Fine tuning a Paramount MX mount

[gregbradley]

I just spent a night and a half tuning up my PMX mount.

I did multiple TPoint runs to tweak the Polar Alignment. Then when it was good I did a 329 point model so I can run Protrack during exposures. I have been told running Protrack does not help but my experience with a 300 point Protrack model on my PME and CDK17 shows otherwise. I find it can be the last little bit to get round stars.

Protrack handles slow flexures and autoguiding fast corrections.

My PMX has been running off a PEC curve generated using Software Bisque Precision PE.

I used PemPro this time after several threads about problems with PEC and PMX mounts from some time ago.

The resulting curve was surprisingly different from PemPro compared to the Precision PEC one.

When testing its effectiveness the PE curve went quite flat so PE is now down around 1 arc second or so.

I then did an imaging run with my AP140 and Proline 16803 using a MMOAG and an STi guide camera. I use a reducer on the STi and sometimes a 740nm infrared filter.

Wow, images were ultrasharp showing off the incredible figure of the AP scopes' lens. Some images have stars as small as FWHM of 1.2 which I have found to be rare.

So PemPro and the Protrack model leading to a more pefect Polar Alignment was time well spent.

I also used a 740nm infrared filter last night. Hard to say if it helped or not as I haven't done a with a without comparison. But when the seeing is a bit rough which it was last night, it seemed to cut through that better than what I have normally seen in the past. So I think it probably does help and perhaps more in times of poor seeing.

I thought I would post this as some have had trouble with PEC on their PMX mounts in the past.

A 300 point Tpoint model and a PemPro PEC are the keys here to 1 arc second guiding and lovely round stars at 20 minutes and 1050mm focal length.

Greg.

[alistairsam]

Interesting read Greg,

I'm contemplating the MyT Mount, so is relevant for me.
Curious to know, with a mount like the PMX, do you really need the 300 point model to get tracking and good pointing accuracy?
what happens if you get it polar aligned accurately within a minute or so but don't have a large model.
How is the pointing accuracy and tracking?
Just trying to understand why it needs a 300 point model if precision is high and polar alignment is very close.
I'm making the jump up from an Eq6 to a MyT mainly for consistent pointing and tracking, not just from an observatory but also from various sites where I can't afford to do a large model.
Just wondering if I should rethink my approach.

Cheers
Alistair

[gregbradley]

PemPro is the main thing. You can also use its Polar Alignment feature which is fast and accurate.

PemPro reduces the PE of the mount which is advertised as 7 arc seconds or better. Mine without PEC correction is about 3.5 arc seconds and I have heard that before.

The 300 point model is not essential and pointing accuracy is high without it.

You can create a 30 point model in a matter of 5-10minutes once familiar with everything.

A couple of 30 point t point models would have your Polar Alignment very close so you could do accurate guiding throughout the night.

Of course the longer the focal length the more demanding all this has to be. But the PEC only has to be done once and is stored in the mount in memory.

So don't worry about guiding accuracy as the PMX is very good at that.

I was looking to make it work the best it can.

A large Tpoint model can improve guiding accuracy as it corrects for slow flexures and so improves autoguiding.

Of more importance than a Tpoint model is to use an off axis guider of some type rather than a guide scope.

Greg.

[Bassnut]

Most interesting Greg, thanks for that.

[alistairsam]

Thanks Greg,

I'm using an AO at 3 to 5Hz at 1m FL and a 685nm IR filter.
I do use PemPro for polar alignment but found that the lodestar on the polar scope of an eq6 yields better results for me.
Do the Paramount's have polar scopes at all?
30 points from a travelling setup perspective sounds reasonable.

Cheers
Alistair

[gregbradley]

I had heard that Protrack does not help guiding which seemed odd as SB were promoting it ages ago that it does.

I do notice on my CDK17 and PME that activating Protrack and enabling tracking corrections from the 300 point model I have on that does seem to make stars that little bit more round. It takes it that little bit further.

Its not dramatic but subtle.

Greg.

[gregbradley]

A 30 point model with 5 second 3x3 or 3x3 binning takes about 10 seconds per point to take once started. So all up its around 5 minutes + .

There is a polar scope available for the PMX.

I used to align my Tak NJP by doing accurate drift alignment (boy that's tedious) and then shining a laser through the polar scope hole in the mount.

I then placed a small bit of electrical tape on the ceiling of the roll off roof.

Next time I installed the mount I placed the laser in the same spot and adjusted it until it shone on the bit of tape. That worked really well and was super fast.

These days I would probably use either T-point and its instructions (there is a fast polar alignment routine for PMX which takes about 2 minutes then followed by a few 15 point models and then follow the instructions it gives you.

Or if you go to the same place every time then place something on the ground to mark exactly where you place your mount and hopefully its all good to go.

Greg.

[Bassnut]

Well, I use protrack but no PEC, works for me. Their has been endless argument on various combinations, as you know. You have found a combination that works, well done.

[gregbradley]

Its also sometimes hard to tell if there is an improvement when its also very windy and you turn it on for some exposures and off for others.

I think at best its a subtle tool.

Greg.

[Paul Haese]

I am not 100% certain but I think with PE and Protrack done correctly you can use direct guide instead of using guide relays and that improves the guiding accuracy.

I have been investigating direct guide recently as a means to manage guide related errors and the above notion has occurred to me as being a means to provide very accurate guiding. Could be wrong.

[Bassnut]

Relay guiding used to be with, well....relays, which would theoretically have delays and inaccuracy. Optical couplers are used these days, Im guessing there wouldn't be much in it now. I still use the relay option and don't see a problem. I could be wrong.

[Joshua Bunn]

I use direct guide, and have Protrack and PEC running together, My guiding on my PME is fantastic. I think they complement each other. bellow is my guide graph (must have been a night of good seeing) at 1.2"/pix for the guide camera with guide exposures of probably 5-8 seconds range for memory.

[gregbradley]

I think the other point worth mentioning here is that once you've got all this setup it may give you better results to turn the aggressiveness of the guider right down. I started seeing some seesawing correction of the last correction at one point which stopped when I reduced the aggressiveness right down to between 1 and 3.

Greg.

[PRejto]

There was a question earlier re the MTY and the need for a large model.

Like Greg said, pointing is pretty darn good even with a small model. What the large model does is allow for the possibility of not guiding. On my MEII I can quite easily go for 5-7 min unguided with no problem and a 400+ point model and this is at .85 src-sec. Many report 10 min + and recently at the Software Bisque Forum are quite unbelievable non guided images of 30 min!

Being portable doesn't mean that you need to generate a large model every tme you set up! If your setup is reasonably the same each time then the essential terms in the large model can be preserved from night to night! It is only necessary to recalibrate into the large model to re-establish ME and MA (and 2 others I forget at the moment). 6 points is reported to be enough but 20 or so would be better and can be done in a few minutes. Thus, you can use and keep a 400 point model from night to night. Given this I think it is well worth doing. My experience with Protrack is that it doesn't hurt guiding. I know people report otherwise but I could not confirm that and I did experiment.

Also, Greg is spot on re PEMpro. I never could get TSX to do a proper PEC. On both my former MX and now my MEII I could get PE down to around .6 arc-sec. And PEM pro also has a very good drift alignment tool which could be just as fast or easier to use than some of the recommended SB methods. When combined with Protrack even Patric Wallace said that polar alignment would be quite good enough to avoid any serious field rotation. And that, combined with Protrack, which addresses flex and other issues, is a perfectly reasonable way to get going quickly.

Peter