Why is my goto not accurate after drfit alignment?
I am wondring if anyone can help me? I have recently just installed a meade ultrawedge on my LX200gps 10". I am using WCS software to drift align.
1. Position scope facing true north (purchased a separate compass from orienterring shop) (compensate for magnetic declination by 12 deg to left facing north). I think I am in the truth north.
2. Level mount using bubble on wedge
3. Do GPS fix
4. Do drift alignment with WCS (Pretty sure I am accurate with this as the software is telling me I have achieved it - no visible drift present)
5. Put the scope in Polar alignment mode by slewing the OTA to the south with Dec reading 90deg
6. Choose two star alignment and the scope slews pretty close to the chosen star both times. Then I center using a ccd for accuracy.
7. Autostar then tells me align successful BUT when I choose a goto object their is about 5 deg miss on the object everytime. So you have to use the arrows to slew and "find" the object. This is really bad as when imaging distant object you need to be sure the goto is accurate.
One thing I did realise is that the MEade Ultrawedge manual instructed me to set my Dec Lat angle for my area, but others on the web say you minus your lat angle from 90 deg and set the wedge at that! Maybe this is why I am out?
Any help on this would be appreciated!
Last edited by CoolhandJo; 10-04-2009 at 11:31 AM.
Reason: Updated information added
I think you need to adjust the WCS instructions to reflect you are in southern hemisphere. I'm assuming you have your wedge polar axis pointing true south - not true north?
I've not used a fork mount or wedge but assume you need to tell it you have a wedge - is this polar alignment mode?
Paul, you need to make sure your mount forks are pointing south not North and that the scope has been rotated so that the finderscope is on the underside and the OTA is parallel to the forks pointing south.
Getting an LX200 to achieve good gotos especially in polar mode on a wedge can be quite difficult. This is quite a common problem with the LX200 especially the earlier versions. The first thing you need to do is to get all the rotational axes orthogonal (at least within an arc minute or so). If your axes are not orthogonal you will never get good results. Forget SMT it only makes it worse.
The Meade telescopes very often are supplied with the dec pivot on one fork arm up to 3mm higher than the other. I have seen many like this. The Optical tube is then adjusted to try to compensate for this. In the two star alt/azm mode the error is often not noticed to be too bad but when you attempt to polar mount the errors become very noticeable.
The attitude of the telescope when mounted on a wedge takes the following form. With the optical tube pointing straight up in the forks (90 degrees) and the wedge set to your latitude the Optical tube will be pointing at the south celestial pole. The control panel will be down and if you stand in front of the control panel you will be looking north.
The instructions for polar mounting start off by asking you to establish the exact 90 degree position of the OTA. This is the most important part and if found correctly will make the rest of the instructions easy. But remember orthogonality is the key to good alignment especially in polar mode.
The actual position of the finder scope during alignment will depend on where Sigma Octans is however the starting position will have the finder scope on the underside. The second alignment star will always end up with the finder scope on the higher side and the OTA pointing pretty much up towards the zenith.
If all this is done correctly you can forget the drift alignment. I have not used a drift alignment for 12 years. If your mount gets set to the true south pole attempting a drift alignment will only make matters worse.
Barry
Last edited by Barrykgerdes; 10-04-2009 at 01:24 PM.
Thanks to all for the inforamitive responses. Thanks for the graphical posts Dennis!
So, I have concluded that my issue with goto is more to do with the precisness of my 90 deg angle and RA being level at start- ensuring true north is chosen, but also setting the OTA to the polar south as precise as possible.
Unfortunately I have to drift align as I have obstructed view of the south.
This means that I must do a two star align after I drift align.
Reading through Barry's informative post, it seems that I may never achieve 100% goto accuracy this way (as I enjoyed in Alt/ALz).
If you can't see the south pole you will need to try a slightly different method but first you must ensure that the orthogonality is good. Even not being able to see the south pole you can still get a perfect alignment provided this is so.
This is the method. Find your OTA 90 degree as per the book method and lock it there so it won't move till the scope is turned on and mark it so you can find it exactly again. A pointer clamped onto the fork pointing at a marker on the rear housing of the scope should give an accurate enough point. Line your wedge as close to n/s and -34 degrees latitude as you can. Set your RA to exactly 0,0 using the scale. You should be able to get this within about 5 arc seconds. Turn your telescope on and start the standard polar alignment. Accept 90 degree point and 0,0 by pressing enter. The scope will slew to Sigma Octans. When the slew is finished accept the position and press enter. The scope will now slew to your second alignment star. When the slew is finished centre this star with the wedge controls only.
This will get the wedge almost exactly correct. Now park the scope and turn off. Put the OTA back to the 90 degree point and the RA to the 0,0 mark again and repeat the procedure. This should get you a little closer.
Test your gotos and maybe now a fine tune with a drift alignment may or may not give an improvement.
As I said first up if the orthogonality is accurate this method will give extremely good results. It is how I do it in the field. I start from the park position (I have it very accurately marked) select a star near the zenith, Goto it, The GPS takes a fix and slews to the star. I adjust the wedge controls to center the star. This gives as good a polar alignment as I can get.
Barry
Last edited by Barrykgerdes; 11-04-2009 at 10:36 AM.
This is extrmely helpful and I really appreciate you taking the time to post this.
I will be definately doing this next time.
Thanks again.
Hi Coolhand jo
If you are located in Sydney and are prepared to bring the telescope around I can set up the orthogonality for you. It takes a couple of hours. Contact me by PM with your location etc.
G'day Coolhandjo
Before visiting Bazza
try this out
Hi
I checked out that doc. It is quite a standard method of doing a polar alignment and is similar to most that I have seen. It will work quite well if the telescope rotational axes are orthogonal.
If they are not it will produce the errors so many people find. The older method of attaching the fork arms to the base in Meade telescopes was often quite poor and produced the condition where the arms were not the same height or tilted to the left or right. This was often compensated by adjusting the OTA in its brackets to make it appear perpendicular to the base. The newer method now supplied uses a one piece lower fork that is much more reliable and I have not seen any of these that have the problem of uneven fork arms. This only leaves the OTA to be tested in its brackets.
The method that Meade uses to find the 90 degree position of the OTA will also show up the OTA offset in the amount the test star drifts from left to right and back while finding the 90 degree point. Once the fork arms are corrected.
It is possible to get the axes within an arc minute of truly orthogonal with a little patience and an effective method of measurement. This done even the simplest of polar alignment procedures will present good goto's. On my permanent mount I could goto anywhere at any time and have my object within 4 arc minutes of the centre of the eyepiece.
I have been advocating the necessity of orthogonality in Meade fork mount telescopes for 13 years now and have received all sorts of negative response from the so called gurus on other forums who said it was not necessary and that you could correct the goto problems with the SMT extra or other ways. Good engineering practice says you don't fix an error with another error, you correct the first error.
Barry
Last edited by Barrykgerdes; 11-04-2009 at 12:18 PM.
Just putting my 2 cents worth in I also have the 10" meade on an ultra wedge, I have found this thread very helpful as my goto's arent the best either so thank you all very much.
One thing I can add is that the bubble level and compass on the meade tripod are useless and so inaccurate I dont even bother with them. I went on the net and bought myself a " Wixey" digital angle gauge, an orienteering compass and a 6inch spirit level for setting up, altho my goto's still need work I'm dead sure its not the initial levels or angles and using these tools put me ahead leaps and bounds.
use your computer running stellarium from your exact location. Look for a star that is easily identifiable close to north with an azimuth of about +2 degrees. Manually point your telescope at this star and track it by hand (scope off). When Stellarium shows this star hitting 0 degrees stop tracking. Your telescope is now pointing due north much closer than you could ever get with a compass.
Barry is right! I tried this myself. I also purchased an better compass - but I must admit that the new release of Meade ultrawedge compass is a vast improvement on the old (as quoted by Michael from Bintel!)
The compass needs to be accurate not "an Improvement" what I do is use the orienteering compass because it has a rectangular base, run a straight edge between the north facing tripod legs then butt the compass up to the straight edge. then mark the spots where the tripod lands so you wont have to realign every time the scope is dragged outside.
once this is done check the meade supplied compass it wont even be close to correct something to do with the fact that it is in the middle of the metal wedge and this throws off or introduces anomalies in the magnetic field.
Same goes for the spirit bubble once properly levelled with an aftermarket spirit level check the meade level.....not even close.
Just one more thing once I'm all set up no drift etc etc my wedge's dec angle reads 36deg Perth is 32deg use the aftermarket wixey angle meter which is deadly accurate and it reads 31.8 deg so ummm ???
dont get me wrong I love the scope and its not a complaint all Im saying is that for me I was chasing my tail on polar alignment for months trusting the supplied bubble and compass, once I put more accurate measuring instruments in the loop everything was gravy no problems.
Fair enough Robbie! Another good tip, however I just picked my wedge up last week (was on back order for months), and apparently the compass is different to the last type. When did you get your wedge?
Anyway, I agree that its not worth risking and needs to be accurate so I too purchased a different one and also different spirit level(s).
I believe the major issue with the Meade compass was the lack of correction for Magnetic Dip. This is casued when you move poles (as in Sorthern) and it hasnt got dip compensation (something that top quality ones do have).
I poured a concrete slab and marked out true N using the solar noon. I then marked a line 90 degrees to this to give E -W directions. It is a simple job to place the tripod and level using a good survey quality digital level which I also use to set the lattitude on the wedge. Sigma octanes is almost always in the FOV of my 26mm EP and its a simple task to align and setup from there. The supplied level and alt scale that come with the wedge leave a bit to be desired.
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