Help needed with 10" Dob

Benno18 (Ben) · #1 29-10-2009, 12:14 AM

Hi all,
Just having a little trouble with the focuser, (i think), On my 10" Skywatcher Dob. It is working fine with the 25mm eyepiece and the 32mm eyepiece I have. I am able to focus in fine with those, even with the Barlow Lens I have. But when it comes to the 10mm eyepiece trouble is brewing.

I cant get it to focus properly, Im either too far one way or the other, which gets quite annoying when looking at Jupiter and star clusters.

Is it just me or the focuser or is the 10mm just a bit too powerful for the scope?
I have seen focusers which have a smaller knob on them to do a fine focus. Is that all I need?

Any advice is good advice here
Thanks

erick (Eric) · #2 29-10-2009, 12:26 AM

Yes, a focusser with a fine control (usually drives the focusser drawer tube 1/10 of the rate of the normal knob) is very useful. You would need to replace your exisiting focusser. However, I worked without one for a year, no problem. Is your "tightness" screw in sufficiently? The draw tube is not slipping? Another way to adjust focus is to lift the eyepiece slightly out of the focusser. Just loosen the screw and while looking lift it up until you achieve focus, then nip the set screw up. But you shouldn't have to do that. 10mm is not too much. I worked with 6mm in an 8" dob without a problem.

Benno18 (Ben) · #3 29-10-2009, 12:43 PM

Ok thanks Erick. Which one is the tightness screw do you think? the silver knob which stops the focuser from moving all together or the allen key one just above it? I dont want to touch anything i dont know about, and the little allen key screw is one of them.

Thanks again

erick (Eric) · #4 29-10-2009, 12:50 PM

An allen key screw? But that will be it. For GSO focussers, the one nearest the scope "locks" the draw tube - the with adjustment wheel spins freely. The one furtherest from the scope adjusts the pressure the adjustment wheel places on the draw tube. Too loose and the tube will slide under a heavy eyepiece. Though this doesn't seem to be your problem - touching the adjustment one way then the other and observing the effect might help?

Before you do, anyone else with this specific focusser who can advise??

astro744 · #5 29-10-2009, 01:13 PM

Ok. You first need to determine if you need to rack in to achieve focus or out. Out is easy, first rack out all the way with the 10mm eyepiece in place and then slide the eyepiece out by loosening the eyepiece lock screw or even the 2" lock screw and move the 2"/1.25" adaptor out a bit. If you can get focus then all you need is an extension tube, 35mm no more.

If you need to rack in to achieve focus but cannot rack in any further then you may be able to 'push' the focal plane out further by loosening all collimation screws on the primary mirror and then re-collimation by adjusting two of the three screws as needed. This may give you enough to reach focus. Another more drastic method would be to shift the entire primary mirror and cell up the tube a small amount.

It would be very unusual if your 10mm eyepiece did not focus and perhaps collimation (or lack thereof) and bad seeing had contributed to what appears to be an out of focus image on the night you were viewing.

Dont worry about the fine focus as it is not essential. In fact sometimes the human eye can adjust to a slightly out of focus image before you get your fine focus spot on. Some people like the fine focus though and it is highly recommended for imaging. If you cannot get course focus on your object then fine focus will not help at all.

Note too if your focal plane is only a little out past the fully racked out position then you can tighten all collimation screws on the primary mirror to bring it it and loosen two only as needed to re-collimate.

mental4astro (Alexander) · #6 29-10-2009, 01:42 PM

I agree with astro744 in checking the collimation of the optics first.

There is another source of difficulty: the atmosphere. Although a 10mm might not be exactly high power, it can start to reveal turbulance in the atmosphere, making focusing near impossible. There is also much less tolerance in the focusing range with the shorter focal length eyepieces. I've been able to see Jupiter under stable atmospheric conditions twice this year to comfortably use my 5.5mm. Most times even my 10mm is not great either.

