The secret to guiding with MaximDL

Few people have expressed interest due to the fact that I am able to guide properly and accurately with such a long focal length as that of the C14 (4 Meters or 13 Feet). Indeed, some people thinks it’s a feat or an achievement to be able to track a star with anything longer than 1.8 Meters (8 Feet). Well, I have learned imaging techniques with my current setup on my own, so I do not think much of it, especially since my only other guiding experience was using a C11 with a DSLR guided by hand!…

So, what’s the secret to proper guiding? As I see it below is a list of preequisite, from the most important to the least important:

  1. A very stable and robust mount well sized for the weight of the OTA.
  2. The mount has to be properly aligned using King or Bigourdan alignment method.
  3. Camera setup must be solid and very rigid so that here aren’t any flexures.
  4. The guiding/imaging Software must be free of bugs and well calibrated.

My current setup fulfill all the above. Losmandy Titan mount is massive enough (it can withstand 90 Lbs loads if I remember correctly) to support the C14 EdgeHD and all it’s accessories without effort. I try to keep the mount well aligned using Bigourdan’s technique (detailed in another post). Finally, even though my OAG (Off Axis Guider) is old, it support both the Atik 11000-CM imager a well as the Atik 16-IC cameras very rigidely and MaximDL works reasonably well. That being said, the most important factor to accurate guidance is: A good Calibration!

Calibration

Calibration is used by the software that is beeing used for guiding, be it MaximDL or other, to tell it by how much and in what direction to move the mount in Right Ascension or Declination in order to put the guide back onto it’s assigned sensor coordinates. So, in that respect it is critical. I usually try to align the guiding camera axes with the axes of the mount as it makes it easier for myself  to see what the mount is doing, and it what direction the guide star tends to drift. That tells me things on how well the mount is set up, but it really is not necessary as any guidance software is smart enough to decide which motor of the mount to act upon. Also, a guide star, with a well set up mount, will only tend to drift along the Right Ascension (RA) axis, because of  ”periodic error” (which is an inherent defect of the wormscrew mechanism) thus it should only necessitate to correct the mount along that axis.  Let’s look at what the calibration window looks like in Maxim:

calibs

What you see here is what the calibration window looks like just after the calibration has been completed. Prior to this, the “expose” radio button will have been used after each small movement of the mount in an attempt to locate a guide star (which is not always easy). I usually use a 2 to 4 sec exposure time to locate a guide star. Of course, if you’ve already performed a calibration on an area of the sky not too far away from the area where you are planning to image, and the cameras have not been rotated, then there is not point to do it all over again. For calibration, what the Software does is pilot the mount to move a few seconds in declination, then the same amount of time back, before doing the same in the RA direction. The amount of time used can be set under  the “setting” button as seen on the picture above under “agressiveness” (see below).

Guide_window

You should aim at displacements around 100 pixels in each direction and adjust the time accordingly. Now, as you can see on the picture, the axes of calibration are nearly aligned with the x and y axes of the guiding camera, since I try to align it with the scope axes. Due to mechanical inaccuracies of the mount and motors, such as backlash and periodic error, it may happen that the star does not quite always go back to it’s original position in between calibration steps, but, there is no much that can be done about it apart from trying to set a “backlash” value, again, under “settings”. Also, because the software has to be able to clearly “see” the star, it has to be bright enough, otherwise, it might pick on a hot pixel of the camera window. If that happens, the best thing to do is to increase exposure time to make the star brighter.

Ok, so now Maxim is calibrated meaning  that it can relate guide star errors in pixel to amount of movement to send to the mount to set the star back to its assigned position.

Guiding

Now is the time to select the radio button “track” and see what happens. In track mode, a small window with the guide star in the center will pop up (see picture below).

