ObservingDetails
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Constraints for ground-based observing
Introduction
Seasonal observability (not sure this is an actual word) for an astronomical object refers to times of the year when that object is above the horizon at night at a viewable height. In other words, you can see it easily in that it is "sufficiently above" the horizon. When the object is low on the horizon, your line of sight will pass through much more atmosphere (air) compared to a line of sight looking straight up. The less airmass you see, the better the photometric quality of your observation. Hence, "sufficiently above" means its above the limit for the amount of air/atmosphere you are willing to tolerate.
Here is a nice online description with Figures:
Most observatories have written their own code to calculate seasonal visibility. I used this one:
How to read Seasonal Observability tables
For observing purposes, astronomer commonly use the quantity labelled SecZ (pronounced Secant Z). SecZ is the secant value of zenith angle, usually labeled as 'Z'. Zenith angle is the angle between straight overhead and wherever your object happens to be on the sky. When the object is overhead, its Zenith angle is 0 degrees. When it is on the horizon, its Zenith angle is 90 degrees. Astronomers prefer SecZ over directly using Zenith angles because it is easier to work with SecZ values. When something is overhead, it has a SecZ value of 1.0, which is the best value it can have. When an object is on the horizon, its SecZ value is very large. It is infinity.
By "sufficiently above", we usually mean SecZ is less than 2. When using the observability table, we pay close attention to when the object has SecZ values less than 2.
That said, here is the explanation for the columns in the seasonal observing tables.
| Title | Explanation | |
|---|---|---|
| Date | The date for which the rest of the values are valid. | |
| Moon | Phase of the moon. F=Full, N=New | |
| Evening, Center, Morning | Start, middle and end of the night, respectively. | |
| HA, SecZ | The SecZ value for the object at the beginning, middle and end of the night. For now, ignore HA values. |
In choosing which dates to select, consider:
- When is the object most visible above SecZ of 2 and 1.5?
- When is the best SecZ value for the object in the middle of the night.
For example, for BRC 27, I would choose Dec-Feb as the best times to observe this object from Hawaii.
Seasonal Observability for BRC 27
RA & dec: 7 01 37.9, -11 18 48, epoch 1950.0 Site: Mauna Kea, Hawaii
| Date | Moon | Evening | Center | Morning | Hours with SecZ | |||||
|---|---|---|---|---|---|---|---|---|---|---|
| HA | SecZ | HA | SecZ | HA | SecZ | <3 | <2 | <1.5 | ||
| 2012 Feb 6 | F | -2 48 | 1.6 | 2 17 | 1.4 | 7 22 | down | 7.1 | 6.2 | 5.0 |
| 2012 Feb 21 | N | -1 43 | 1.3 | 3 16 | 1.9 | 8 14 | down | 6.0 | 5.2 | 4.2 |
| 2012 Mar 7 | F | -0 39 | 1.2 | 4 12 | 2.9 | 9 03 | down | 4.9 | 4.1 | 3.1 |
