What is good pointing with a PME and narrow field of view?

[Paul Haese]

Hi again,

One thing I noticed the other night with the RC12 and the STL11K is that despite a pointing accuracy of 12" my pointing was still a little off the target I was aiming toward. I attach an image here to illustrate the point.

I was aiming for the middle star in the running man on the right hand side of the nebula. M43 is on the right hand side of the image for reference. You can see that pointing is a bit off and I would have expected to be a bit closer. Is this 12"? I would have said it was further but I could be wrong.

Just for reference this is 74 points but not all the sky. Only 3 extra terms added to the base terms. About 1/3 of the sky, so I know the model is not strong but it got me fairly close.

Bottom line, with a longer focal length do I need say 300 points, protrack turned on and better pointing accuracy?

[TrevorW]

Can't comment on the PME but I get that close with my G11 after alignment with Alignmaster etc and I've no permanent setup so I would envisage that you'd want to put the middle star dead center.

Cheers

[gregbradley]

Paul, I am at 2958mm and PL16803 which is a large FOV chip.

I have a 150 point T-point model and I consistently get the object about 1/8th of the image from the centre of the view. This is plenty good enough for me.

It also lands the object in the FOV of the 8300 chip which is a lot narrower field of view.

I hear of others who get their pointing to centre of an image routinely but I frame the image anyway so it isn't that important to me. I just want the object in the image without having to hunt for it.

Doing a larger model, tweaking polar alignment and making sure there is no flex in your imaging system would be the things that would improve the pointing accuracy.

Greg.

[frolinmod]

With the Paramount ME with an EdgeHD 14, at f/11, FL=3911mm, mirror locked down, ~180 points mapped in TheSkyX with Tpoint Add On using supermodel to compute my model, I get 8 to 9 arcseconds RMS all sky pointing. Nothing special. I've seen people do better and I'm a bit envious. With an 8300 chip, objects land within the central box of the CCDsoft image cross hairs every time, all over the sky. Good enough for me (though I would like the 3 arc seconds RMS one guy gets with a large refractor).

Mind you, that's with things basically perfect on the mount. I have played around with how I mount the scope, how far back or forward I place it, how much DEC balance counterweight I use, dew shield on and off, how much RA balance counterweight I use and where it's placed on the counterweight bar, etc. In awkward situations with weights hanging far off axis I can get really bad pointing if I try hard enough.

ProTrack won't affect your pointing accuracy, but it may smooth your tracking. I've seen ProTrack nicely compensate for slow movement from flexure. I've found that it needs lots of points to work well. At least about 70-90 to not make things worse and at least about 150-180 to work very well.

[rally]

Paul,

I think you'll find that error is in the order of 12-14 arc minutes not 12 arc seconds.

Was your target SAO132331 or SAO132320 ?
If it was the latter then the pointing error is more in the order of 15-16 arc minutes, so I am not sure how you derived the 12" number.

The P-ME should get you sub 10 arc second pointing accuracy with only a 10 or 20 point model no trouble.

It won't make any difference what the focal length of the OTA is, the pointing accuracy will still be the same since that is mount dependent, however longer focal lengths are more often mirrors, so you'll have more OTA movement to contend with.

Are you sure your time is accurate between sessions or you didnt forget to resync the mount, or Polar Alignment is way out because that sort of error is way outside the normal mechanical limits of a P-ME ?

Rally

[frolinmod]

Pfft, it may say your accuracy is 10 arc seconds, but without more points, it won't have enough data to determine model terms needed to actually get you there everywhere in the sky. You may get the stated accuracy somewhere, but not everywhere.

[rally]

Frolinmod,

My comment was based on my own and associates repeated experiences with a P-ME and refractors in the field.

The mount is accurately polar aligned and setup with a digital inclinometer with an initial 6-10 point model and adjusted then a second model of around 10-20 points.
Adding another 20 or 30 points didn't seem to make any difference to the pointing.

Using MaxIm at 400% magnification across the sky above say 45 degrees altitude, the cross hairs after a slew to a star and image acquired for mapping purposes will either be on the star or within a star's diameter.
At 1.7arc secs per pixel on mag 4.0 - 5.0 stars that is about the mark.

This is using the standard 6 terms with everything rigidly mounted or attached via custom threaded adapters - there is no measureable flexure.

Rally

[Paul Haese]

Rally, I got the figure from the Tpoint model which says 12.2 arc seconds after 74 points.

With the TSA I get very accurate pointing. Typically only a few seconds ( what looks like a mill or two on the screen) at most from the cross hairs to the target.

Time I did a sync around 5 pm with my usual server. I doubt that would make much of a difference.

Yes sub 10 second pointing at 10 points, I get around 4" from 10-20, then as it gets further into the model is blows out a little to around 40" prior to adding terms. Once terms are added it get more accurate and I follow the significant differences ideas we have discussed in the past (ie watching the sigma etc in the model).

PA looked fine after 20 points and the mount has not moved since my Tpoint run on the TSA.

Since Greg gets a similar result I suspect that focal length does influence the pointing accuracy, so I will hunt down what is flexing if anything.

[gregbradley]

I am not sure if this may affect it but the manual for the Sky X seems to indicate you need to do a synch after every power on. Its a little unclear on this point. I also though per the T-point manual that if you did a synch you had to do a new model. I'd have to read it again.
I don't know if this is a change with the Sky X but I suspect not.

But I was just doing as Rally was saying and after the synch the go-tos were on the star or really close by. I was putting together an initial T-point model on the PMX. This though is with an AP140 and reducer and about 750mm focal length.

I haven't been synching every time I turn on the mount. It appears you are supposed to. Is that something you have been doing?

Greg.

[Paul Haese]

Greg I don't use the SkyX. The sky 6 with TPoint will force a new model if you try to sync. My pointing in the past has been great with the TSA.

[gregbradley]

That is what I thought but wondered if I missed something.
That appears to be different then in the Sky X. Unless I have read it wrong.

Also another cool feature which I don't recall in T-point is the ability to recallibrate your T-point model if you move your mount or have a portable setup.

That is useful for me as I take my PMX to my dark site every now and then and don't fancy doing a new 150 point model every time.

Greg.

[frolinmod]

If your mount doesn't have a homing sensor like the Paramounts do, then you have to do a sync every time you power on the mount or disengage worm, etc. With a Paramount you just home the mount, no sync needed.

TheSkyX permits you to "Sync into model." It works very well (I've tested it) and permits you to use the same model and data with just the sync reset. That would be the one to use for sure with an mount that can't be homed.

The SkyX is quite an improvement over TheSky6. You guys who are still running TheSky6 are falling further and further behind ever day. You really should upgrade sooner rather than later. At the moment I personally think the most convenient way to do that is by purchasing a Universal Subscription at upgrade pricing. That way you get a serial number that can activate everything old and new without any fuss or muss.

[frolinmod]

Okay, my hat is off to you guys with the perfect super rigid refractors that don't experience anything that's not perfectly correctable with the standard six terms. Very cool. In that case I'd expect you do be able to reach 3 arc seconds RMS or better like one other refractor user I know. So what's holding you back? I'm quite envious. :thumbsup: