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Need some advice on alignment and tracking

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#1 mostlyemptyspace

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Posted 26 March 2014 - 11:23 PM

I've had my Nexstar 8SE for a few months now. I've never gotten the goto and tracking to work well. I've read several articles and tried many things to get it to work better.

So I started by using a level app on my phone every time to make sure the mount is level. I use the exact GPS coordinates, and the time down to the second. I then use the SkyAlign mode and pick three stars quite a ways away from each other. I often choose Sirius, Betelgeuse, and Capella. To align, I start with a 21mm EP and track to the star. I make the scope out of focus so I can align the donut with the field of view. I then hit Enter, and replace the 21mm EP with an 8mm EP. I align again and hit Align. I then put it in focus to confirm that it's centered. I do this for all 3.

So after all this, the goto gets in the neighborhood of the target, but I always have to do some adjustment to get it in view. It's just so so.

As for the tracking, it pretty much sucks. At high power, Jupiter goes out of view within a minute or so. If it's tracking at all, I can't tell.

So I tried messing with the anti-backlash settings. I used very low numbers like 05 or so, and any amount of improvement in inertia causes it to jump. I wasn't able to get it to react any faster without making it jump, so I just set it back to 00 for all 4 settings.

So, is there anything else I can do to improve the goto and tracking, or do I just have a *BLEEP* mount?

#2 Tel

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Posted 27 March 2014 - 05:03 AM

Hi Mes,

It sound as if you are doing all the right things but there remains a doubt in my mind that, (and forgive me if I'm wrong), your 8SE has not been optimised to achieve maximum GoTo accuracy.

Again forgive me if I'm wrong in making the following statements and suggestions, but:

1) Have you balanced the 'scope off the mount and then replaced it with a necessary small amount of imbalance; (i.e. either with a weight bias towards either the fore-end or rear-end) ?

2) Assuming the above and the tube has been placed back in the mount's dovetail to give a weight bias in either of these directions, have you selected the appropriate "GoTo Approach" Altitude setting to complement your choice. (Normally fore-end heaviness associates itself with a "Negative GoTo Approach" Altitude setting while rear end heaviness, with a Positive one. Perhaps check ?

3) You don't say where you are located but if in the Northern Hemisphere, then a Positive "Azimuth GoTo Approach" setting must be selected. (Conversely, if you live in the Southern Hemisphere, a Negative one).

4) These settings, plus the necessary imbalance of the OTA will also affect your backlash, .... (which incidentally needs to be set with a full quota of the usual accessories and be tracking),.... and hence your GoTo accuracy.

Additionally, your choice of Altitude GoTo Approach will determine how you must centre alignment stars and objects in your eye piece(s).

I don't know whether you've read this text, (see below), but it may help if you are not quite au fait with what is required to tune these 'scopes to perfection. Here's the link. If there's anything you don't understand then please do ask !

http://www.cloudynig...&Number=5421776

Best regards,
Tel

#3 mostlyemptyspace

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Posted 27 March 2014 - 11:34 AM

Hi Tel. Thank you for your thoughtful response.

I just balanced my tube, and I definitely had a significant forward weight bias before. With my heavy Baader Hyperion EP's, I had to move the tube forward so the bottom of the dovetail bar was flush with the mount arm.
When you say to give it a small forward weight bias, do you mean like 1cm or so?

As for the approach, they were both set to positive, so I set the alt approach to negative.

As for anti-backlash, it seems that the slightest increment above 00 causes it to jump. Even 03 or so. Do I need to increment positive and negative evenly, or what's a good strategy?

What do you mean by "your choice of Altitude GoTo Approach will determine how you must centre alignment stars and objects in your eye piece(s)"? I usually follow the method in the link you posted. I defocus the object and center it with a 25mm EP, then center it again in an 8mm EP. That's about it.

There is one thing in that post which concerns me. The last paragraph says it will track poorly when viewing objects in the western region of the sky. Well, that's where everything is right now! Orion sits directly to the SW, so all the fun stuff is in that area. Does it make a difference how I orient the mount? In other words, could I rotate the mount more than 360 degrees without causing the gears to lock up?

#4 barbarosa

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Posted 27 March 2014 - 12:49 PM

So, is there anything else I can do to improve the goto and tracking, or do I just have a *BLEEP* mount?