| 2012 Mar 21 | N | 0 21 | 1.2 | 5 04 | 6.3 | 9 46 | down | 3.9 | 3.1 | 2.1 |
| 2012 Apr 5 | F | 1 25 | 1.3 | 5 58 | down | 10 31 | down | 2.9 | 2.0 | 1.1 |
| 2012 Apr 20 | N | 2 30 | 1.5 | 6 54 | down | 11 17 | down | 1.8 | 1.0 | 0.0 |
| 2012 May 5 | F | 3 37 | 2.1 | 7 51 | down | -11 56 | down | 0.7 | 0.0 | 0.0 |
| 2012 May 20 | N | 4 45 | 4.3 | 8 50 | down | -11 05 | down | 0.0 | 0.0 | 0.0 |
| 2012 Jun 3 | F | 5 47 | down | 9 47 | down | -10 14 | down | 0.0 | 0.0 | 0.0 |
| 2012 Jun 18 | N | 6 52 | down | 10 49 | down | -9 14 | down | 0.0 | 0.0 | 0.0 |
| 2012 Jul 2 | F | 7 49 | down | 11 47 | down | -8 15 | down | 0.0 | 0.0 | 0.0 |
| 2012 Jul 18 | N | 8 48 | down | -11 08 | down | -7 04 | down | 0.0 | 0.0 | 0.0 |
| 2012 Aug 1 | F | 9 36 | down | -10 13 | down | -6 01 | down | 0.0 | 0.0 | 0.0 |
| 2012 Aug 16 | N | 10 24 | down | -9 16 | down | -4 55 | 5.2 | 0.0 | 0.0 | 0.0 |
| 2012 Aug 30 | F | 11 06 | down | -8 24 | down | -3 54 | 2.4 | 0.4 | 0.0 | 0.0 |
| 2012 Sep 15 | N | 11 53 | down | -7 27 | down | -2 46 | 1.6 | 1.5 | 0.7 | 0.0 |
| 2012 Sep 29 | F | -11 25 | down | -6 36 | down | -1 48 | 1.3 | 2.5 | 1.7 | 0.7 |
| 2012 Oct 14 | N | -10 38 | down | -5 41 | v.low | -0 45 | 1.2 | 3.5 | 2.7 | 1.7 |
| 2012 Oct 28 | F | -9 51 | down | -4 48 | 4.6 | 0 15 | 1.2 | 4.5 | 3.7 | 2.7 |
| 2012 Nov 12 | N | -8 57 | down | -3 49 | 2.3 | 1 20 | 1.3 | 5.6 | 4.8 | 3.8 |
| 2012 Nov 27 | F | -7 58 | down | -2 46 | 1.6 | 2 26 | 1.5 | 6.7 | 5.9 | 4.9 |
| 2012 Dec 12 | N | -6 54 | down | -1 40 | 1.3 | 3 34 | 2.1 | 7.8 | 6.9 | 5.0 |
| 2012 Dec 27 | F | -5 48 | down | -0 34 | 1.2 | 4 41 | 4.1 | 8.5 | 6.9 | 5.0 |
| 2013 Jan 10 | N | -4 45 | 4.3 | 0 28 | 1.2 | 5 40 | 81.1 | 8.5 | 6.9 | 5.0 |
Seasonal Observability for BRC 34
RA & dec: 21 34 35.8, 58 18 10, epoch 1950.0 Site: Mauna Kea, Hawaii
| Date | Moon | Evening | Center | Morning | Hours with SecZ | |||||
|---|---|---|---|---|---|---|---|---|---|---|
| HA | SecZ | HA | SecZ | HA | SecZ | <3 | <2 | <1.5 | ||
| 2012 Feb 6 | F | 6 40 | 4.9 | 11 45 | down | -7 10 | 7.0 | 0.0 | 0.0 | 0.0 |
| 2012 Feb 21 | N | 7 45 | 13.9 | -11 16 | down | -6 17 | 4.0 | 0.0 | 0.0 | 0.0 |
| 2012 Mar 7 | F | 8 49 | down | -10 20 | down | -5 29 | 2.8 | 0.2 | 0.0 | 0.0 |
| 2012 Mar 21 | N | 9 49 | down | -9 28 | down | -4 46 | 2.2 | 0.9 | 0.0 | 0.0 |
| 2012 Apr 5 | F | 10 53 | down | -8 34 | down | -4 01 | 1.9 | 1.6 | 0.3 | 0.0 |
| 2012 Apr 20 | N | 11 59 | down | -7 38 | 11.7 | -3 15 | 1.6 | 2.4 | 1.0 | 0.0 |
| 2012 May 5 | F | -10 55 | down | -6 41 | 5.0 | -2 28 | 1.5 | 3.2 | 1.8 | 0.2 |
| 2012 May 20 | N | -9 47 | down | -5 42 | 3.0 | -1 37 | 1.4 | 4.0 | 2.7 | 1.0 |
| 2012 Jun 3 | F | -8 44 | down | -4 45 | 2.2 | -0 46 | 1.3 | 4.9 | 3.5 | 1.9 |
| 2012 Jun 18 | N | -7 40 | 12.1 | -3 43 | 1.8 | 0 14 | 1.3 | 5.9 | 4.5 | 2.9 |
| 2012 Jul 2 | F | -6 43 | 5.1 | -2 45 | 1.5 | 1 14 | 1.3 | 6.9 | 5.5 | 3.9 |
| 2012 Jul 18 | N | -5 44 | 3.1 | -1 40 | 1.4 | 2 24 | 1.5 | 8.0 | 6.7 | 5.0 |
| 2012 Aug 1 | F | -4 56 | 2.4 | -0 45 | 1.3 | 3 27 | 1.7 | 8.4 | 7.7 | 5.3 |
