VIII. Data Gathering Methods

B. Sketch Method

1. Basics

During each observing session the user makes a sketch on paper of the positions of the Moons as seen in the eyepiece. Later the separation on the sketch is measured and is then used as one data point in the Jupiter Project data analysis. This method requires no special equipment, but more observations than normal ( try to get around 20 ) are recommended so that the inherit inaccuracies in any sketch may mitigated by the abundance of data.

2. Eyepiece Selection

Because the separation of the satellite in any given observation is to be compared to other observations, there is a need to have a consistent FOV from one observation to the next. The focal length of the eyepiece directly influences the FOV (as do other parameters that vary between eyepieces ). Consistency of the sketches is enhanced when the same telescope and eyepiece combination are used for all sketches.

3. Use a high power eyepiece for sketching.

The use of higher power eyepieces generally reduces the field of view (FOV). The smaller FOV reduces the proportionality of the satellite separation to the FOV, making the sketching as easy as possible. Details of the planet surface features, while interesting, are not needed for this project. Select an eyepiece that as that enables the maximum separation to be still in the FOV during subsequent observations without having to change eyepieces.

Observers using a telescope lacking tracking capabilities will need to use a lower power eyepiece. That enables Jupiter and it’s selected satellite remain in the FOV long enough to make a reasonably proportioned sketch ( it is harder to make a good sketch the faster the object moves through the FOV.).

4. Eyepiece FOV

It is helpful if the field of view (FOV) of your eyepiece is determined before the observing sessions begin. This is not a required step, but may aid the observer in estimating separations when the FOV is known.

To determine the FOV, select a star near the celestial equator (within ±5° of zero declination ) and time the star as it crosses the diameter of the field of view. Multiply by 15 to convert that clock time into arc-seconds (or arc-minutes if timed in minutes).

5. Use same size sketch circles

Because this method will obtain the separation between the satellite and the center of Jupiter by placing a ruler on a sketch, each sketch should be similarly sized. That is, for a given eyepiece that has a FOV of say 30 arc-minutes, the sketches should consistently use the same scale for the FOV.

For ease of recording data, it is suggested (not required) that the observer use "The Astronomer’s Journal, An Observing Log and Sketch Book", available from AAAA’s own www.AstroMax.com. That Journal has a convenient and consistent method of recording field observations that encourages consistency and completeness of observations.

6. Advanced Eyepieces

It is helpful in making sketches if an observer has a reticle eyepiece ( an eyepiece with a built-in scale). This is a help, not a necessary piece of equipment to the successful completion of Project Jupiter. The scale on the eyepiece need not be calibrated in order to make to scale sketched.

For those interested, both Meade Corporation ( see www.Meade.com and look for the Astrometric eyepiece at about $150 ) and Celestron ( see www.celestron.com and look for the Micro Guide Eyepiece #9471) make such eyepieces. Another option is to see if another member of your local astronomy club would be willing to loan you one for this project. As stated, these eyepieces are a sketching aid, not a requirement for the successful completion of Project Jupiter.

7. Data Preparation

Measure the separation of the selected Moon from the center of Jupiter on each of the sketches made. It is recommended that the familiar inches and fractions be avoided by measuring the separations on the sketch in mm. Avoid the tendency to "smooth" the data at this stage. If on observation #5 the separation is 5 mm, on observation #6 it is 8 mm, and on observation #7 it is 6mm (went back down), that is OK. Report the data as sketched. For each observation, record the separation measurement ( in mm ) on the data sheet (Attachment A).