A Reflector, 80mm extension tube and a 32mm EP

[skir]

And I can't focus! So is this extension tube simply too long for a 32mm EP?

If so would a 50mm or 35mm extension be better for it?

My 8mm was able to attain focus and wow what a lot of stars there were up there!

[badgerchap]

If I'm using my 90 degree and my eps on my refractor, I have to pull the eyepiece half an inch or so out of the holder to achieve focus, and the 90 degree has a long old light path!

[skir]

So do you think the 80mm is too long or should I be ok with that?

[badgerchap]

I would try and work out roughly where the focal plane is by moving the ep in and out of the ep holder. If you can roughly measure the distance between the ep and the holder during focus, you should be able to work out what length extension you need. What scope is it?

[skir]

Yes good point. Could I also say cut up bits of cardboard in different lengths roll them into cylinders and then try the EP with these to see where focus was attained?

I know it's a bit 'Heath Robinson' but if it does the job.....

oh, scope is in my sig.

[badgerchap]

Yeah that should do the job. I love Heath Robinson approaches, loads of fun!

[skir]

Right well I'll give it a go tomorrow. It seems extensions are 35mm, 50mm and 80mm in size. Are there other lengths besides these?

the 32mm did focus beautifully on a chimney pot of a nearby house 50 metres away (I'm short of good objects to focus on a long distance off in the daytime). But when it came to anything in the heavens it was hopeless, but it did do a good job of showing up the lp better

[skir]

If I used a Barlow could this also help the 32mm to focus?

[rwilkey]

That's very unusual, you should be able to focus the 32mm without an extention, are you sure you have the ep adaptors set up correctly ie. you only have the 1.25" adaptor in the focuser and not the 2" one? Make sure you are using only the one adaptor....

[skir]

I had it setup as: 2 inch focuser tube- 2inch to 1.25 inch adapter - EP.

should that configuration work?

[skir]

Is there a possibility that the collimation and lock nuts on the primary could all be too tight and that this is pushing the primary slightly too far up the ota and causing issues for focus? Or that if too tight it could slightly deform the primary mirror?

I've not checked to see if they are but if it is a possibility I will check and if they are loosen everything off, re-collimate and tighten it back up.

[haitch]

You could be maxing out the distance of the primary up the tube but I wouldn't have thought it had enough travel to cause your problems if you have the original focusser on it. You won't deform the primary as the mirror cell is constructed so that the mirror shouldn't be under any pressure from bolts/screws etc.

[skir]

You could be maxing out the distance of the primary up the tube but I wouldn't have thought it had enough travel to cause your problems if you have the original focusser on it. You won't deform the primary as the mirror cell is constructed so that the mirror shouldn't be under any pressure from bolts/screws etc.

thanks for that. So should I try a shorter extension with the 32mm?

[haitch]

thanks for that. So should I try a shorter extension with the 32mm?

Could work - can you post a picture of how you've got it set up? With a reflector, with original equipment attached (i.e. mirror cell and focusser) you shouldn't need to be extending to reach focus.

[skir]

Yes, I'll see what I can do.

[skir]

Https://www.dropbox.com/s/ib5cwmai4a9m96v/Photo%2031-01-2013%2018%2029%2031.jpg

Thats it with the 32mm ep on the 2-1.25 adapter on the draw tube. Ignore the decor its not my house!

[rwilkey]

Hi Skir, that sounds correct, still can't understand why it doesn't work....sorry, just seen the photo - looks good!

[skir]

Well I've had a good look at the Collimaton again and if I move the primary knobs one by one the black dot comes out of the centre circle on the primary. But I can put it straight back in the centre again. The cross hairs are very slightly off vertically and horizontally and the black circle is slightly concentric. Which from reading astro baby's guide would seem to be right for an F5.

I'm quite happy to believe I'm doing something wrong here. But has anyone got any other thoughts?

Feel free to pm me if you want to.

[Moonshane]

one way or another you need to establish where the focus point of the scope with the eyepiece in question is. personally I would use the following method:

take out the 2"-1.25" adapter

rack out the focuser as far as possible

put your left hand on the top of the focuser

hold the eyepiece in your right hand

use your left hand as an eyepiece holder and hold the eyepiece with your right hand and look through it.

move it gradually out until you get a sharp image (of a star or moon etc) and measure the distance above the focuser.

you'd need to start out and work in if the problem is in focus.

once you do this you have options

if a standard adapter will bridge the gap (I'd expect an 80mm extension to be too much for most newts) then get one and sell the 80mm

if it's a small amount of out focus then you could screw a 28mm Baader fine tuning ring (http://www.firstlightoptics.com/baader-planetarium/baader-hyperion-finetuning-ring.html) into your 2-1.25" adapter. this allows you to pull it out a little

if it's a lack of in focus then it's harder to solve. a light pollution filter might help as this drags the focus point in a few mm.

[skir]

Thanks I'll give that a go. I think I'll wait until a night when the moon is at least partially visible so I've got something large and obvious to work with.

Unless of course someone has a better suggestion to focus on?

[Moonshane]

it doesn't really matter. if you can see night sky then use anything the scope points at

[skir]

Ok, thanks. It's looking partially clear for at least part of tomorrow evening so I'll make it my one task to see what's going on.

[haitch]

Looks set up ok but it does look like a fairly low profile focusser so it's possible it has been upgraded from a taller one. The focus point of a 32mm plossl is usually further out from the field lens than shorter f/l ones (because it's a longer focal length) so if other eyepieces focus I would have thought your problem was more likely lack of out focus as you say if other EPs come to focus.You can get some taller 1.25" ep adaptors which may be enough to solve it. The 80mm extension is likely to be way too much. The shortest 2" extension you can get (I suggest 2" because if you ever get any 2" eps it'll be handy plus they should be more stable) is 35mm I think. Don't bother trying to test focus on anything on the ground - the focal point will be completely different to infinity, if I remember rightly further out.

HTH

[skir]

I'm pretty sure the focusser is the original one as the seller didn't advertise it as having been upgraded. I didn't get an instruction manual with it and I don't think one exists from research. However I think the scope is this one:- http://www.optcorp.c...ogies/8newt.pdf just sold under a different name.

When you say field lens is that the same as the secondary? Sorry, just trying to get used to all the terminology.

[skir]

Well I used a cut up length of bog roll cardboard to make a 35mm extension, but I couldn't get the 32mm EP to focus on a chimney pot 30-40 metres away with this, it got close but wasn't there.

Thinking about it when I turn the focuser I hear and feel some resistance at points of turning the focuser wheels. Should I be getting this at all? When the package was delivered there was a small hole in the box and one of the thumbscrews attached to the focuser was badly bent.

If the focuser tube had been bent out of alignment would this cause focussing problems?