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how to use a classic Celestron C8

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#1 ANM

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Posted 19 October 2020 - 03:57 PM

I've mucked around in astronomy on and off for 40-50 years. When I started I taught myself to use setting circles, but when they invented electronics I thought 'that's for me! no more trying to figure out sidereal time or changing J2000 RA/Dec to local coordinates.'

 

But now I find myself with multiple pre-electronics Celestrons so I'm making the effort to go back 40-50 years so I can use these telescopes as they were intended. I have a few questions.

 

So:

1. let's assume I can get the mount set up and correctly aligned on the NCP...

2. can I use the sidereal time app on my rotary dial phone, get the local sidereal time and set the meridian pointer and RA circle on my C8 to that time? Well I know I can, but is that the right way to do it? Here's a picture of the meridian pointer and RA circle:

IMG_2005.jpeg

 

3. after doing that I should not touch the RA circle setting again that evening, right?

4. to go to an object simply (I love that word!) rotate the telescope in RA to the object RA (not using the meridian pointer, but the one on the base of the fork), and set the telescope Dec circle to the object Dec. The object should now be centered in the eyepiece, right?

 

Okay now as hard as it is to believe, it is possible that I may get the alignment on the NCP a bit off, so:

1. if my first object is something bright and easily identified, but it's not centered in the eyepiece, I do not want to alter the RA/Dec of the telescope since it is already set correctly for the object, right?

2. what I need to do is alter the NCP alignment of the wedge until the object is centered, right? My wedge does have fine adjustments for that...

 

And I think that's it, right?

 


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#2 dcollier

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Posted 19 October 2020 - 04:39 PM

That's a way to do it.  Another way is if you can establish where the meridian is located.  Set that to zero on the scale and use it as a hour angle scale.  The problem with setting it to the local Sidereal time is that it is constantly changing so you have to keep readjusting.  You will be able to read the local hour angle straight from your planetarium App.

 

             -Dave


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#3 kathyastro

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Posted 19 October 2020 - 05:05 PM

You've got it!  I love those old fork mounts.  My single-arm C-90 mount has the same RA setting circle arrangement, where the circle is driven by the clock drive.  It is big enough that you can read it to the nearest minute, and you never have to re-set it all night.  Getting 120VAC in the field is a little awkward, but a small inverter takes care of that.

 

If you always observe from the same location, and always level the mount, you can keep the wedge angle the same.  Polar alignment it, as you describe, merely a matter of adjusting the azimuth.


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#4 ANM

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Posted 19 October 2020 - 06:11 PM

You've got it!  I love those old fork mounts.  My single-arm C-90 mount has the same RA setting circle arrangement, where the circle is driven by the clock drive.  It is big enough that you can read it to the nearest minute, and you never have to re-set it all night.  Getting 120VAC in the field is a little awkward, but a small inverter takes care of that.

 

If you always observe from the same location, and always level the mount, you can keep the wedge angle the same.  Polar alignment it, as you describe, merely a matter of adjusting the azimuth.

I am really getting into them also. I've had a couple of them apart and I love the machining of the parts, perfectly machined, and then anodized to minimize friction and give a hard wearing surface, some of the parts go together and there is no clearance but they go together easily, talk about precision! I guess that's what happens when your machining is done in-house.

 

I always use a battery (automobile lithium jump-starter) and an inverter, it's good for about 3-4 hours or so, and if I want to observe for a longer period I have 2 extra batteries.

 

99% of my observing is from the patio in my backyard so 'permanently' setting the altitude on the wedge is possible, and I know I'll wind up putting markers down which will take care of the azimuth. It will be like having a permanent pier.



#5 jgraham

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Posted 19 October 2020 - 07:10 PM

I never had much luck with setting circle even back when they were the only game in town. For me, nothing beats a good right angle finder...

 

Sandcast C8 Setup (3-23-2018)-1.jpg

 

...my all-time favorite star-hopper!

 

Give me a nice finder and a copy of the S&T Sky Atlas and I'm a happy camper. smile.gif


Edited by jgraham, 19 October 2020 - 07:10 PM.

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#6 charlesgeiger

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Posted 19 October 2020 - 07:54 PM

Also, make sure your declination circles are set the same (may need to adjust with the screw in the center of each) and read your proper latitude when the scope is pointed on polaris.  


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#7 Beeham

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Posted 20 October 2020 - 06:05 PM

I'll toss another idea in to the mix - this is what I did with my C8:

 

1) remove the aluminum RA circle, use it as a template, and cut another one the same size.  (Save the original in case you change your mind.)

