Spider veins

[Wv18]

Tonight I was trying to look at the moon with my Celestron Omni XLT 150 Newtonian reflector and I got it lined up on the moon and whenever I looked through at it I would see the spider veins and a black dot and I was not able to get it to focus at all is this due to needing more collimation or something else?

[Ricochet]

As with your posts yesterday, the problem is with reaching focus. I doubt that there is enough travel on the collimation bolts for this to have been caused by the collimation of the telescope. I also doubt that Celestron would sell a telescope/eyepiece combination that can't reach focus so it is likely that you have somehow set the telescope up incorrectly.

If you point your telescope at a star you will see a disk with the shadow of the secondary mirror. Now turn the focusing knobs and watch what happens to this disk. To reach focus you want to make it as small as possible. Determine which way makes the image get smaller and keep turning it that way until the disk becomes a focused point. If it starts getting bigger again you have gone too far. If you reach the end of the focuser travel before you get a focused point take note of which way you have been turning. If the focuser is at the limit of inward travel then you have an extra piece in the focuser that you need to remove (but as it is a 1.25" only focuser I doubt this). If you have reached the limit of outward travel then there is an extension piece missing that you need to insert into the focuser. With the focuser at its most extended undo the screw(s) holding the eyepiece in place and slowly withdraw it from the tube while looking through it. If you find that you can reach focus this way take note of how far out of the focuser you have pulled the eyepiece: this is the size of the extension tube that you are looking for.

You can also try focusing on objects during the daytime. If you do this point the telescope at the furthest object you can see. Your telescope is not designed for terrestrial viewing and so it is normal if it cannot focus on nearby objects.

If you bought your telescope from a specialised dealer you might want to contact them and see if they know what the problem is as you're unlikely to have been the first customer with this problem. Alternatively, there is probably a local astronomical society who would be happy to help you get your telescope set up correctly if you take it along to a meeting.

[Stub Mandrel]

Make sure the focuser isn't locked in position.

[rwilkey]

Seeing spider veins usually happens when you are using a very low power ep or that you have not achieved full and accurate focus.

[Wv18]

I'm just not very sure what you mean because before I fixed the problem by adding the 2" adapter to the 1.25" eyepiece because now I'm actually able to see stuff rather than the black with a white ring. But now I can't focus it whenever I look at anything. 

[Louis D]

Try this to determine where the focus point lies for your telescope:

1. Remove all adapters from the focuser

2. Lower the focus tube all the way in

3. Aim the tube at the moon

4. Hold a sheet of thin white paper (tissue paper in a frame works well for this) or frosted transparent plastic over the open focuser tube.  You should see an image of the moon on it, even if very out of focus.

5. Start moving the paper away from the focuser tube like a sheet of film in a camera.

6a. Is the image getting smaller and sharper?  Keep pulling it away until it sharpens into a nice projected image of the moon.  That's how far out your image plane lies and where the shoulder of most eyepieces will have to be to reach focus.  If it's above the highest point of your focuser tube and extensions, you'll need to get more extensions.

6b. Is the image getting bigger and blurrier?  If so, your image plane lies somewhere below the lowest point of your focuser tube.  You'll need to move your mirror forward on its collimation bolts to achieve focus.  There's a limit to how far you can go with this approach.  If this is the case, you should contact your dealer.

Give this approach a try and report back on what you find.

[John]

23 minutes ago, Wv18 said:

I'm just not very sure what you mean because before I fixed the problem by adding the 2" adapter to the 1.25" eyepiece because now I'm actually able to see stuff rather than the black with a white ring. But now I can't focus it whenever I look at anything. 

Usually, you should use either the 1.25" adapter OR the 2" one but not both at the same time, as in that picture.

You might be able to reach focus on things relatively close to you with both adapters in place (eg: trees, chimneys etc) but you won't be able to rach focus on astronomical targets.

When stars are in focus with a scope they appear as a tiny point of light. As they go out of focus that point grows into a disk and eventially you see the shadow of the secondary mirror and the spider vanes projected against the bloated star disk - the scope is way out of focus at this point.

It can seem counter-intuative when using a telescope but things are generally at focus when they appear their smallest though the eyepiece.

[rockystar]

Ahh, the double extension trap, designed to fool all beginners (including myself). As John says, you need to split the extension tube: just undo the middle thumb screw and they should slide apart, then pop one of them back into the focuser add an eyepiece, and then (probably) about half way through the travel of the focuser you will reach focus.

[Louis D]

Why isn't the 1.25" extension as long as the 2" extension if they're meant to be used separately to bring each size of eyepiece to focus?  1.25" eyepieces don't inherently focus more inwardly than 2" eyepieces.  Maybe I'm misunderstanding the purpose of each extension.  I'm guessing your remove both to affix a DSLR for prime focus photography.

[John]

3 minutes ago, Louis D said:

Why isn't the 1.25" extension as long as the 2" extension if they're meant to be used separately to bring each size of eyepiece to focus?  1.25" eyepieces don't inherently focus more inwardly than 2" eyepieces.  Maybe I'm misunderstanding the purpose of each extension.  I'm guessing your remove both to affix a DSLR for prime focus photography.

The stock 2" and 1.25" adapters are actually the same length. Here they are out of the scope:

 

 

[Louis D]

I was wondering if that was just an optical illusion in that other photo.  It seems silly to provide two tall adapters.  All they needed to do was provide a 2" to 1.25" step-down adapter of negligible height along with the 2" extension tube.  That would also eliminate some confusion about when to use it.

[John]

6 minutes ago, Louis D said:

I was wondering if that was just an optical illusion in that other photo.  It seems silly to provide two tall adapters.  All they needed to do was provide a 2" to 1.25" step-down adapter of negligible height along with the 2" extension tube.  That would also eliminate some confusion about when to use it.

I agree that Synta's approach is more confusing than it needs to be. Many other brands provide a 2" fitting focuser that you insert a 1.25" adapter as required.

[Louis D]

5 minutes ago, John said:

I agree that Synta's approach is more confusing than it needs to be. Many other brands provide a 2" fitting focuser that you insert a 1.25" adapter as required.

Yes, I was thinking along the lines of the Meade Telescope 2 inch Extension Tube 07474 #868 for some of their 1980s and 1990s newts.

[rockystar]

I suspect it is easier and cheaper to manufacture the longer 1.25 extension tube; but I agree, it would be much simpler if it was just an adapter that slotted in the 2".