JonathanA

24 minutes ago, michael8554 said:

You have Dec backlash. 

So to force the mount to take up the backlash by itself, balance it to "flop" in the same Dec direction as the slight PA error you mention. 

So PHD2 will send constant small Dec corrections to straighten out the Dec trend, and with judicious choice of MinMo and Aggr the Dec axis should remain on one side of the axis instead of yoyo-ing over and under the axis. 

Michael 

Got it, thank you, Michael. That makes perfect sense. First time out it will be a pain working out which end to bias towards but after that it seems quite straightforward.

17 hours ago, michael8554 said:

First off your black on black text is quite hard to read.........

You could setup to avoid dec backlash comp being sent at all.

Balance to be slightly heavy one way in Dec, then Dec shouldn't cross the axis and require big corrections.

Michael

Yes, sorry about that - it wasn't intentional!

In know that with a slightly poorer PA in dec I could engineer things to guide in one direction only and that may well get round the problem. At the moment that is my backstop - which I'm guessing will work quite well. I'll try to get to the bottom of the issue first though if I can.  Can you explain what you mean about being heavy in dec - I'm can't quite see how that would help (I understand how it helps with RA)?

On 01/09/2019 at 00:42, discardedastro said:

It is certainly mechanically possible, more or less depending on exactly what you've got on the mount and where.

Reducing guiding speed could help, yes. Those corrections also look quite large - it may be worth tweaking the Dec algorithm to prefer more, smaller movements over larger movements (smaller min-move would be a good start). I'd start there and then look at reducing guide pulses to .3x or .2x if the problem persists.

Yes, my minmove was quite large (but it was the recommended value at the time). The behaviour has been quite consistent for a little while though even when using quite small min move values.  I'm mostly thinking for now that there is some mechanical vibration that is large enough to disturb RA everytime I get a dec backlash correction.  The Dec axis has always been quite stiff and holding my ear up to the mount I could hear a faint scraping at some points in the cycle when manually turning the axis. Anyway, I have loosened up the thrust bearing slightly and I've now got freer movement and no noise. I'll have to wait an see whether this has improved matters (or created a whole bunch of other problems!)

Really frustrating problem with my EQ6-R. What's happening is that every time a backlash compensation pulse is sent in pHD2 I am getting a large deflection in RA. The graph below shows it quite well. The frustrating thing is that guiding is pretty good in between times (RMS about .6-.8). Is this some sort of vibration that's being set up in the mount by the backlash correction? Would reducing guide speed help? Can anyone suggest an adjustment I could make that might improve this? I know that I can set it up to guide dec in one direction only but I'd rather avoid this, if I can. Any thoughts would be much appreciated.

21 hours ago, Waldemar said:

The only way to get this sorted is by trial and error. Too many factors are  involved for a manufacturor to give you the exact distance.
 You need to be able to take steps of max 0,1 mm to find the ideal distance. An adjustable extension like this: https://www.firstlightoptics.com/adapters/baader-t2-extension-12-16mm.html or the Baader Varilock (2 lengths available) are the easiest way to do this.

Hi Waldemar,

I think at the level of precision that you are talking about, tenths of a mm, you might be right about there being many variables. But the basics are pretty well defined - focal ratio being the key. Certainly other manufacturers and sellers seem able to provide detailed information. We have a situation here where on the USA version of the ES website they suggest 55mm back focus and on the European version they suggest 65mm! There's definitely something wrong there.

Fortunately, someone very helpful from ES USA did respond quickly to my enquiry - see below.

'I consulted with one of our optical engineers to confirm: The measurement starts at the rear flange (flat surface) of the corrector. NOT from the CCD M24x0.75 plate -- take that off, then start the measurements.

The M42x0.75 plate adds approximately 3 mm.

The backfocus is 55 mm. There is an additional 2 mm either side where the correct is acceptable.

You cannot judge T-rings by their total thickness – because that is not the amount they add. They have threads on one end that screw onto the M42x0.75 adapter – so those threads don’t count. The T-ring also have a bayonet (or other) mounting system that, when coupled with the camera, subtracts from the T-ring’s overall measurement. All those variables have to be accounted for in creating your system.'

It is puzzling that ES do not support some of their products with clearer information. No guidance at all comes with this product. A similar reducer supplied by Willams Optics comes with very detailed information about back focus and other parameters.

Thanks for suggestion of the variable extension tube - I had thought about that approach but unfortunately to get the correct backfocus with a Nikon DSLR I am going to have to go down the low profile T adaptor and very thin extension ring route.

Hi,

All the above useful but I'm hoping to resurrect this thread as I have been having exactly the same problem and have spent hours trying to sort it.  I’ve tried various suggested distances between the focal plane of (Nikon DSLR) camera and the reducer haven’t achieved a flat field yet. I’m hoping someone has finally resolved this.

A couple of things to add to the discussion

The ES USA website (and some of the forums) are saying that 55mm is the back focus distance but there is no indication where this is measured from on the reducer. If it is from the lens surface, then would this be from the edge of the optical element or from the centre (it is obviously very curved)?

Puzzlingly, I know that the reducer used to come Nikon and Canon specific adaptors (arrowed in blue on the image below).  Studying images of these, it looks to me that using them would have given way more than the suggested 55mm of back focus.

To make this even more confusing on the ES Europe site they have the diagram below which seems to say something altogether different about the back focus required - ‘about 65mm’. (I’ve tried it – doesn’t work).

Has anyone got a definitive answer to where to measure from on the reducer? There are various suggestions in the forums but, looking at some of the posters’ setups and the absence of actual images taken through the reducer, I’m a bit doubtful of these especially as I've tried most of them!

Thanks.