2) Print out a new label for the new ring from step 1, graduated in degrees (not hours), stick it on, and reinstall in the mount

3) Stick a digital angle finder on the OTA, optionally on a fine-adjustment gadget like the one shown below.

 

You now have a fantastically simple Alt/Az setup.  The rotating azimuth (formerly RA) ring makes it easy to zero out to a known target.  I get alt/az coordinates in real time from a cell phone or tablet, point, and usually get the target in view of my 15mm eyepiece on the first shot.

 

If you don't like it, you can always go back to the original ring graduated in hours.

 

You can't do astrophotography, and you don't get tracking, but it's fast and easy to set up and (other than the AAA in the angle finder) doesn't require any power.

 

ref: fine-adjuster widget

inclinometer holder

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#8 ANM

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Posted 21 October 2020 - 10:56 AM

This is a report of how my first observing session (using the above procedure) went.

 

I made a sequential list of easily visible targets: the Moon, Jupiter, Saturn, Mars, and M31, all chosen because there would be no question whether I was really looking at it.

 

I had the C8 set up by 20:30, and I used my iphone to get the local sidereal time and set the RA circle to the meridian pointer. My first target was the Moon, I manually slew to the moon using the setting circles. It was maybe 5° off, in both RA and Dec so I used the fine adjustments on the wedge to center it in the eyepiece. I can't see Polaris from my patio so 5° off was not unexpected. Then I manually slew to Jupiter and it was off in the eyepiece! I assumed that I probably didn't get the wedge adjustments correct so I again adjusted it and then slew to Saturn. Saturn was way off! I started to wonder if I did something wrong. I again adjusted the wedge and slew to Mars. It was way off too... about then I glanced down at the inverter... wasn't there a light that came on when it was running normally? DOH foreheadslap.gif After switching it on started to run through the list again but then decided to call it a night...

 

So the upshot of all this is that this will work if:

1. The Dec setting circles are accurately positioned. If they are not I will be compensating for the error by adjusting the wedge.

2. I also think that leveling the mount, while not critical, will make later adjustments simpler.

3. And I switch on the inverter. 

 

I have some things to do but I'm going to try and get out again tonight.


Edited by ANM, 21 October 2020 - 10:58 AM.


#9 ANM

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Posted 21 October 2020 - 11:06 AM

I'll toss another idea in to the mix - this is what I did with my C8:

 

1) remove the aluminum RA circle, use it as a template, and cut another one the same size.  (Save the original in case you change your mind.)

2) Print out a new label for the new ring from step 1, graduated in degrees (not hours), stick it on, and reinstall in the mount

3) Stick a digital angle finder on the OTA, optionally on a fine-adjustment gadget like the one shown below.

 

You now have a fantastically simple Alt/Az setup.  The rotating azimuth (formerly RA) ring makes it easy to zero out to a known target.  I get alt/az coordinates in real time from a cell phone or tablet, point, and usually get the target in view of my 15mm eyepiece on the first shot.

 

If you don't like it, you can always go back to the original ring graduated in hours.

 

You can't do astrophotography, and you don't get tracking, but it's fast and easy to set up and (other than the AAA in the angle finder) doesn't require any power.

 

ref: fine-adjuster widget

I have thought about this but it almost seems as though it would be more difficult to use since if you're doing an extended observation of an object you will have to be using both slow motion controls at the same time in order to keep the object centered. If using a GEM with an RA motor then centricity happens automagically. Even a GEM without an RA motor you would only have one axis to adjust.



#10 Tom Stock

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Posted 21 October 2020 - 12:11 PM

... an easier method is to polar align, move to a known star and set the RA to the RA for the star and verify that the DEC is also close.

 

Done.

 

I used the setting circles on my C8 all the time. I loved them!  Move to RA (it's the most accurate) and then just pan in dec to find the target if it's not in the FOV.

 

Before leaving each target, set the RA (should not need to adjust DEC) again for accuracy.

 

Work from one side of the sky to the other if possible for best accuracy.


Edited by Tom Stock, 21 October 2020 - 12:52 PM.

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#11 kathyastro

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Posted 21 October 2020 - 12:43 PM

Is your sidereal time corrected for your location within your time zone?  It does need to be specific to your location, not a nearby city, especially since the setting circles can be read to the nearest minute.

 

With a wedge, levelling the mount is crucial to avoid having to adjust the wedge angle every time you set up. 

