MX Question

[PRejto (Peter)]

Well, I hope this isn't too obvious, but I can't quite get my head around two things. First, it has been 6 months + since I looked at my polar alignment (I'm permanently set up) and pointing has been a bit off even when I recalibrate into the model. But before getting on with a new model I wanted to check out unguided tracking. With the scope pointing south and not too far from the pole I could do 4 min tacking with no star distortion, But, if I pointed overhead or to the north I cannot do even 1 min without serious trailing in RA. Yes, I get the fact that the velocity of tracking would be slower closer to the pole due to the smaller circumference, and faster progressing away from the pole, but the motor speed is the same in both situations. So, why am I seeing such a huge issue? Would it just be due to PA being off? (Protrack was on.)

Second part....ran a 67 point model and found that both Azi and Alt were off by around 80 arc sec. Is that enough to explain the poor unguided results away from the pole, or is it because the model has only 200 points (I only see about 50% of the sky from my location.)

3rd part. I went to adjust the altitude and tried a method that had worked easily for me 6 months ago, and I totally failed. This feels bad because I've been preaching about how well this "photographic" method works! Anyway, I think I've forgotten what it is I exactly did which is really frustrating. I picked a star on the meridian pointing south. Then I centered it photographically and plate solved the photo. This showed the yellow bullseye in TSX right on the star. I slewed the mount down 80 arc sec, so the yellow bullseye moved off the star 80 arc sec down. After that I went out and adjusted the mount up 1 tic and took another photo, plate solved it, but the bullseye did not move at all. I expected to see it move after the plate solve but obviously something is messed up in my reasoning. Anyway, there has got to be an easier way to do this. Perhaps I should put the camera in focus mode with a smaller frame? But, I'm still trying to figure out how to measure the movement the exact distance. A crosshair showing the center of the photo would certainly help. Any ideas? I can use CCDSoft or the TSX camera add on. THANKS!!
Peter

[Visionoz (Bill)]

Originally Posted by PRejto

... I picked a star on the meridian pointing south. Then I centered it photographically and plate solved the photo...
Peter

Peter

I'm not sure if I should tell you how to do it -- but shouldn't you be pointing to a star that is on the meridian pointing north rather than south if checking drift aligning is what you're doing?? Sorry if I'm wrong as I don't own any of the fancy MX mounts etc and will leave this to the others who would chime in

HTH
Cheers
Bill

[PRejto (Peter)]

Bill,

I see now that I was quite unclear about what I am attempting to do!

The polar axis of my mount needs to be raised 80 arcsec to obtain optimal polar alignment according to T-Point. Moving the axis such a small amount can be a hit or miss proposition if one depends entirely on the MX adjustment control, so I am attempting to confirm such a move photographically. Using the slew control of TheSkyX I first lowered the scope to point 80 arcsec below a star centered in a photograph. Then I raised the mount physically by 1.2 tics on the MX adjuster. Then taking a second photograph I wanted to confirm that the physical adjustment of the mount was actually 80 arcsec up. I had hoped to confirm the move by comparing the first photograph to the second photograph. But in trying to do this I have become stuck. In other words if I had successfully raised the mount by 80 arcsec the star would be recentered. As I mentioned, 6 months or so ago I did this easily but 2 nights ago I got bogged down in actually trying to compare the photos. What I'm asking is if there are any experienced users of TSX out there that might propose a clean way to do this seemingly easy task. If the photo viewer in TSX actually had a cross hair defining the center of a photo this task would be fairly simple, but if there is one I have not found it. I have figured out a fairly task intensive way to do it in photoshop using a grid pattern where the pattern shows divisions of 1 arcmin. By using this method it should be possible to just move the axis up/down or in azi and read the change by comparing how far the star moved from centered and it wouldn't be necessary to slew the mount in the opposite direction first. Any ideas are welcome. I'm sure there is something obvious I'm missing!!!

Peter

[rat156]

Hi Peter,
There is a cross hair checkbox on the image viewer, I use it all the time. I'd take a screenshot, but I'm away from my system for a while. The crosshair has a box in the middle, but you should be able to move the mount whilst in focus mode and see the star move, then adjust it back.

Cheers
Stuart

[PRejto (Peter)]

Originally Posted by rat156

Hi Peter,
There is a cross hair checkbox on the image viewer, I use it all the time. I'd take a screenshot, but I'm away from my system for a while. The crosshair has a box in the middle, but you should be able to move the mount whilst in focus mode and see the star move, then adjust it back.

Cheers
Stuart

See, I knew it was something simple and I obviously missed that!!! Many thanks!

[Joshua Bunn (Joshua)]

Hi Peter,

Since the PA report says you are x arc minutes above or bellow the pole, try doing your method while pointing at the SCP. Im really not sure if it will make a difference where you point on the sphere, but why not try?
What method are you using to guage your 80 arc min move, are you using the angle separation tool in TSX?

Keen to hear how you go.

Josh

[PRejto (Peter)]

Hi Josh,

Well I cannot see the South Pole so that option is out!

I'm using the slew command to move the 80 arcsec. Bit tricky to know if you are moving up or down relative to the mount....

I've now noticed that the crosshair in the photo viewer is not exactly precise! The box seems rather large which doesn't exactly lend to very small moves.

Peter

[Joshua Bunn (Joshua)]

Just point your scope to the south pole so the digital setting circles read -90 S, that aut to get you close enough.

i dont know if your bellow or above the pole, but if for example your bellow the pole, so you need to raise the alt axis, then slew the scope south (check your going the right way by looking at the NSEW cross in the top right of the window), you can gauge your movement by measuring out 80 arcmin with the angular separation tool. Then, as you know, try to get back to that same place with plate solves before and after.

Having said this, PA doesn't need to be spot on, there is no spot on PA for your rig. There will be different PA for different aspects of the imaging system you want to optimize. And, Protrack will compensate for some miss PA.

You may find it easier to do a new large tpoint model and use that. Sorry if ive covered what you already know. .

Josh

[Logieberra (Logan)]

Peter, why not post this at SB forum?

[PRejto (Peter)]

Josh, Thanks for your input. First off, I'm not 80 arcmin out...I was 82 arcsec low, and that is now fixed to the point that a 65 point T-point run super model indicates PA is spot on and has no recommendation. I'm pretty happy about that. I used a method in photoshop that I made up. I noticed that the FOV indicator in TSX showed my camera sensor in arcmin, so I divided the pixel count in the y axis by the arcmin in the y axis and derived that about 1 arcmin was equal to about 26 pixels. Then I created a grid where the squares were all 1 arcmin. I used the "free transform" tool to show the center of the photos. With the grid, and knowing exactly where the center is it was fairly easy to plot the move in altitude. I kind of like this method because I did not need to slew at all. All I needed to do was open an initial photo in CS5 and note the starting point of the star movement, then take a second photo to see the effect of raising the polar axis.

Logan, sure I could post at SB. I have noticed that sometimes replies are slow in coming and I was hoping for something quick here, which is pretty much what I got.

[Joshua Bunn (Joshua)]

Yep, 80 arcsec, now, hows the protrack tracking?

[PRejto (Peter)]

Josh, since you asked....

I had a terrific night last night. I managed to get Azi to 7 arcsec, and Alt to as close to perfect as I think is possible. T-Point had no recommendation in Altitude after a 285 point run. I now have RMS of 8 arcsec.

No problem with unguided exposures. At first I thought I had a big problem unguided overhead and to the North, but then realized what I was seeing was PE. After turning on PEC all the problems went away!

Now, finally I might get to try imaging again....

Peter

[Joshua Bunn (Joshua)]

Great to hear Peter.