I am also assuming that the scope is an f/5 instrument. It might help a little also if you stop it down a little with a mask made out of stiff cardboard with a hole cut out in it smaller than the diameter of the primary, yes reducing the apeture, but increasing the focal ratio. Remember, good planetary scopes a refractors, Maksutov's & Schmidt-Cassegrains that are much slower than your newtonian. Cheaper and less complicated than changing focuser.

The following link is to a photo of a 13.5" f/4.5 dob. It shows the 'mask' cut into the plywood cover of the primary mirror. Get the idea,

. This is purely a device to assist with planetary viewing. It overcomes the supposed 'problem' with fast newtonians not being good for the planets. I've done it for my 10" f/4.9 dobbie using the waxed cardboard boxes cabbages & lettuces come in at vegie shops. Works too. I did, though, keep my circle cut-out co-centric with the primary.

http://www.atmob.org/library/member/bsmith/bs5big.jpg

Do some calculations to start with, say, an f/15 mask. You can then cut it larger. You can't make it smaller after it's been cut, though a vegie box should give at least two masks. Keep in mind the central obstruction in you calculations.

See how you go.

Mental.

Benno18 (Ben) · #7 29-10-2009, 10:07 PM

Hey thanks guys for your help. I dont think the problem is with the collimation. I did what you said Astro 744 and i can get to s stage where i go in and out of focus. Just cant get it on the perfect spot. But still Could well be the collimation, i took it straight out of the box and away i went, was told i shouldnt have to touch the collimation screws for at least a year, and even then it would be a touch up.

So im going with my extra strong fingers taking the focus wheel to far when its not needed, Also the viewing. some pretty average nights but just thought it would be better than what ive got it so far (focus wise that is. What i can see through this amazes me)

Thanks heaps guys
Happy viewing

astro744 · #8 30-10-2009, 08:50 AM

Both your primary and secondary telescope mirrors can easily shift during transport and collimation could easily be lost. You should ensure your telescope is properly collimated and then let us know if your are still having focus problems. Also ensure your 10mm eyepiece is clean and has no fingerprints or dust on the eye lens.

To check for gross collimation just look down the tube without an eyepiece installed. Rack out fully and insert the 2"/1.25" adaptor. Remove any cap from the front of the tube too and point the telescope at either blue sky or the garage ceiling, something with a uniform brightness (NOT THE SUN!). I find my white gyprock garage ceiling works well.

Look down the focusser and you should see your eye, the secondary, the primary & spider vanes reflected in the secondary but most importantly everything should be concentric. If not your telescope is out of collimation.

You can collimate approximately at least enough to give you proper focus with your 10mm eyepiece without any collimation tools by simply tightening or loosening the primary mirror screws and by enusring the secondary is centrered under the focuser and showing the full primary mirror reflection and also showing the end of tube as a concentric ring.

Search for collimating a Newtonian on the web and you'll find lots of links. You may read about secondary offset but do not be concerned with it at this time. To get even better collimation you can invest in a Cheshire collimation tool or make a simple sight tube using an old 35mm film canister.

However if you cannot get a 10mm eyepiece to focus after basic sight collimation without any tools then you may have other issues and I suggest trying a different 10mm eyepiece.

Benno18 (Ben) · #9 30-10-2009, 02:21 PM

Hey guys
I tryed the tightness screw that erick thought of and that seemed to work quite well, only needed a little tweek and the focuser winds slower and easyer to focus. Also yes a 'smudge' on the eyepiece dosnt help when focusing.

just had a look down the scope as well astro, and it seems slightly out. but dont want to touch anything with out someone with me to show me.

thanks heaps for all your help guys.

erick (Eric) · #10 30-10-2009, 10:27 PM

Good news! Now pack it up and get to a starcamp or a New Moon weekend with a gang in WA and someone will look into the collimation for you.

But don't be hesitant about doing it yourself. Newtonians are easy to get to reasonable collimation and easy to unscramble if it all goes pear-shaped on you when you first try. The mechanicals are quite simple, really.