 

Check that the dec circle is accurately positioned when you set up.  Don't use the wedge angle to compensate for inaccuracy: that will make things worse.  To set the dec circle:

1. You do not have to be polar aligned for this.  All directions are relative to the mount, wherever it happens to be pointing.

2. With the RA axis set to an hour angle of 0, swing the scope in dec to point at some identifiable object north or south.

3. Note the declination of the object.

4. Rotate the RA axis exactly 180 degrees, and again point the scope at the same object.

5. Again note the declination. Both declinations should be the same.

6. Loosen the declination circle and, without moving the scope, adjust it to split the difference between the two readings.

7. Repeat the procedure until the dec circle reads the same on both sides of the fork.

8. Your dec setting circle is now positioned accurately.


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#12 ANM

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Posted 21 October 2020 - 03:22 PM

... an easier method is to polar align, move to a known star and set the RA to the RA for the star and verify that the DEC is also close.

 

Done.

 

I used the setting circles on my C8 all the time. I loved them!  Move to RA (it's the most accurate) and then just pan in dec to find the target if it's not in the FOV.

 

Before leaving each target, set the RA (should not need to adjust DEC) again for accuracy.

 

Work from one side of the sky to the other if possible for best accuracy.

One of my problems I have is that I can't see Polaris or the NCP, my house is in the way. That's why I was doing it by setting the sidereal time, going to an object, and then doing a final adjustment of the wedge. Once I can get the C8 reliably aligned on the NCP you procedure would work reasonably well. 

 

Somehow I have to make sure the Dec circles are set accurately. The only way I can think to do it is focus the C8 on a stationary, distant object, read the Dec circle, rotate 12hrs in RA, flip the OTA over, focus on the object again, and get the new reading. They should be the same if the Dec circles are set correctly.



#13 ANM

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Posted 21 October 2020 - 04:00 PM

Is your sidereal time corrected for your location within your time zone?  It does need to be specific to your location, not a nearby city, especially since the setting circles can be read to the nearest minute.

 

With a wedge, levelling the mount is crucial to avoid having to adjust the wedge angle every time you set up. 

 

Check that the dec circle is accurately positioned when you set up.  Don't use the wedge angle to compensate for inaccuracy: that will make things worse.  To set the dec circle:

1. You do not have to be polar aligned for this.  All directions are relative to the mount, wherever it happens to be pointing.

2. With the RA axis set to an hour angle of 0, swing the scope in dec to point at some identifiable object north or south.

3. Note the declination of the object.

4. Rotate the RA axis exactly 180 degrees, and again point the scope at the same object.

5. Again note the declination. Both declinations should be the same.

6. Loosen the declination circle and, without moving the scope, adjust it to split the difference between the two readings.

7. Repeat the procedure until the dec circle reads the same on both sides of the fork.

8. Your dec setting circle is now positioned accurately.

I'm using an app called: Sidereal Time v1.7 on my iPhone which uses my location longitude to come up with the time. The longitude it's using is within 2-3 seconds of that reported by various Internet applications so I assume it is correct.

 

Today I spent almost 3 hours adjusting and getting my tripod level, setting the Altitude on the wedge, and marking the position on the patio for the tripod feet.

 

Thanks for the Dec procedure Kathy, that is what I'm going to use! Even though I messed up last night I could tell that the alignment and observing procedure will work, I just have to remember to turn the inverter on...



#14 ANM

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Posted 21 October 2020 - 05:45 PM

And I have set both Dec circles. If it's clear tonight I'll give it another try...

 

P.S.:

As soon as it was dark enough I set up the C8 on my patio; the patio was marked where to place the tripod feet, then the C8 went on the tripod, I plugged it in, and switched on the inverter!!! Using my iphone I got the LST and turned the RA circle to align with the meridian pointer.

1. I manually slew to the celestial coordinates of the Moon, and adjusted the wedge (it was only off a little) Az and Alt to center it in the eyepiece.

2. Then I manually slew to the celestial coordinates of Jupiter, it was off by about 1-1/2°. At this point I don't know what constitutes acceptable.

3. Saturn was next, it was also off by about the same amount.

4. Ditto for Mars.

5. Next was M31 which was harder to visually locate after slewing than a nice bright planet. After moving the scope around a little I noticed M31's faint fuzzy blob moving in the field. It too was within 1-1/2° to 2° of the center of the field.

 

I have a couple of problems:

1. Most objects I slew to were off by ~the same amount and ~same direction which to my interpretation indicates a possible setup error.

2. The Dec setting circles are subject to interpretation, as well as being difficult to setup accurately.

3. I'm not sure how much atmospheric refraction (is that the right term?) is going to alter the coordinates of an object.

