Posts Made By: Paul Preto

Greed, treachery, winnning at any cost.... seems like that's what the law is all about these days. As we just become collateral damage....

Summing up what I've read so far, it appears there are two schools of thought. I originally read about cables on the Yahoo bigdob group, with Nils Olaf being one of the chief proponents. Dave sums up that school of thought nicely:

What you want is something that has as little friction against the glass as possible, does not stretch even a millimeter with temp or tension, and something that does not go near the face edge of the glass...

But Floyd and others, including myself I think, believe:

It seems obvious that the cable being narrow would not distribute the weight over as large an area as a strap or sling.... You want to place the weight over the largest area to minimize distortion...

So is either of these a proven fact, or is it merely a matter of preference? Also, once again, I wonder whether the cable method, if valid, only applies to mirrors over a certain size?

Darryl,
Let me preface this by saying that I have no first hand experience yet. But this is my understanding. Yes, you can use your barlow. However, you will need to make a white mask that covers the end of the barlow closest to the secondary. You will have to cleanly cut round hole in the center of that mask (size?) to allow the laser to shine through. What will happen is the the barlow will spread the laser beam out to form a large disk on your primary. This will be reflected back to your secondary and onto the mask that you put on the barlow. In the middle of that reflected image will be a dark shadow that is produced the center spot on your primary, since it should not reflect the light. This shadow is then aligned with the hole in your barlow mask to achieve primary collimation.

Why is this better than what you have? Apparently the laser housing will wiggle somewhat in the focuser, such that your point of light can hit the primary at a slightly different spot each time you place it in the focuser. When reflected back at your laser and appearing on the 45 degree surface you see from the side, the % error at the primary will be even greater by the time gets back to the laser. This can cause alignment errors. Everything I've read says that the spread out beam produced by a barlowed laser returns the dark shadow of the primary center spot in a consistant manner that will not change even if you wiggle the laser in the focuser. That's why I'm going to a barlowed laser setup by Howie Glatter called the Blug. Check collimator.com for more information on that device, if you're interested.

I too would make sure I have the money in hand (accept only paypal?), buyer pays the additional shipping, and shipment is insured at buyer's expense. Buyer must accept all responsibility for damage during shipment in other words. Also, dated pictures showing the scope in good condition prior to shipment would be smart.

As for the GSO 30, you can get an extender for you focuser fairly cheap. Re your other question, the center of the field will be good to excellent in just about any of the eyepieces sold today. It's the edge of the field that will suffer in a fast scope. In your dob, you may want the extra field of view, however, just to make it easier to keep your object in view. But realize the edges won't be sharp with an economy eyepiece.

Thanks, Dan. Excellent review. I was considering this for my f/4.5, but will probably pass now.

I have the DGM 5.5" scope. It produces very bright images for the aperture, very contrasty. Drawbacks are the long tube requires careful balancing for different eyepieces. For planetary viewing, I've found I need to run a cooling fan most of the night, since the primary is quite thick for its size. This is because it is a segment of a much larger mirror (14"?). Also collimation is very different from a standard newtonian and requires some getting used to. Standard collimation tools are useless with this scope. I would not recommend it as a "quick look" scope for the above reasons.

Interesting question. I agree with you. Also, all internal reflections should be eliminated. But mostly I'm writing to get you back on page 1 of this forum, since someone replied to a months old string, bumping you down to page 3.

I had this problem with a scope and noticed that the 1 1/4" to 2" adapter in my 2" diagonal was about 3/16" high. In other words, the adapter was moving my eyepiece that distance further back. A flush sitting adapter solved the problem.

Just to update, I followed Floyd's tip about installing a 2nd pin to keep the UTA from swinging around the locking pins. Actually, James Grigar at Astro Sky recommended that I not drill, but rather use flat pieces of wood glued to the truss pole connector pieces just below the locking pins, as shown in the attached photo. Net result is that I can now get the scope collimated quickly after removing and replacing the UTA, since this preserves angular placement of the UTA to the poles from one observing session to the next.