GeorgeCarey

I wanted to change an old post but it looks like this can't be done. Could the Forum be altered to allow this? George

Reply from ASA today. They suspect that it is indeed due to a bearing problem. The mount will go back to Austria next week. Thank you to ASA for good service!

I use Cartes du Ciel which is free. It may not have some of the bells and whistles of The Sky, but does everything I need. Some people use Stellarium, which is also free.

Hi Ian, I think you are getting muddled up between 'Home position' and 'Park position'. I was similarly confused when I got my mount. You need to set a new park position. This is found under 'Mount... Park Positions... Set new Parkposition'. Move your mount to where you want it to 'sleep' and then follow this sequence. 'set new Homeposition' is for when you are polar aligned, you have a good pointing model, and you want Autoslew to remember where the stars are. Do a synch on a star and then 'Set new homeposition'. There is even more confusion because when you switch the mount on you do a 'Homefind' which is nothing to do with the previous steps. The startup Homefind is for the mount to find refernce marks on the encoders. George

No reply from ASA. I have sent another email today.

Can you post a picture of the front end of the scope? It should be possible to check if there is misalignment. You will need to get the camera on the central axis of the tube. When I was adjusting my homemade Newtonian I used this image to check the vanes:

Hi Pavel, Are you talking about the apparent 'splitting' of the spikes? This is due to the secondary support vanes on opposite sides of the secondary not being perfectly in line. Each vane produces a spike that runs through the bright star. When sharply focused the two spikes will be superimposed. Try an out of focus shot and you will see the spikes separate even more. I don't have an ASA scope so I do not know if their is any adjustment that you can make to line the vanes up precisely.

The graph was made with the data when the mount behaved perfectly. There were no spoiled images and I don't think there were any excursions. I would need to aim at a very bright star to check this. The deliberate slews after the over current errors are almost certainly what caused my 30,31 Com B stars. The odd thing is that this happened on 8th May with no errors reported by Autoslew. When I do a 'Reset', I switch off Autoslew, turn off the power supply to the mount for 10 seconds and then start again. I do a manual homefind. Sometimes when I move the mount to the reference points by hand the homefind fails and the mount jumps around a bit. Because of this, after a manual homefind, I always offset the mount a little with the controls and then do another homefind. The motor speed is low and the mount moves more smoothly than I can push it by hand. I suspect that you are correct in thinking that the magnetic angle in the log file may be a different representation than the magnetic angle found after a homefind. I put in a support request yesterday - we will see what transpires.

This is a graph of current for the good tracking period.

I then switched to counterweight West and started again. Apart from a 'Velocity exceeded' error the mount is behaving and currents are stable. Over 2 hours of good tracking.

I should never have said that I had cured the over current error - I got several tonight. Problems started as soon as I slewed to ngc4725. Over current errors and position errors were frequent. I had to seset and start again. The error log is here: http://geoastro.co.uk/may2013/autoslew/ I don't really understand 'magnetic angle'. Once it has been established (manual homefind) why does it vary? George

Thanks, Bernd. I will set up the log recording next time.

The clouds started coming over but I left a second exposure of 10 minutes running. Low and behold - it did it again!

I managed to reproduce almost exactly the same effect. I started a 10 minute exposure on ngc4725 and then did a rapid slew northwards and then back to ngc4725. 30 and 31 Coma Berenices showed up in the right places. The trace was not as clean as in the previous examples and it looks like there was a small wobble - the return trace was slightly displaced. However, the guiding was not affected and no deviation showed on the graph. The slew was easily completed in 5 seconds. Conclusion: it is possible for Autoslew to do a rapid excursion and return!

Thanks for you help with this, Bernd. I will be watching my mount closely to see if it does it again.

Average of 21.83 - I am so envious! My best ever was 20.11

Strangely there is a 10.92 magnitude star over on the right that has a beautiful straight line showing above and below it. The star is colour index 0.57 which may be why it has produced a bright trace,

I think we have a clear picture of what happened. This is the picture of HIP62778

Superb! Has anyone taken an SQM meter to the Tivoli site?

I had a look at the relative intensities of the two stars earlier today. Unfortunately the left one is fully saturated with many pixels at 64000 so accurate values are not possible. If the right one is taken to be mag 5.75 the left came at near mag 6. The fact that the left one is saturated gives us a clue as to how long Autoslew stayed on it before going back to ngc4725. I can do some test exposures to find the minimum time to reach saturation. Also if the star was saturated then RBI is less likely. In my experience Autoslew can make sudden and unpredictable excursions. However, usually it reports an error. This was not the case last night. My RA axis never misbehaves, but the DEC can give problems. Some weeks ago I was getting 'over current' errors that could not be corrected. I seem to have cured this by doing two things. First I tweaked the tuning and altered the values slightly from those found by auto tuning. Secondly, and this may not really be doing anything, I swing the DEC axis through about 80 degrees by hand before switching on the power. I do this about 8 times. This is because the DEC axis seems to have some mechanical stiffness, and there maybe some tight spots that benefit from 'warming up exercises'. Since I started doing this I have not had any over current errors, and position errors are much fewer. I will have a look tomorrow at your later suggestions to see if the are any spikes on HIP62778. PS - the scope is a refractor so there should not be any diffraction spikes.

I think we have to accept that the ghost stars were 30 and 31 Comae Berenices. How the scope pointed to them is a mystery. I have taken the guiding log from Maxim and very carefully isolated the part that goes with the errant image. I did a test image to confirm that the exposure time recorded in the FITS header is indeed the start of the exposure. The guiding was without any sudden jumps. The previous image was also without any jumps. What about this as a possibility? I use 5 seconds exposure with the guide camera. Autoslew makes a rapid Northward movement and returns, in less than 5 seconds. This might not show up on the guiding log. None of the stars around ngc4725 are bright enough to leave a trail. 30 and 31 Coma Berenices ARE bright enough so trails are made. I will try and reproduce this next clear sky.

I took the DSS images for ngc4725 and the two Coma Berenices stars and did some overlays. I had to rotate the Coma Berenices star image by 1.4 degrees anticlockwise. The fit is very good. All I need to figure out is how this could happen - twice!

The angular separation on 5th May was 1923.3 arc seconds. On 8th May it was 1923.8 arc seconds. The separation of 30 and 31 Comae Berenices is 1922.20, 1923.30, 1923.80 and 1923.4 depending on the source of the coordinates. So agreement is better than 1 part in 2000. Next clear night I will slew the scope North and see where these two stars are. Your solution to the problem has a lot going for it, but why were no stars in the ngc4725 area trailed?

I will check those two stars you mention. It certainly looks like a 'double image' of some sort, but I have checked the images taken before and after the 8th May image. They are both normal. The anomaly was number 4 in a sequence, so I am puzzled how the scope could have wandered off to look at 31 and 32 comae berenices. Autoslew seemed to be behaving OK. I had dithering set between images with a delay of 90 seconds to allow time to re-establish guiding. On May the 5th the faulty image was the first in a sequence, so with that one there is more chance that the shutter was open at a different location. However, I still find this hard to accept because I use autoguiding, and set the scope guiding before any images are taken. I may be able to inspect the guiding log from last night to see if somehow the scope did a sudden deviation. George

This is the two images stacked with alignment on the left anomaly. The right hand anomaly is perfectly aligned.