 

I did not try to adjust the Az/Alt settings after the first slew to the Moon, but I think I should, although with atmospheric refraction I may end up chasing that error back and forth and never accomplish anything.

I mights try some later, Ultima fork arms with their altered Dec setting circles, seems like they maybe more accurate as well as easier to setup.


Edited by ANM, 22 October 2020 - 11:27 AM.


#15 ANM

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Posted 23 October 2020 - 01:26 PM

...

I did not try to adjust the Az/Alt settings after the first slew to the Moon, but I think I should, although with atmospheric refraction I may end up chasing that error back and forth and never accomplish anything.

...

If I go to an object close to the zenith would it have minimal atmospheric refraction? 



#16 rmollise

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Posted 23 October 2020 - 01:37 PM

I've mucked around in astronomy on and off for 40-50 years. When I started I taught myself to use setting circles, but when they invented electronics I thought 'that's for me! no more trying to figure out sidereal time or changing J2000 RA/Dec to local coordinates.'

 

But now I find myself with multiple pre-electronics Celestrons so I'm making the effort to go back 40-50 years so I can use these telescopes as they were intended. I have a few questions.

 

So:

1. let's assume I can get the mount set up and correctly aligned on the NCP...

2. can I use the sidereal time app on my rotary dial phone, get the local sidereal time and set the meridian pointer and RA circle on my C8 to that time? Well I know I can, but is that the right way to do it? Here's a picture of the meridian pointer and RA circle:

attachicon.gifIMG_2005.jpeg

 

3. after doing that I should not touch the RA circle setting again that evening, right?

4. to go to an object simply (I love that word!) rotate the telescope in RA to the object RA (not using the meridian pointer, but the one on the base of the fork), and set the telescope Dec circle to the object Dec. The object should now be centered in the eyepiece, right?

 

Okay now as hard as it is to believe, it is possible that I may get the alignment on the NCP a bit off, so:

1. if my first object is something bright and easily identified, but it's not centered in the eyepiece, I do not want to alter the RA/Dec of the telescope since it is already set correctly for the object, right?

2. what I need to do is alter the NCP alignment of the wedge until the object is centered, right? My wedge does have fine adjustments for that...

 

And I think that's it, right?

Why make life complicated? Polar align the scope, point at a star of known RA and set the RA circle.


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#17 ANM

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Posted 23 October 2020 - 02:22 PM

Why make life complicated? Polar align the scope, point at a star of known RA and set the RA circle.

Hey Uncle Rod, 99.99% of my astronomy is done in my backyard and the house blocks access to the NCP.  I've marked the patio for the tripod feet so once I get the wedge set correctly I shouldn't have to do any adjusting ever after.

 

I've got a polar scope on the mount so if I had NCP access I'd be all set...



#18 Tom Stock

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Posted 23 October 2020 - 04:29 PM

Hey Uncle Rod, 99.99% of my astronomy is done in my backyard and the house blocks access to the NCP.  I've marked the patio for the tripod feet so once I get the wedge set correctly I shouldn't have to do any adjusting ever after.

 

I've got a polar scope on the mount so if I had NCP access I'd be all set...

Use a compass... compensate for magnetic variation to point true north.  You can align edge of compass card with side of wedge, or OTA if set centered.

 

use an angle meter on the OTA to measure your tube angle to set latitude

 

Move to a known star and dial in RA

 

You could drift align after this but it's really not necessary. You only need the setting circles to get you close.


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#19 charlesgeiger

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Posted 24 October 2020 - 09:30 PM

To Uncle Rod's comments; yes, that is what I did.  Just polar align and point the scope to a know RA and Dec and set the hour circle.  Also, make sure the Dec circles are both set at the same setting as the circles easily rotate and can be adjusted with a cross type screwdriver.  All my scopes came new with the Dec circles a little off from each other.

Charlie



#20 RalphMeisterTigerMan

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Posted 25 October 2020 - 08:14 AM

What-ever you do, do NOT under any circumstances mix the Classic 8 with the New celestron 8. Terrible things could happen. As long as you keep the NEW Celestron 8 far from the CLASSIC Celestron 8, you should be fine.

 

Oh the 1980's!

 

Clear skies!

RalphMeisterTigerMan


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#21 ANM

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Posted 27 October 2020 - 11:19 AM

And I too have a bit of advice: my wedge has a small built in level but it is at least 2-3 degrees off. So Uncle Rod's method would probably be best...




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