The size of this window is selected under “options” in the guidance window. Generally, if all is well, the guide star should remain firmly in the middle of the window so it does not have to be that big. But, if you knock the mount or if for any other reason the star was to drift out of the window, then Maxim “can’t see it” anymore and is therefore unable to pilot the mount to put it back in the center. Normally, the star will drift by only a few pixels around the center of the window (right click on the window and select “crosshair visible”). It may happen, because of turbulence, that the star jumps around the center of the guidance window. There is not much that can be done about it appart from trying to increase guidance exposure time to try to average out turbulence. Exposure time will also have to be set according to how quick periodic error of the mount is causing the star to drift. If periodic error curve is very steep, then it might be necessary to reduce exposure time in order to correct the mount more often. For example, with a 4 second exposure time, the mount is only corrected once every 4 seconds, whereas, with a 0.5 Second exposure, the mount is corrected twice per second…

Tracking can also be improved by setting “agressiveness” parameters to their correct values. No, do not worry, even if you set it too high, the mount is unlikely to bite you, but it sure will destroy your image… What agressiveness does,  is multiply calibration parameters some so that the mount is over or under corrected. For example, if you notice that after each correction made, the star does not move back to the center of the crosshairs, then it is undercorrected and agressiveness has to be increased for the applicable axis. On the other hand, if aggressiveness is set to high, on oscillation can occur where star movement is over corrected and star moves around the crosshair. I generally find that a value of 5 on both axes works for me.

Guiding is not such a dark science, even trying to guide with a C14 native focal length. But, tyrying to understand what the software is trying to do goes a long way toward reliable and accurate guidance… Remember to set the appropriate audible alarms in case guide star is lost, use “autosave” function to program the number of exposures, set the alarm clock,  and go for a snooze, either in your bed or under the stars!

When you get back up again, the chance is that the guide star is still being faithfully maintained in the center of the crosshairs, and your hardrive will be full of wonderful deep sky sub images!

Happy Imaging

Serge

VN:F [1.9.7_1111]
Rating: 8.9/10 (9 votes cast)
VN:F [1.9.7_1111]
Rating: 0 (from 2 votes)
The secret to guiding with MaximDL, 8.9 out of 10 based on 9 ratings

4 Responses to “The secret to guiding with MaximDL”

  • Jose Verdugo says:

    Hello ,Serge

    which values do you use for Max and min move in Maxim?

    I have a LX 200 ,which I know is not well adapted for guiding….
    min speed is 2x sidereal ,so usually overcorrects…
    so I am trying to use as max move 0.1 seg and for min move..0.02 segs.

    what do u think?

    thanks

    Jose

    VA:F [1.9.7_1111]
    Rating: 0.0/5 (0 votes cast)
    VA:F [1.9.7_1111]
    Rating: -1 (from 1 vote)
    • admin says:

      I’m sorry, Jose, but I am not familiar with Meade’s moun’s at all. 2x sideral speed correction sure does seem a bit excessive. It depends how often your guiding software corrects and on how good your calibration is…

      VN:F [1.9.7_1111]
      Rating: 0.0/5 (0 votes cast)
      VN:F [1.9.7_1111]
      Rating: -1 (from 1 vote)
  • Dick Krause says:

    I hope I can add a photo to show you guiding graph from the other night. Guiding within one pixel, I know, hard to believe. Can’t figure how to send pix, if you send me your e-mail I’ll be happy to send and tell you about my improvements to my G-11 mount.

    Dick

    VA:F [1.9.7_1111]
    Rating: 0.0/5 (0 votes cast)
    VA:F [1.9.7_1111]
    Rating: -1 (from 1 vote)
    • admin says:

      Dick,
      What I’ll do is publish your modifications to the G11 if you can write something up, the graph you sent me seems too perfect to believe! I am sure we all want to know wha you did to you mount!

      VN:F [1.9.7_1111]
      Rating: 0.0/5 (0 votes cast)
      VN:F [1.9.7_1111]
      Rating: -1 (from 1 vote)

Leave a Reply