Seeing stars

fok4tel · #1 11-06-2009, 05:48 PM

I find that I can’t bring the stars to a fine focus under 200x in my telescope, the stars appear to be disc-like. I think I don’t have problem with collimation, and I use Nagler 9mm EP. If everything else being in order, does it mean I have actually seen airy disc of those stars? If it is true, then I will never be able to see pin-point stars in my Nagler 9. But I remember people claim to be able to see pin point stars even with a shorter focal length. That puzzles me.
Appreciate your comments on this.

Baron von Richthofen (Vaclav) · #2 11-06-2009, 06:44 PM

What scope do you have?

Lismore Bloke (Paul) · #3 11-06-2009, 08:43 PM

I find it's easier to achieve a pin-point star image with
wide field eyepieces rather than shorter ones.

If the focuser is a Crayford, is it slipping with a heavier weight
eyepiece inserted?

fok4tel · #4 12-06-2009, 12:26 AM

Thanks for your reply.
To Vars191: I own a LightBridge 16", F4.5.

To Lismore Bloke: Yeah, I find it that way too, much easier to achieve pin point star image with a 28mm ep. My scope comes with a Crayford focuser, it works well when the ep is there, smoothly without slipping.

So, is my problem common among my fellow star chasers out there?

wavelandscott (Scott) · #5 12-06-2009, 03:15 AM

I would expect that you would be able to make 200x work in your scope.

Assuming that your scope has had time to cool and that collimation is good, I suspect that you may not have quite enough travel left in your focuser.

Do you have it racked all the way in or out?

You may need to adjust the height of your mirror a bit or add an extension to your focuser.

While this sounds scary, it is not. Simply back off (or tighten up depending if you are working to bring the mirror up or down) the collimation bolts an equal amount and re-collimate at this new lower (or higher) position...or add an extension to your focuser to bring it to focus.

As you say that your stars are disc shaped that sounds to me like you are only lacking a little bit one way or the other.

gmbfilter (Geoff) · #6 12-06-2009, 08:10 AM

Quote:

Originally Posted by fok4tel

I find that I can’t bring the stars to a fine focus under 200x in my telescope, the stars appear to be disc-like. I think I don’t have problem with collimation, and I use Nagler 9mm EP. If everything else being in order, does it mean I have actually seen airy disc of those stars? If it is true, then I will never be able to see pin-point stars in my Nagler 9. But I remember people claim to be able to see pin point stars even with a shorter focal length. That puzzles me.
Appreciate your comments on this.

Cant avoid Airy Disk
These guys full of help full information http://www.oldham-optical.co.uk/index.htm
Check it articles on the left

Robh (Rob) · #7 12-06-2009, 11:13 AM

I assume your collimation is correct.
Check you can move the focuser further in and out beyond the point at which you judge focus to be best. Adjust the focuser tension for fine focus.
Try a different eyepiece.
Compare what you see through someone else's scope, it will give you a better idea.

Focussing a star to a "pinpoint" is rather a subjective assessment. You cannot actually focus a star to a literal pinpoint due to diffraction of light. Theoretically, the smallest point of light you can get is the size of the airy disk.
In practice, seeing conditions will affect what you see more than anything else. The brighter the star, the larger the glare and the more bloated the star will look. Stars will appear as specks of light about 1 or 2 arcsec diameter. If you compare stars through your scope, they don't all appear the same size. Also, in a fast reflector you will get more coma which can add volume to stars.

Regards, Rob

fok4tel · #8 12-06-2009, 11:35 PM

Thanks wavelandscott, gmbfilter and Rob for your valuable input.
Wavelandscott, I need to say I have no problem with the focuser and eyepiece. Nagler9 T1 has a dual size skirt. When I throw it into the 2" focuser , it does not have enough travel out to reach focus, so I can only use the ep 1.25" skirt with the focuser 1.25" adapter. This makes the ep 30mm more extended from the focuser to enable focusing in my eye. With this arrangement I have more than enough travel in and out of the best focus. You should be right, I should be able to make 200x with 16". May be the atmospheric turbulence is the cause . Thanks to gmbfilter's recommendation I visited the webpage and think that airy disc shouldn't be discernable at that magnification. I would take Rob's advise to compare again the star images between eyepieces, and I will also throw in the barlow this time. Comparing what I see with that of other scopes is also a good idea.