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Can't Center Polaris after Aligning IEQ45

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#1 Lew

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Posted 03 November 2012 - 02:37 PM

A question from someone who is very much a newbie and has never used a GEM before. I've had the IEQ45 out five times since I got it last month (living in Pgh, the norm is cloudy skies esp. at this time of the year). I've tried it twice with the 80mm CFT, twice with the 120mm EON and latest was with the C11"XLT. I'm using them on a wooden deck off of my home. Each time I use the polar scope, then put the telescope on the mount and then did a two star align. The goto is okay (typical error reading is 00-02 low and 00-07 east with the 80mm and higher with the 120mm and higher yet with the C11" - the weight of the latter two seems to have some effect). I've read the need to check polar scope alignment but will not have a clear sight during the day to do this until the leaves fall off some oak trees. Each time I've tried at some point to look at Polaris. With the 80 mm it is in the FOV but low. With the 120mm it is low and barely in the FOV and with the C11" not in the FOV. I tried using the arrows on the controller to center it and no matter which direction I try, it will not center - basically Polaris stays low in the FOV. Why can't I get Polaris to center in any of the telescopes? Both the C11" and the 120mm have two knobs at opposite ends of the dovetail on the scope. I'm not sure what those are for or why they are there (other than it would prevent the scope from sliding off the mount). Do they serve any other purpose?

Thanks for any advice.

Lew

Ioptron IEQ45M; Celestron 11" XLT; Orion 120mm EON; Orion 80mm CFT; Meade ACF LX 90; Meade ACF LS-6; Meade ETX 90PE. Various Celestron X-Cells and Meade 5000 HDs, Orion 12mm reticle, Astrotech 1.25" dialectric diagonal, Antares f/6.3 SCT reducer and Meade plossl set. Orion Mini-guider package; Orion Off-axis guider. Meade DSI II color and Orion Starshoot Deepspace video camera. Maxim DL v.5.

#2 mgwhittle

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Posted 03 November 2012 - 02:43 PM

Have you tried telling the mount to go to Polaris? I'm not sure what you are trying to accomplish, are you trying to center Polaris because you are checking polar scope alignment or seeing if your telescopes are aligned with the RA axis of the mount? Those are two different issues.

If you want to check your polar scopes alignment, you need to center Polaris in the CENTER of your polar scope then rotate the RA axis as much as you can in either direction and see if Polaris stays centered. As far as getting Polaris centered in the telescope, that is another issue altogether but does not necessarily let you determine if your polar scope is aligned with the RA axis.

#3 Phillip Easton

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Posted 03 November 2012 - 03:30 PM

It could be due to cone error. Try looking at this procedure here.

http://www.youtube.c...h?v=WatdQlPp22Y

Cheers!
Phillip

#4 Phil Sherman

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Posted 04 November 2012 - 08:59 AM

Don't forget that Polaris is located around 3/4 degree away from the pole. It's location in the scope with the mount in the home position will depend of the FOV of your scope/eyepiece combination.

Phil

#5 Cliff Hipsher

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Posted 04 November 2012 - 09:07 AM

Lew:

You're making some rookie mistakes.

1. Get thee to solid ground. The deck may be convenient, but you need to put the tripod on stable ground.
2. You need to attach the OTA to the mount and then do your setup, starting by leveling the mount, balancing the OTA, and then do your visual Polar Alignment.
3. You are fighting Cone Error. Watch the video...

You can check the polar scope alignment during the day. You need to set the mount up so you can see a terrestrial target. Put the center cross on the target. Rotate the mount in RA and watch the target. If the field wanders, or the cross will not stay on the target then you need to bore sight the scope. Consult the user manual or call the manufacturer...

#6 Midnight Dan

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Posted 04 November 2012 - 09:48 AM

Hi Lew:

Just to explain a bit more about cone error. Imagine replacing your polar scope with a green laser pointer and aiming your mount so the pointer hits the wall of some distant building at night. If you spin your mount around the RA axis, and if everything was perfectly aligned, the laser beam would not move and should always look like a perfect line.