Things that you might consider and try:

Three star alignment is not more accurate than two star. The third star is a pass-fail check. Two star auto align should work just as well.

Balance—confirm whether this is a factor by either moving the OTA rearward (EP toward you and away from the dovetail clamp) or checking the result or put in a lighter EP such as the one that came with the scope.

Anti-backlash— Try it again making small incremental changes. The setting you want is one that does not cause the target to jump and still shows movement within a few seconds.

Alignment—setting the time to the nearest second will not have a practical impact. The apparent movement of the stars is once around every about 24 hours or 15° /hour, or .25°/minute or .004° /sec. Similarly close is good enough for location.

Leveling—in alt-az mode a level mount should make the slew to the second alignment star more accurate, but is not going to effect go-to accuracy if the mount is “eyeball” level.

Try the best pairs list.

Calibrate Go-to from the menu. Do this after alignment.

Less likely but possible even on new scope is play because the arm was not correctly tightened at the factory. I have seen Nexstar mounts with visble play in the arm. It may flex but should not rock. An easy fix if you are up to removing the cover. Even less likely is the arm is not orthogonal to the base. At least one such case was reported on CN.

Celestron says that pointing accuracy “For Celestron’s NexStar 6 and 8SE mount and similar mounts like the 8i and i-SE, this is usually about 5-10 arc-minutes (300–600 arc-seconds), well within the mount’s specification of 20 or so arc-minutes.” They go on to say this, “Flexure of the mount and optics can cause the pointing accuracy to be off by arc-minutes over the entire sky. Additional error is caused by backlash.”

Celestron does not promise spot on accuracy from the Nexstar mount, 5 to 10 arc-minutes usually and up to 20 arc-minutes with additional errors possible. A full moon is about 30 arc-minutes, so the factory really is saying that what we can expect is that the target will be within the FOV of a 25mm Plossl.

*BLEEP* mount? Unlikely. Always spot on? No, even the CPC series is not quite that good.

#5 mostlyemptyspace

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Posted 27 March 2014 - 01:30 PM

Thanks Barbarosa. So a couple things, what is the best pairs list you mentioned?

The GoTo isn't that bad, it gets in the neighborhood, and a little bit of slewing will find it. The main problem I see is tracking. If I say, go to Jupiter and pump up the power using an 8mm or so EP, it won't stay in the FOV for more than a couple minutes.

I've also tried taking some AP of M42 using my DSLR and an f/6.3 focal reducer. In a single 30sec shot, it could get a good frame, but between shots, I noticed M42 was noticeably moving. So after about 5 shots it was out of the frame. I'm not talking about field rotation here, it just crept slowly out of the FOV.

#6 Tel

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Posted 27 March 2014 - 02:28 PM

Hi Mes,

I'll take each of your comments and concerns in turn. I might however duplicate a lot of what David has advised.

However, for what I can contribute, here goes,........

1) As long as the imbalance, (in your case a chosen fore-end "heaviness"), is not over-excessive, (and many owners ensure that whatever diagonal and EP combination they use will not make contact with the base of the mount), then this will be fine.

Therefore, move the loaded OTA, (i.e. with finder, dew shield, diagonal and EP etc.), forward in the dovetail clamp to achieve this.

Aim for a point which will give about a half an inch of clearance between the diagonal/EP and the battery container of the mount when the OTA is at the Zenith.

Note that the ability of the OTA to move in altitude with no tendency to slip or fall must be determined by taking the OTA to the Zenith and then bringing it smartly back down to the horizontal, (slew rate 9).

If there is any suggestion that the axis might be "loose" observed by the OTA failing to stop abruptly when the direction button is released, then adjustment of the altitude clutch will be necessary.

It surely goes withiout saying that in bringing the OTA smartly back down to the horizontal will require you to place a hovering hand beneath it.

2) Assuming all according to 1) to be in order, I would propose, (as you have already done), setting your Altitude Goto Approach to Negative. There is no reason why you shouldn't experiment with this setting, (i.e. trying a Positive setting), but leave it set any experimental change to a future date.

3) Antibacklash:

You will inevitably find that the Altitude axis is far more sensitive than the Azimuth so it's important to get this just right.

You don't say whether your original attempts were made with a fully loaded OTA and/or whether the 'scope was tracking at the time or indeed how you carried out the exercise but as David advises, I would review the settings, using the terrestrial "brick wall" grid to aid the process.

