Apr 13

Measuring Periodic Error

Periodic error of my Star Adventurer

x-axis: time in seconds, y-axis: arcseconds

The peak-to-peak error of my Star Adventurer is about 87 arcsec (10 minutes period), or 8.7 arcsec per minute.

I did some further measurements, all with fully loaded equipment (DSLR, 400mm lens, Guidescope). The first one is done on Feb 12, 2017.

2. measurement

February 14, 2017, peak-to-peak: 78 arcsec (peak-to-peak) per 10min period, or 7.8 arcsec per minute.

3. measurement

April 13, 2017, peak-to-peak: 88 arcsec or 8.8 arcsec/min

4. measurement

April 13, 2017, peak-to-peak: 71 arcsec or 7.1 arcsec/min

I never achieved such a good DEC curve in the following measurements… the first one was a very lucky polar alignment 🙂
Curves from the 3th and 4th measurement:

Duration of this measurement: 2 periods (20 minutes)

Anyway. The (bad) DEC curve (peak-to-peak: 17 arcsec/period or 1.7 arcsec/min) wouldn’t be noticeable even on a 200mm, 60 seconds shot.

Regarding these values I could exposure unguided as follows with my Star Adventurer (at 0° Declination):

Periodic Error (best achieved value): 7.1 arcseconds/min
Resolution formula: (Pixel Size / Focal Length) * 206.265 = arcsec/px

Example: A star moves 7.1 arcsec during a 1 minute shot. The resolution of my DSLR is 2.11 arcesc/px at 400mm focal length.

Calculation: 7.1/2.11 = 60s/18s

  • 400mm: Resolution: 2.11 arcsec/px ~ 18sec exp.
  • 280mm: Resolution: 3.02 arcsec/px ~ 26sec exp.
  • 200mm: Resolution: 4.23 arcsec/px ~ 36sec exp.
  • 100mm: Resolution: 8.46 arcsec/px ~ 71sec exp.
  • 50mm: Resolution: 16.91 arcsec/px ~ 143sec exp.

(Canon EOS 70Da pixel size: 4.1 micrometer, Astronomy Tools Calculators)

My Star Adventurer performs bad after 2 years heavy use… On the net some other Star Adventurers have around 50 arcsec peak-to-peak PE.

I really thinking about a new SA 😉 A comparison between a brand new and a 2-year-heavy-use Star Adventurer would be interesting…

 

Video

Measuring the periodic error of the Star Adventurer with the MGEN-II. See in this video the whole process.
Download latest MGEN firmware and Windows application here.

Used equipment in this video:

 

Step-by-Step instructions

Setup Star Adventurer and start star tracking.

  1. Setup MGEN
  2. Download data from MGEN to PC
  3. Create graph (in MS Excel)

 

1. Setup MGEN

  1. Set date and time (it creates the file timestamp).
  2. Next create and open a new file.
  3. Go to guiding menu and search a star near to 0° Declination by aligning the guide scope.
  4. Go to page 5/5 in Guiding Menu and select “more..”
  5. Enable “Save pos.”. From this moment the absolute position of the star is beeing recorded.
  6. Wait at least one worm gear period (Star Adventurer: 10 minutes)
  7. Disable “Save pos.” to stop recording data.
  8. Close the file


  9. Power off MGEN

 

2. Download data from MGEN to PC

  1. Connect MGEN to PC via USB and power on the MGEN
  2. Start Lacerta MGEN PC app (download here) and go to “File control”
  3. Select file and set parameters. Check at least “Generate Periodic Error” and set focal length of your guide scope, worm gear period of your mount and the star’s declination coordinate.
    Then click “Download + process file(s)”.

 

3. Create Graph (in MS Excel)

I used MS Excel to create a graph. Excel interface only in German… sorry.

  1. Open pe1.csv (can be found in the created sub directory in the MGEN app folder)
  2. Select graph type
  3. Select data
  4. Add row title (RA)
  5. Select RA data range
  6. Add next row title (DEC)
  7. Select DEC data range
  8. Now you should see something  like this:

5 comments

  1. Jan Schubert

    Hi Karol,
    Danke für die interessante Messreihe. Hat eigentlich die Verstärkung, also die Aggressivität, Einfluß auf die Messergebnisse?
    Heute Nacht soll der Himmel rech klar sein, also werde ich mich heute mal an M101 versuchen. Ich bin gespannt, wie das Ergebnis wird, ich hoffe es Endet nicht enttäuschend.
    Bei nächster Gelegenheit werde ich auch mal eine Messung machen, mal schauen wie die SA auf “mein” Übergewicht reagiert! 😀

    CS Jan

    • star-watcher

      Hi Jan. Während der Messung ist das Guiding komplett ausgeschaltet, somit hat die Aggr. auch keinen Einfluss. Interessant wäre sicherlich mit unterschiedlicher Beladung zu messen (zb. 1kg, 3kg und 5kg). Das werde ich bei Gelegenheit noch tun. Bin gespannt auf Deine Ergebnisse (welche sicher besser sein werden als bei meinem “verbrauchten” SA…). CS, Karol

  2. Jan Schubert

    Hi, der Sinn und Zweck hat sich mir dann jetzt auch ergeben! 😀
    Ich werd bei der nächsten Gelegenheit mal die Genauigkeit protokollieren. Bis dahin hab ich hoffentlich schon das Kontergewicht halbieren können.
    CS Jan

  3. James

    Great blog post. This inspired me to measure my lightly used SA. I have never tried guiding with it previously but it worked well. My measured peak-peak over 1 sample was approx 40″.

    With guiding turned on I was getting an RMS of 3″ and a peak of 11″. A seriously impressive little mount, especially considering my PA was off by 30′.

    • star-watcher

      Thanks. It’s really an impressive little mount. Precise, small, portable and very quick setup. I like it 😉

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