However, if your laser pointer is off kilter an not aligned with the axis of rotation, then when you spin the mount around RA, the laser pointer will move in a circle on the wall of the building. The beam itself will no longer stay in one place like a line, but instead will create a "cone" when you spin it. Hence the term "cone error". And if you think about it, there is no place you can aim your mount to make the beam point at the middle of the circle on the wall. Sound familiar? :grin:

Cone error can occur in two places. As Cliff points out, the polar scope can be misaligned and experiencing cone error. Or the main scope can be misaligned and experiencing cone error. Each of these is independent and will not affect the other. If the polar scope has cone error, then your polar alignment will be off and your tracking will suffer. If the main scope has enough cone error, then you will not be able to reach polaris, as you are experiencing, and your tracking will also suffer.

You should check both situations and compensate for each. For the polar scope, there are three set screws around the mounting ring that allow you to adjust its aim relative to the mount. Spin the RA axis back and forth 180° and adjust the aim so that the target in the cross hairs doesn't move.

The video that Phil linked shows an excellent technique for adjusting out cone error for the main scope. Note that the adjustment occurs at the dovetail on the scope. This means if you use more than one scope, you'll need to adjust each individually, and if you remove the dovetail you'll have to readjust when you replace it. You may find that ALL of your scopes are off a bit in the same direction. This most likely means that the mount itself has some cone error and therefore all the scopes are experiencing the same error. You might be able to adjust that out by shimming the dovetail saddle, but it's still probably easier to just adjust all the scopes.

-Dan

#7 Astronewb

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Posted 04 November 2012 - 07:27 PM

Lew, you can't center Polaris because Polaris is always +/- 41 degrees from NCP. Polaris is always rotating around the North Central Pole point, and the iOptron polar sight reticle offsets Polaris to the correct position so your mount axis IS pointing to the NCP. Any variation from this is referred to as 'Polar Offset'.

Heres a post from another thread that is relevant, I couldnt figure out how to link it, sorry:

A accurate initial polar alignment is critical, like the foundation of a house.

Some suggestions, make sure your polar scope is accurately adjusted and aligned. There are several files on the iOptron Yahoo group explaining and illustrating how to do this.

Using appropriate levels, square the mount in both axises. To minimize cone error in RA, set up your mount with the counterweight shaft aligned with the front (north) facing leg..use a plumb bob off the shaft perfectly intersecting the front leg and mark your RA basic setting. I use a small white tag on the RA joint, put a fine line on it and cut it at the joint with a razor knife, do the same with an OTA in the saddle and mark the DEC joint the same way. Now you have a 'base' repeatable setting.

When performing a PA, to double check..center Polaris in the crosshairs in the polar scope, with an EP in the OTA, Polaris should be centered in the EP also. It may be off vertically, but you can't do anything about that unless your OTA plate has cone error adjusting screws. If it's off significantly horizontally in the EP, adjust the DEC manually and remark the joint. Turn the mount on and off to set the zero position again.

Now you can adjust Polaris using the handcontroller position indicator to the correct position using the alt/az adjusters. Perform the 2 star align, or, if you are going to be slewing from Capella to Betelguese, perform the 3 star align routine for better pointing accuracy. I use the 2 star routine because I can't always see the 3d star from my viewing location.

After each star align routine, return to the 'zero' position after adjusting the polar offset before doing another iteration.

When you get to '00-00', or as close as you can, tighten the alt and az adjusters with the allen key...they will slip if just finger tightened, and it will ruin your perfect alignment. (I replaced the thin nylon washers with 1/8" nylon washers for this reason)

Re; synching to stars. Any synch to a star will override your 2 or 3 star routine. It will improve pointing accuracy in that section of sky. I'm always synching to stars since I primarily image and need to frame up my targets..when I slew to the next target for imaging, it's always in my camera's FOV (2.25 x 1.50 degrees) so it works for me.

Once your mount is setup and marked as described, every time you set it up it will become easier and faster with better results.

Sorry for the long post, you probably know and/or do all of this, I just wanted to touch all the bases.

My mount is the V2 with the 8407 controller also.


