bottom of the focuser, which from what I can tell, acts as a barlow lens. Yet from my understanding, this wont give an evictive 2x focal length, but will rather just double the magnification of the eye pieces, and also cut light coming in.

I am wondering whether I would be better taking out this lens, and just having a clear focus tube or not. And also, would it be possible that this lens is ruining any sharp focus, which thus far has eluded me?

[iceman]

Hi Chr.

You're right - the lens screwed into the bottom of the focuser is a cheap way of getting a longer focal length scope, while still having a short tube.

These type of scopes are best avoided, as they are generally inferior quality and will simply not deliver the sharp images you want.

I doubt that taking the barlow out will work - the eyepieces will not come to focus. The mirror figure would be designed with having that (usually plastic) barlow in place.

If the scope isn't giving you the images you want, perhaps it's best to cut your losses and get something like an 8" dob for approx $400?

[rmcpb]

I wonder what taking the plastic barlow out and using a good barlow in the usual way would do. Reckon it would be worth a try.

[Chr]

From initial testing in the daylight, it would appear this is correct :(
Alas it also appears that replacing it with a standard barlow doesn't work either.
On the plus side, taking it out did give me a nice red dot with the laser collimator rather than a weird bar shape =D

Cheers
Chris

[Jarrod]

i have a reflector with a lens in the focuser. i was considering removing the lens and giving the 'scope a full length tube, would that improve the view?

[Chr]

You will most likely find that you have to have the lens in. otherwise you won't be able to get anything in focus :(

[Jarrod]

but if the tube is made to match the focal length of the objective then the lens in the focuser is not needed. so if we were to lengthen the tubes to match the objective's focal length we could remove the 2x barlow lens. won't the two actions cancel each other out?

[mill]

Don't do this because you will end up with a very bad scope.
They stop grinding the mirror just before the parabolizing.
Figuring that last little bit cost too much time for small mirror manufacturers and that is why they use a barlow in the focusser (cheap and nasty).
You could still collimate the scope using the laser.
Use the barlowed method, there are many websites explaining the barlowed laser technique.
Since you already have a barlow in your focusser you don't have to use the barlow as they show you on those websites.

[Jarrod]

are you saying the mirror isnt parabolic? is there any way of improving the view through a telescope with a built in 2x barlow? im still using the crappy eyepieces supplied with the 'scope, would a good set of eyepieces improve things really dramaticly?

so many questions, so little time!

[mill]

Even good eyepieces wont improve the scope much :shrug:
I had an meade 2114ds once and it was an piece of ^#^$&&%^*
Do yourself an favour and sell the scope and then get an 6" dob or bigger.
Sorry to burst your bubble :(
I had one and learned my lesson very quick.

[Jarrod]

so if i lengthened the opticle tube, i would end up with a scope suffering from spherical aberration? but would it improve the sharpness of the images, brightness etc. a scope with allright views and spherical abberation sounds better then a scope with no abberations but really crap views. i also read somewhere that spherical abberation becomes less noticeable as the focal ratio increases. my scopes an f:8, how bad would the spherical abberation be?

[mill]

If you only had a parabolic mirror it would be good , but with the mirror you have the focal ratio doesnt do anything.

[Jarrod]

but would i get bright and sharp views? i once made a 50mm refractor from some crappy lenses, its got the worst abberations possible (spherical and chromatic) but it shows incredible detail. im happy to put up with spherical abberation if i get nice sharp and bright views in return.

the image used as my 'avatar' was taken through that 50mm refractor.

[cristian abarca]

Hi Jarrod. I have one of these scopes. The biggest problem with these telescopes is collimating them. I found that I couldn't collimate the thing without removing the barlow. I tried putting a 2x barlow at the eyepiece and it didn't work, i also tried an extension tube and that didn't work either. It become inpractical because of the length required and the focuser can't handle it either. I thought about making a long tube and use it without the barlow but there are too many things that can go wrong with this. For one the mirror is barely parabolic if at all. When you start getting towards these focal lengths you can get away with mirrors not being exactly parabolic. Secondly the length of the tube will make the mount wobble too much. It already wobbles with a short tube so imagine a tube 1.2m long. Thirdly you will probably need to change the size of the secondary. The size needed for an F8 is smaller than the one the telescope has in it. Fourthly there is the cost involved in modifying the thing. You will spend money on something that may not give you better views that you are already getting. Remember that Reflectors don't give you as sharp a view as refractors due to the central obstruction (the secondary and spider vanes), and the central obstruction in this scope is not small. The only way I found that I could get reasonable views was to collimate it really really well. All said and done I did get a lot of use out of this scope even if it is cheap and nasty. This scope is the reason I looked into telescope making.

Regards Cristian

[Jarrod]

thanks cristian, its nice to know my scope has still got some potential. is there any minor things that would help improve the view through my scope?

maybe it would help if you know what my idea of "improvement" is...

through a 76mm newtonian jupiters cloud bands and five moons are clearly visible, OK. through my 127mm newtonian the cloud bands are only just visible during perfect seeing, and only four moons are ever visible. i was very dissapionted when i saw that a far smaller scope was offering such superior views.

i just want the view through my scope to match its aperture.

[cristian abarca]

I think that collimation is most critical in this scope. The bands in Jupiter should be clearly visible as the four moons. I never saw the red spot in Jupiter. Saturn I saw pretty well but I couldn't see the Cassini division. As far as the other planets I saw Neptune as a blue disk at high magnification. Mars I only ever saw as an orange to yellow disk. As far as star clusters ther was no problem nor was there a problem with planetary nebulas. Tuc 47 looked pretty good through it as well, that is until I saw it through a 25 inch from a deep sky site, but that is another story. One of the things that I did change were the screws at the back of the secondary. Instead of the phillips head screws I got allen head screws, they seem to hold a little tighter. I also played around with the focuser because I found that it wasn't in line with the secondary. I placed a washer where the focuser touches the tube. This may not be necessary with yours. As far as the mount goes I put a bottle of water dangling from the centre of the tripod to steady it a little. The sight tube can be a bit of a bugger to get aligned but you can always put a washer underneath to get the alignment easier. But I must reiterate that collimation is the most important and this is not an easy task in this telescope. You have to learn this and learn it well.


Regards Cristian