| 2012 Aug 16 | N | -4 08 | 1.9 | 0 12 | 1.3 | 4 33 | 2.1 | 8.7 | 8.4 | 5.3 |
| 2012 Aug 30 | F | -3 26 | 1.7 | 1 04 | 1.3 | 5 34 | 2.9 | 9.0 | 7.7 | 5.3 |
| 2012 Sep 15 | N | -2 39 | 1.5 | 2 01 | 1.4 | 6 42 | 5.0 | 8.3 | 6.9 | 5.3 |
| 2012 Sep 29 | F | -1 57 | 1.4 | 2 52 | 1.5 | 7 40 | 12.2 | 7.6 | 6.2 | 4.6 |
| 2012 Oct 14 | N | -1 10 | 1.3 | 3 47 | 1.8 | 8 43 | down | 6.8 | 5.5 | 3.8 |
| 2012 Oct 28 | F | -0 23 | 1.3 | 4 40 | 2.2 | 9 43 | down | 6.0 | 4.7 | 3.0 |
| 2012 Nov 12 | N | 0 31 | 1.3 | 5 40 | 3.0 | 10 48 | down | 5.1 | 3.8 | 2.1 |
| 2012 Nov 27 | F | 1 30 | 1.3 | 6 42 | 5.0 | 11 55 | down | 4.1 | 2.8 | 1.1 |
| 2012 Dec 12 | N | 2 34 | 1.5 | 7 48 | 14.9 | -10 58 | down | 3.1 | 1.7 | 0.1 |
| 2012 Dec 27 | F | 3 40 | 1.8 | 8 54 | down | -9 51 | down | 2.0 | 0.6 | 0.0 |
| 2013 Jan 10 | N | 4 43 | 2.2 | 9 56 | down | -8 52 | down | 0.9 | 0.0 | 0.0 |
Seasonal Observability for BRC 38
RA & dec: 21 39 10.3, 58 02 29, epoch 1950.0 Site: Mauna Kea, Hawaii
| Date | Moon | Evening | Center | Morning | Hours with SecZ | |||||
|---|---|---|---|---|---|---|---|---|---|---|
| HA | SecZ | HA | SecZ | HA | SecZ | <3 | <2 | <1.5 | ||
| 2012 Feb 6 | F | 6 36 | 4.7 | 11 41 | down | -7 14 | 7.6 | 0.0 | 0.0 | 0.0 |
| 2012 Feb 21 | N | 7 41 | 12.8 | -11 21 | down | -6 22 | 4.1 | 0.0 | 0.0 | 0.0 |
| 2012 Mar 7 | F | 8 45 | down | -10 24 | down | -5 34 | 2.9 | 0.1 | 0.0 | 0.0 |
| 2012 Mar 21 | N | 9 44 | down | -9 33 | down | -4 50 | 2.3 | 0.8 | 0.0 | 0.0 |
| 2012 Apr 5 | F | 10 48 | down | -8 38 | down | -4 05 | 1.9 | 1.6 | 0.2 | 0.0 |
| 2012 Apr 20 | N | 11 54 | down | -7 43 | 13.5 | -3 20 | 1.6 | 2.3 | 1.0 | 0.0 |
| 2012 May 5 | F | -10 59 | down | -6 46 | 5.2 | -2 33 | 1.5 | 3.1 | 1.8 | 0.1 |
| 2012 May 20 | N | -9 52 | down | -5 47 | 3.2 | -1 42 | 1.4 | 3.9 | 2.6 | 1.0 |
| 2012 Jun 3 | F | -8 49 | down | -4 50 | 2.3 | -0 50 | 1.3 | 4.8 | 3.5 | 1.8 |
| 2012 Jun 18 | N | -7 44 | 14.0 | -3 47 | 1.8 | 0 09 | 1.3 | 5.8 | 4.5 | 2.8 |
| 2012 Jul 2 | F | -6 48 | 5.4 | -2 49 | 1.5 | 1 09 | 1.3 | 6.8 | 5.4 | 3.8 |
| 2012 Jul 18 | N | -5 48 | 3.2 | -1 44 | 1.4 | 2 20 | 1.4 | 8.0 | 6.6 | 5.0 |
| 2012 Aug 1 | F | -5 00 | 2.4 | -0 49 | 1.3 | 3 22 | 1.7 | 8.4 | 7.7 | 5.3 |
| 2012 Aug 16 | N | -4 13 | 2.0 | 0 08 | 1.3 | 4 29 | 2.1 | 8.7 | 8.5 | 5.3 |
| 2012 Aug 30 | F | -3 31 | 1.7 | 0 59 | 1.3 | 5 29 | 2.8 | 9.0 | 7.8 | 5.3 |
| 2012 Sep 15 | N | -2 43 | 1.5 | 1 57 | 1.4 | 6 37 | 4.8 | 8.4 | 7.0 | 5.3 |
| 2012 Sep 29 | F | -2 01 | 1.4 | 2 47 | 1.5 | 7 36 | 11.3 | 7.7 | 6.3 | 4.7 |
| 2012 Oct 14 | N | -1 14 | 1.3 | 3 42 | 1.8 | 8 39 | down | 6.9 | 5.5 | 3.9 |
| 2012 Oct 28 | F | -0 28 | 1.3 | 4 35 | 2.1 | 9 38 | down | 6.1 | 4.8 | 3.1 |
| 2012 Nov 12 | N | 0 27 | 1.3 | 5 35 | 2.9 | 10 43 | down | 5.2 | 3.9 | 2.2 |
| 2012 Nov 27 | F | 1 26 | 1.3 | 6 38 | 4.8 | 11 50 | down | 4.2 | 2.9 | 1.2 |
| 2012 Dec 12 | N | 2 29 | 1.5 | 7 43 | 13.6 | -11 03 | down | 3.2 | 1.8 | 0.2 |
| 2012 Dec 27 | F | 3 36 | 1.7 | 8 50 | down | -9 56 | down | 2.1 | 0.7 | 0.0 |
| 2013 Jan 10 | N | 4 39 | 2.2 | 9 51 | down | -8 56 | down | 1.0 | 0.0 | 0.0 |
