Pointing a dob?

[0tt0maddox]

How do you aim these things?

I recently bought a 10" Gso Dob from the Ice Trade Classifieds (thanks Rob). It's my first reflector scope.

I've been dying to take it out since I bought it, but it's been pretty muggy until tonight.

I geared up and headed out and things were fine optically. Crystal clear images and after a while I got used to topsy turvy movement. The only problem is I had so much trouble cetring the image. It would slip too far one way, then too far the next. No matter how hard I tried I couldnt adjust it the right amount. I was on very low magnification as well. I've done the teflon furniture feet, milk bottle and car wax thing, but it didnt seem to affect the movement.

Is this a common problem? If so, are there any tricks to avoid it?

Thanks

[ballaratdragons (Ken)]

You'll get used to it.

You only need to slightly nudge the scope by tiny fragments.

If it jerks when it takes off or stops then it isn't slippery enough. But if it glides nicely you'll be OK. Just be very gentle. Tiny movements.

Start with something like a 25 or even 30mm EP and get used to the feel. Then gradually work your way up to 15mm and 9mm etc.

[mswhin63 (Malcolm)]

Originally Posted by 0tt0maddox

The only problem is I had so much trouble cetring the image. It would slip too far one way, then too far the next.

If you are overshooting, does you DOB feel sticky, If so loosen the tension a bit, If it feels too smooth and easily overshoots then loosen it, once you have found the equilibrium then it will work well for you.

I found the Teflon bearings have an initial stickiness (not a lot) to them so once you got the right image then the drift required more adjustment, this is when it become a bit harder the force needed to overcome the initial movement was more than once it was started so you would overshoot. I personally replaced these bearing with roller bearing (not advisable now) and now it is really easy. Unfortunately it is now too easy and moves much too easily. Once I get back to using the DOB again I suspect I will investigate a way of creating a tiny bit more friction to stop overshooting. The other problem with roller bearings on the ALT is when you look at low altitude fields it DOB want to continue downwards.

[kitsuna (Adam)]

practice practice practice. That'll be your first option.

Another suggestion (though pricey), is to get a nice wide angle eyepiece. The wider the field of view, the less impact excessive movement will have. It also means that your target will stay in your field of view longer, meaning you don't have to move the scope to follow it as much.

I'll give an example.

I have 2 eyepieces of nearly identical focal length;

25mm plossl, with an apparent field of view of 52 degrees. In my 10in dob with a focal length of 1200mm, it gives me a magnification of 48x.

and

24mm Televue Panoptic, with an apparent field of view of 68 degrees. In my 10in dob with a focal length of 1200mm, it gives me a magnification of 50x.

To find the actual field of view that an eyepiece gives on your scope, you divide the eyepiece's apparent field of view by the magnification it provides.

So, my plossl would provide 52/48 = 1.08 degrees.

my panoptic would provide 68/50 = 1.36 degrees.

The result is that with the pan, I have 25.9% more actual field of view at the eyepiece.

Thus, you can see how the wider the APPARENT field of view, and the lower the overall magnification translates into a wider (and therefore easier to manage) actual field of view.

The absolute lowest you can possibly go for a telescope is to multiply the scope's f-ratio by 7 (7mm is the diameter of a fully dark adapted youthful eye. Any bigger than this, and not all light will make it into your eye, wasting it).

So on my f/4.7 scope (4.7x7 = 32.9). therefore, I wouldn't want to get an eyepiece with a focal length of longer than 32.9mm. If I did, not all the light would get in my eye, and because it's a newtonian, the secondary mirror might appear to obstruct the view.

I have to say that good wide angle eyepieces are not cheap, but they will make your life with a dob easier, and they provide some excellent views of the sky. I strongly recommend you try before you buy.

Some possible choices (listed from generally most expensive to least) include:

Pentax XWs
Televue Ethos
Televue Naglers or Panoptics
Vixen Lanthanum LWs

but there are a few others out there. Keep in mind though, that if you get a good, reasonably wide angle eyepiece at a proper focal length, you'll use it quite a lot, and keep it, even if you sell your scope to upgrade later.

[stevejack (Steve)]

To get mine the way I wanted I added a lazy susan bearing. That made it way too slick so I added some 3M stick-on felt pieces (the stuff you put under chair legs) to give it more friction. That cleared up my Az movement problems.
As for the Alt, I found that as long as I pulled the springs down onto the base (assuming yours has springs and not a tension knob??) that as long as the azimuth motion was smooth, I could put up with some minor jerkiness in the Alt. and just tried to compensate for any overshoot. I'd put the object to in the upper left and let it drift into the centre for example.

You'll never keep the object centred - without tracking, all you can do is put it in roughly in one corner or even slightly outside of the field of view, let it drift through, then repeat. Doing this also allows the scope to stop shaking by the time the object reaches the centre.

Good luck

[Rob]

Hi 0tt0maddox, the other thing you could try is removing one or even both of the altitude springs. If you remove both be careful it doesn't tip over and damage itself. Obviously this will only help with altitude movement, not azimuth.

Cheers,
Rob