How to focus the telescope

[Outbackles]

Hello,

I have had a long running interest in astronomy and finally after reading through these forums carefully I took the plunge and have just purchased a telescope. I bought a GSO 6" reflector on an eq mount with crayford focuser. I went this route because I would have trouble fitting a dobsonian in the car with camping/other equipment.

I have set it all up and lined up the finder scope during the day, BUT last nite I took it out for its maiden run.

To my disappointment I cannot get it to focus on anything, not the moon the stars or planets ! I only tried using a 25mm plossol and I can only focus the objects down to a very blurry ball/disc shape.

When focusing on a star I can see a blurry light and the 'crosshairs' from the front of the telescope superimposed on the image? Is that right?

Appreciate any help, I have not tried collimating the telescope, it may have had a traumatic journey with TNT from Sydney up to Darwin where we live.

Thanks for any help!

[deadsimple]

I can't help you with your problem since I'm only just learning about telescopes myself, but to be honest I always expect the worst from couriers after receiving this package from Amazon a few months back:

[dannat]

I would try and focus on something in the distance during the day.

You will also need to check the collimation of the mirrors (whether they are lined up with one another) - if you go to andys shot glass there is a good tutorial video there

Re-post after you have tried daytime use

[erick]

Try during the day - looking at a distant hilltop or antenna etc. Of course, the image will be rotated 180 deg (basically it looks upside down). Now it should be easier to see something and work out what is happening. Can you see the focusser draw tube moving smoothly all the way in and out as you turn the knob? All the way to a stop point at each end of the travel. The lowermost screw on the focusser should be loose. It is a lock screw and, if tight, prevents the draw tube from moving. The upper screw should be adjusted until the draw tube is moving and not slipping. If it is too loose, the tube is not driven by the wheel. Is your eyepiece sitting all the way down against the focusser?

So what is happening to the image? Do you go from very unfocussed, through a focussed point back to very unfocussed again? If so, then we'd better start looking at the mirrors. But before that, have a good look at the eyepiece - anything odd about it - something out of alignment? Try another eyepiece?

OK, look down the tube and through the focusser with eyepiece removed. Is the secondary mirror in the right place? Has it rotated so it is not "facing" directly towards the focusser? Does the Primary mirror look to be tilted too much one way or the other.

There's a bunch of ideas to work through.

Please DON'T point scope at the Sun on purpose or by accident. Don't leave it unattended with children present while you have in out in sunshine.

I'm sure you'll have it sorted soon - these tube units are quite simple mechanically.

Eric :)

[Outbackles]

Thanks folks for the speedy replies!

Erick, just checked and the bottom screw on the focuser was hand tight, I have loosend it. How loose should it be? what is the point in it?

Hope that is it !

I'm not sure I understand what it does as the tube appeared to be moving in and out anyway?

Hope that is it! Always the simple things to those in the know! but frustrating to the newbie.

[erick]

Yep these GSO crayford focussers are not obvious at first.

If the bottom screw was hand tight, I suspect the tube was struggling to move. It is for locking the tube at focus point. Would be useful if you were trying to take photos through the focusser. Maybe useful to stop inexperienced people knocking the focussing knob and having it out of focus. BUT most people focus at slightly different places anyway. I always put a person's hand on the focus knob and say twiddle away until it is right for you. Many unscrew it right out and put it away somewhere - never use it and you don't want it falling out during bumpy transport to site.

Note that as the temperature changes, the performance of the Crayford changes - how it is working now in the warmth of the day may be different to what it was doing last night in the cool (well, it's all relative in Darwin :P ). Often through a night, one adjusts the tension screw a little to adjust for temperature change as well as for eyepiece weight.

[Outbackles]

Got it to focus during the day. The instruction booklet said to pick a target 200m away which I did (slightly more actually), but the only way it would focus was to pull the eyepiece out away from the focuser (not all the way out) which makes me thinks 200m is not enough distance.

I'll try it again in the evening and see if we have any more luck on the moon.

Les

[erick]

That's good progress, Les. Yes, next step is the Moon. Bit odd if you have to wind to full out, then lift your eyepiece further out? See what you learn tonight.

[Outbackles]

Eric,

Just spoke to the supplier and they reckon it probably needs an extension tube.

Apparently the manufacturer has been trying to improve the short length focusing for people that hang a camera off it and may have gone a bit too far.....

Les

[erick]

:scared: I hope they give you one for free! Plus an apology for your wasted time. However, 35mm, 50mm, 80mm 2" extension tubes all seem to be selling for around $29.