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real amateur question on CG4 dec setting

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#1 nightowl

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Posted 07 January 2010 - 11:32 PM

Howdy folks,

I have a question that shows what happens when you use goto much too frequently. In this pic, shouldn't the zero be center-aligned to where the recessed screw area is between the two hex bolts? Currently it's at "10" as you can see in the photo. Am trying to brush up my use of setting circles and I've been off. :roflmao:

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#2 hudson_yak

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Posted 08 January 2010 - 09:38 AM

I can't really tell, is that the circle pointer over to the right?

Anyway, what you should do is polar-align the mount, then point the scope at an object with a known declination and rotate the circle (will have to loosen that set screw you see near the 16 degree mark) so that the indicator points to the correct reading. Make sure to take into account whether your object declination is + or - and set the ring accordingly. It should then indicate 90 when the scope is swung to point north.

Mike

#3 nightowl

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Posted 08 January 2010 - 09:06 PM

Hi Mike, thanks for the tips. I plan to do this tomorrow evening, and for the heck of it, utilize the sidereal method just to put everything in order. Clear skies to you... Jack.

#4 Adrian Lopez

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Posted 08 January 2010 - 11:17 PM

Where your mount says 10, mine is closer to 5 (between the 4 and 6 marks). I think the setting circles on the CG-4 are mostly decorative, and I haven't attempted to use mine.

#5 nightowl

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Posted 09 January 2010 - 01:21 AM

Hi Adrian, I have the same view also. With so many GOTOs mounts along with good easy to use dobs out there, I think some might no longer seriously consider the potential that could be built into the cg4 mount.

#6 hudson_yak

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Posted 09 January 2010 - 07:41 AM

Those circles will work but, being so small, accuracy of better than about 1 degree in declination and 5 minutes in RA will be a challenge.

Mike

#7 Starhawk

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Posted 16 January 2010 - 11:42 AM

The best thing to do is to start by doing a careful polar alignment (yes, level it and do a drift align). Then take the scope and point it at a unitary object with precise Declination and right ascension coordinate. This time of year, an ideal object would be Sirius.

Right ascension 06h 45m 08.9173s
Declination −16° 42′ 58.017″

So you can look at the coordinates on the mount and adjust accordingly. It will be - 16 and 2/3 degrees down. Or you can look around for something with a more exact even degree declination. For example, if you stay up late, you could use Pollux:

Right ascension 07h 45m 19.4s
Declination +28° 01′ 35″

This one is so close to 28°, there isn't much point in worrying about the 1/60 degree error on your scale. Just keep in mind Pollux is on the south side, Castor is on the north.

Personally, I have yet to find the setting circles on any of the four mounts I have had with them to be especially useful- I always find it easier just to release the clutches and just point by eye.

-Rich

#8 nightowl

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Posted 16 January 2010 - 12:08 PM

Hi Mike and Rich, thanks for your tips. I'll get to this soon enough--we southern Californians, after being blessed by warm weather when much of the US was experiencing blizzards, are now slated to receive a series of intense El Nino storms that will begin tomorrw and last much of the coming week! Clear skies to all of you.

Jack.

#9 John J

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Posted 16 January 2010 - 12:14 PM

Those setting circles are indeed mostly decorative. They are to small in diameter to really be useful or acurate. In the old days setting circles were some times 10" in diameter or more.

JJ

#10 Eddgie

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Posted 16 January 2010 - 12:40 PM

In the old days, refractors were f/10 and f/15, and eyepieces were .965 with 35 degree afov.

For small, fast refractors and reflectors, even thes small circles can be more than sufficent to put a target into a two degree field of view IF the user is careful to take the necessary steps to calibrate the circles properly and do things like remove the cone error and do good polar alignment.

Now, most observers don't want to DO all of those things every time they set up their telescopes, and I think that is why DSCs and Go-To mounts became so popular, but I have used setting circles like these before and easily put targets into the field of view.

Using the vernier scale (again, someting a lot of amateurs won't do) can get you within a degree of any target in the sky with no problem.

When used with small telescopes, It is not the size of the setting circles that limits their utility so much as it is the patience and persistence of the user.

Regards.

#11 Eddgie

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Posted 16 January 2010 - 12:54 PM

Isn't there an idex pointer on the mount?

Actually, the declination scale (to be accurate) needs to be set specifically to each mouted telescope. Just like with cone error where the polar axis and the optical axis of the telescope may not be perfectly perpindicular and has to be matched to each scope, the scale of the Declination axis may not line up precisely with the optical axis of the telescope.

The little screw hole in the picture you provided allows you to simply loosen the scale and make the needed adjustment.

To do this, you need to accuratly polar align and then move your telescope to a target of known declination near the meridian, center it as accuratly as possible, loosen the screw, move the dec circle to the right reading, and tighten it.

I think mostly, these circles are crudly aligned from the factory. It really is important to calibrate the dec circle to the scope it will be used with.

It is also vital to check the cone error of course.

Regards.

#12 nightowl

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Posted 16 January 2010 - 05:52 PM

Thanks for the 2 cents and informative tips fellas.

Jack.






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