Guiding at low altitudes - My solution

[swag72]

I have recently battled with my HEQ5 and guding setup. Previously, it has given me night after night of 20 minute subs and then I put another camera on the back and it seemed that everything stopped working at the same time. I have used guide graph examples here, not real world subs. I know that I often bang on about forget the graph, look at the data, but I knew that these guide graphs were a good indication of the problems I was facing. The graphs here are on a scale of +/- 0.5 pixels and from 60-420 seconds.

This is just documenting how I sorted out my guiding issues which were so bad that I was thinking that the HEQ5 was terminal and had even convinced hubby that a new mount was on the cards.

I think that the new camera, which was the same weight as the other one, but of a different design (cyclindrical) probably changed the balance on the mount far more than I thought possible.

I guide using Maxim and a QHY5 on an Altair Astro 10x60 guider package. I use EQMOD and so ASCOM pulse guiding. Here is an image below of the guide graph that is normal for me. This was achieved at a fairly high 62 degree altitude. With my previous camera, this was a standard dgraph fro me at all altitudes.

I found that the lower my mount went, the worse the guiding became. Here's an example of 34 degrees. As you can see the mount didn't even settle down until 2 minutes in and still this graph was something of a shock to me. The Maxim settings have remained the same on these two examples and the balance was the same. The OTA was balanced along the DEC axis and the mount was East heavy at the weights.

Then I went to 20 degrees and it became very apparent that as it was, I was not going to guide at this altitude. The mount only acted on perhaps 1 in 3 or 4 of the pulses, the guide graph would suddenly veer of to +/- 10 for no apparent reason. I thought that perhaps I had some electrical issues in the guide part of the mount. I changed all cables, power supplies and yet was not seeing any difference. I didn't take a guide graph shot at this stage - I was just too frustrated and I didn't think of it. Trust me when I say that it wasn't working at all and over the course of a night, I lost every single one of my 20 subs. This happened on two consecutive nights, when normally I don't drop a sub.

I changed the balance of the OTA - On advice from Ken (Merlin66) - and made it very camera heavy at the back. That on its own didn't sort the issue. I upped the Maxim aggressiveness setting from 2 (my standard setting) to 8 - From this I got 20 minute subs again, at a low altitude pointing South. The guide graph made me a little uncomfortable, but it worked.

I don't know if PHD has a similar aggressiveness setting. This may not be everyone's problem at low altitudes, but it certainly cured the issues I had.

I hope that this will be useful to others at some time when everything else has been exhausted. Just a bit of food for thought really.

[opticalpath]

Hi Sara.

I'm actually surprised you originally used aggressiveness settings of 2 successfully. If you calibrated the guider normally in Maxim, that would mean you were getting only 20% of the calculated required correction impulse each time there was a deviation. That sounds almost like no guiding and the mount was doing pretty well on its own! At only 20%, I would have expected it to take many correction cycles to correct a tracking blip. I think Maxim recommend a starting value of 7 or 8 in order to get most (but not 100%) of the calculated correction on each cycle. I normally set both axes to 8.

Have you looked at the Maxim guide log? I find it's the most useful source of info to see what's going on. You can see at each guide cycle what was the measured tracking error and the correction applied to each axis; the numbers for the next cycle in the log then tell you how well the correction worked (or not). Did you recalibrate the guider when you changed the camera and stick to the same guider settings - binning etc - after the change?

Adrian

[Merlin66]

Sara,

When the telescope is pointing high in the sky it's bottom is closer to the mount...less out of balance to look after....

When you point low in the South it's bum is now in the air and further away from the mount...more to be balanced...

Make any sense?

Anyway, seems like you're getting on top of it!

Onwards and upwards.

[swag72]

I'm just glad that it's all working again!!!

Yes Adrian, everything was recalibrated and the settings were the same. It was a bit of a head scratching moment!!

[opticalpath]

Hmm, seems to confirm what we all know in our hearts: auto-guiding really is a dark art and not science!

Glad it's working again anyway.

Adrian

[Tom OD]

I use an EM200 which is rated for +/- 5 arc seconds,and I have guiding issues to at that altitude. The lower the worse it gets. I think its inherent in most scopes. Balance and the guide hopping about a bit more with all that atmosphere to shine through.

Tom

[opticalpath]

My guess is:

At low altitude seeing is terrible and atmospheric refraction effects are much worse. Your guide star would be moving around with much bigger deviations than at higher altitudes, so would need correspondingly much larger corrective impulses. With aggressiveness set at only 2, the mount is unable to react strongly/ quickly enough to the large deviations.

Aggressiveness setting '2' sort of implies 'slightly aggressive' but actually means 'very passive' i.e. you will only be getting 20% of the correction Maxim thinks you need on each cycle. Maybe that works reasonably well when the amplitude of the corrections required are smaller at higher altitude, but I think a setting of 6-8 is quite usual, and is what Maxim recommends.

Adrian