Increment each setting and direction equally but separately, (i.e. getting one single direction "right" at, say, the Positive value before moving to the Negative. The link I provided should tell you how to go about this.

Don't think though, that if you have established a particular Positive value, the Negative value MUST be the same. It may well turn out to be so, but not necessarily.

4) Choice of Altitude GoTo Approach / Centring in the EP:

You will notice that if you have selected a Negative Altitude GoTo Approach setting, your 'scope will finish its slew to any object with a downward movement of the OTA. This motion will take up backlash in the gears. To maintain this "backlash absorption", rather than recreate it, it is imperative that in centring alignment stars and objects they are moved within the EP's FOV in a certain way.

This, in the case of an adopted Negative GoTo Approach, suggests you place the star or object in the bottom left quadrant of the FOV of your EP and then move it, using the HC's direction buttons, to the right and upwards to centre it. If you overshoot the centre of the EP, just return the object to the bottom left quadrant and begin the process again.


5) As to tracking in the Western sky; some time ago, I discovered that there was a slight tendency for objects to begin to "slip" from the FOV within an unacceptable time span when my old Nexstar 8i was up and running. I therefore submitted the attached article on the topic to CN.

It was based on my own setting of a Positive Altitude GoTo Approach but essentially advises that if you have aligned your 'scope using a Negative setting and thus have been centring objects with a right and upward movement, you can counter any slippage in the Western Hemisphere by reversing this action.

Here's the link to the article:

http://www.telescope...hp?item_id=1697

Hoping the above helps you a little further,

Best regards,
Tel

#7 barbarosa

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Posted 27 March 2014 - 09:06 PM

...what is the best pairs list you mentioned?

The GoTo isn't that bad, it gets in the neighborhood, and a little bit of slewing will find it. The main problem I see is tracking. If I say, go to Jupiter and pump up the power using an 8mm or so EP, it won't stay in the FOV for more than a couple minutes.

I've also tried taking some AP of M42 using my DSLR and an f/6.3 focal reducer. In a single 30sec shot, it could get a good frame, but between shots, I noticed M42 was noticeably moving. So after about 5 shots it was out of the frame. I'm not talking about field rotation here, it just crept slowly out of the FOV.

http://www.ilanga.com/bestpair/

I agree that you should not have to slew around to find a target. Are the errors consistent by any chance, e.g., always low or left or high?

Once you center a target, the mount should track with little or no perceptible drift for many minutes. When a target drifts and the drift slows or stops after a time the anti-backlash, settings become the prime suspect.

I should try doing the best alignment that you can, sticking with 3 star or which ever you use most, then do the Calibrate Go-to. If either the go-to error or the drift persists then try changing your setup and alignment routine.

Setup the tripod keeping the legs as short as you can and setup on a paved area if possible. It is not necessary really but if you want to level the mount level the tripod not the mount and do not use a bull’s eye level.. Make sure that each of the three of the hold down screws is snug. From experience, two out of three is not good enough.

If there is a city on the site setup list that is within 50 miles, use it rather than your coordinates.

Check to ensure that your time source is within a minute of being correct.

A star’s apparent movement is 260°/ 24hrs or 0.0042°/ second. A one-minute error is 0.25° and would be within the FOV. The same idea applies to location. An error of several miles will not by itself put objects outside the FOV. One working definition of the knot is a minute of latitude/hr.

Try all three methods in succession, one star, two stars, and two star auto.

Clearly, the idea of switching setup routines is to see if the problem is in the mount or next to the mount. Pilot and operator error really do happen, I could tell you an embarrassing story. :crazy:

Alternate diagnostic procedure-- Get someone else to read the manual and set up the scope.

Alternate diagnostic procedure 2—check with a local astronomy club for another SE owner; experienced help might be close by.


#8 mostlyemptyspace

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Posted 28 March 2014 - 12:10 AM

Ok there's a lot of great feedback here. I'm going to test it out once this *BLEEP* weather passes and get back to you. Thanks!

#9 astronomania

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Posted 28 March 2014 - 01:13 PM

One thing that I would add, is when centering alignment stars,
defocus enough so that the star makes a large doughnut.
This will make centering a breaze.
Otherwise your eye doesn't have a good point of reference. :)






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