Cheers,

Paul

#8 Midnight Dan

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Posted 04 November 2012 - 07:34 PM

Hi Paul:

Lew is talking about not being able to center polaris with the main scope, not the polar scope. If the mount is properly aligned and does not have cone error, you should definitely be able to move the mount in such a way as to center Polaris in the main scope.

-Dan

#9 Astronewb

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Posted 04 November 2012 - 07:46 PM

Hi Paul:

Lew is talking about not being able to center polaris with the main scope, not the polar scope. If the mount is properly aligned and does not have cone error, you should definitely be able to move the mount in such a way as to center Polaris in the main scope.

-Dan


Right Dan, info on centering the OTA is in the info above. Basically, first start with centering Polaris in the 4' center ring in the polar scope. This must be done with a perfectly leveled mount with the front facing leg pointing true north.

Once you're at that point, then align the OTA to perfect center (side to side). You CAN NOT do anything about vertical alignment (top to bottom) UNLESS you have cone error adjusting screws built in to your OTA plate. (My Comet Hunter has these...very convenient).

Bottom line, you will always be able to obtain a good polar offset number using these numbers, and the position of Polaris vertically, in the OTA is not relevent. Once you are polar aligned, you can center Polaris in the OTA by selecting Polaris as the target object, and watching the mount slew in RA and DEC until Polaris is in the center of your FOV.

Hope that clears it up a little...?

Paul

#10 Dan Finnerty

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Posted 05 November 2012 - 12:22 AM

Hi Paul:

Lew is talking about not being able to center polaris with the main scope, not the polar scope. If the mount is properly aligned and does not have cone error, you should definitely be able to move the mount in such a way as to center Polaris in the main scope.

-Dan


However, moving the main scope may involve more contortion than Lew may be thinking.

Imagine starting at the traditional Counter Weight Down (CWD) position where the counterweight shaft is pointed down to its lowest position, and the telescope is pointed straight north. It will be pointed at the pole if the mount is accurately polar aligned (and there is no Declination cone error). The telescope will be at an Hour Angle of 6 hours.

Now imagine that Polaris is at an hour angle of -6 hours, I.e. directly below the pole by its 3/4 degree offset from the pole. You will need to slew the telescope 180 degrees (12 hours in RA) to be able to center Polaris in the telescope field of view. Now, depending on the FOV of your eyepiece, you may be able to see Polaris in the eyepiece ( in this case you would need a FOV 1.5 degrees or more ( 2 x 3/4 Degrees) when you are pointer to the north pole, but you cannot center it in the eyepiece unless you slew to the correct Right Ascension. In practice Polaris can be at any angle relative to the meridian depending on when you are observing. You just have to look up where it is to know what angle you need to slew the telescope to (or do a goto if your mount supports it)

(I have rounded the declination of Polaris to 89 1/4 degree for simplicity)

Did I make any sense or just confuse the issue?

#11 Lew

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Posted 05 November 2012 - 06:20 PM

Phillip,

I watched the video and it is very educational and helpful.

Thank you.

Lew

#12 Lew

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Posted 05 November 2012 - 06:55 PM

Dan & Paul,

Thanks for the additional explanation. I've read some of your and Paul's other posts on aligning the polar scope (and various other tips on how to use the IEQ45) as well as well as some in Yahoo and am going to check out the polar scope alignment (I tried same over a week ago, but now realize from a post that I should have used the controller to rotate the polar scope rather than move the entire axis manually).

With regard to the OTAs I've guessed from the video that the knobs in the middle at each end of the dovetail on the Celestron 11" must be for cone error adjustment since those bolts don't attach it to the OTA but if tightended would impact the tube itself. The 120mm EON similarly has two bolts at each end that do not attach the dovetail to the rings and if tightened hit the tube. Am I correct in guessing I would use these and not have to insert shims?

I was trying to look at polaris not for any alignment reason, but because I'm used to starting aligment with my LX90 by first centering Polaris in an eyepiece and I knew I had the correct star when I could see its double. I was curious to see how its double star looked in the C11" and the EON120.

Thanks for all the advice.

Lew






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