4. Imaging with Unistellar eQuinox

https://www.unistellar.com/equinox2

Select an astronomical object to image using https://telescopius.com.

The numbers below follow the inset numbers given in the figure above.

1. Targets: select Deep Sky.
2. Location and date: either select ‘Use my current location’ or enter the coordinates of Dhaka (lat 23.8, lon 90.4) manually. The date is here 17 Nov.
3. Moon: the moon is in ‘waxing crescent’ phase, not very bright, which is good.
4. Sunrise and sunset times, and the astronomical sunset (from when it is totally dark) times are given.
5. Weather: average weather on this night is bad.; ‘very bad’ cloud condition and ‘bad’ seeing.
6. Search parameters need to be set for selecting a sample of your desired targets. The target should be visible from around 6:30 pm (astronomical sunset) to 9 pm.
7. Minimum altitude: the target should have a minimum height from the horizon of around 50 degrees for at least 15 minutes, so that we can take a picture when it is well above in the sky and close to our zenith.
8. Object type: select galaxy, globular cluster or interstellar matter.
9. Distance from the moon of the target should be at least 90 degrees, as far as possible.
10. Apparent magnitude (brightness) of the target should be between 5 (barely visible with naked eye) and 10 (faint but observable with our telescope).
11. Apparent size of the target must be between 10 arcminutes and 40 arcminutes because the field of view of our telescope is around 40 arcminutes. The object should fit within the field of view, that is the frame of the picture.
12. 7 results have been found with these filters, that is 7 galaxies to observe.
13. The list can be sorted by various things, here sorted according to ‘popularity’ .
14. M 110 is the first galaxy in the list. It is an elliptical galaxy, with an apparent size of 19 arcminutes ($19'$), apparent magnitude of 8.1, located in the Andromeda constellation.
15. On 17 Nov, it will rise at 06:02 pm, reach its highest altitude at 08:50 pm and set at 11:48 pm.
16. The track followed by the galaxy in our sky is shown here. Its maximum altitude from the horizon will be 72 degrees at 08:50 pm and it will be toward north during that time.

The visibility has three numbers, the first one is the rising time, the second one the transit time, and the last one the setting time. The difference between the rising and setting time is usually around twelve hours, and the transit occurs when the object is right above our head, when the object reaches the highest position in the sky on a given night.

It is always better to observe an object around the time of transit. Not exactly at the time of the transit, but around that time. If the difference between rising and setting time is twelve hours, then it takes approximately six hours for an object to reach the transit position after its rise. So, for example, if I see that the Andromeda galaxy will transit at 12 am, 16 Oct, then it must have risen at around 6 pm, six hours earlier. If it was near the horizon six hours ago, then it would be halfway toward its transit around 3 hours ago, meaning at around 9 pm.

The rising, setting and transiting can be explained using a protractor or goniometer shown above. It has 180 degrees marked from left to right and right to left. If we are facing toward north, east would be to the right and west to the left. There are 180 degrees from right to left, east to west.

If the observer is located at the center of the horizontal line, then the object is toward 0 degree during the time of rising, toward 90 degrees during the transiting time, and toward 180 degrees during the setting time. An astronomical object might go up or down by an angle of around 15 degrees in one hour, so a total of 180 degrees in 12 hours. The actual time taken depends on many other things which we will skip for now.

We can observe an object for around 4 hours on either side of its transit. If Andromeda transits, reaches its highest point, at 12 am, then we can observe it from 10 pm to 2 am, which suits us very well because our classes are typically from 10 am to 11 am.

The size of an object is also very important. It should not be too small or too big compared to the field of view (FoV) of the telescope. The size and FoV are measured in angles, so we need to understand the units of angle degree, arcminutes and arcsecond.

The angles within a complete circle are conventionally measured in degrees. A circle has a total of 360 degrees, each of the four quadrants having exactly 90 degrees. One degree is divided into 60 arcminutes (‘arcmin’ in short) and 1 arcmin is further divided into 60 arcseconds (arcsec). Arcmin is sometimes expressed using a single quotation mark ($'$), and arcsec sometimes by a double quotation mark ($''$). The symbol for a degree is $^\circ$. So we could write

$$ 1^\circ = 60' = 3600'' $$

$$ 1 \text{ deg } = 60 \text{ arcmin } = 3600 \text{ arcsec } $$

because $1'=60''$, that is 1 arcmin = 60 arcsec. Remember that 1 deg is the size of our pinkie finger, so 1 arcmin is a very small angle, and 1 arcsec even smaller.

In this course, you will encounter astronomical objects that have sizes of around a degree or a few arcmins. For example, the size of the sun or the moon is around 0.5 deg, that is around 30 arcmin. On the other hand, the size of the Andromeda Galaxy in our sky is more than 3 deg, too big for our telescopes.

Our telescopes have FoVs of around 1 deg.