- What celestial objects should I request?
- Why don't my planet images look that good?
- Why can't I take picture of a constellation?
- Why are images of bright stars and some other objects not allowed?
- My observation request has been in the queue for a while, why has not not been completed?
- How does the telescope decide what requests to observe?
- What are all these filters and which ones should I use?
There are literally thousands and thousands of objects to choose from! Here are some tips:
- You have to choose objects visible from Nova Scotia - objects too far south can't be observed! The telescope will tell you this if you try.
- The objects you choose must be reasonably visible in the next 4 weeks.
- Choose objects visible in the current season or the next one. When we say current season it refers to what is visible in the evening sky (when humans are awake), but since the telescope does not sleep, it can observe things until dawn, or about a season or two ahead.
- Don't choose objects that are too big! The camera has a field of view of about the size of the Moon, or 0.5 degrees (30 arc-minutes).
- Don't choose bright stars (they are boring, will be over-exposed, and are not allowed - see this FAQ) and don't choose a constellation. They are too big!
Even planets, while we can take pictures of them, don't look that good.
The telescope does a great job on "deep sky" objects - these are open and globular star clusters, nebulae, galaxies, etc.
- SIZE: choose less than 30'
- DEC: choose objects greater than -10 (first 3 digits). Below that and they are too far south (DEC is the same a latitude in the sky).
- RA: choose objects in season or the next season (first 2 digits). RA is the same as longitude in the sky.
- Winter: 03 to 08
- Spring: 09 to 14
- Summer: 15 to 20
- Autumn: 21 to 23 and 00 to 02
- NAME: this is the name of the object that you use, either beginning with M or NGC. M stands for Messier and NGC stands for New General Catalog.
You can do a web search of the object names to learn about them before you request them.
Taking good images of planets requires special techniques and manual processing. Most good images of planets are made by taking video movies and selecting the best (sharpest) frames and using those to make the final image. The observatory's robotic interface can't do that.
The camera attached to the telescope has a field-of-view of about 0.5 degrees. That is about the size occupied by the Moon. Constellations are much larger - typically 5 to 40 degrees in size.
The short answer is that bright stars (and stars contained in some other objects) are too bright for our large telescope and they will be overexposed, even when the shortest exposure time is used.
The long answer is:
- images containing bright stars will be overexposed and leave ghost images that are visible on subsequent images taken by the telescope. In a shared telescope like the BGO, we cannot allow that to happen.
- some objects, such as large or bright star clusters, contain individual stars that are too bright. An example object in this category is the Pleiades star cluster.
- if you have an educational or scientific reason to observe a bright star, contact a #human.
- the telescope will automatically reject observations of stars brighter than magnitude 6.5 and any other observation where a star brighter than magnitude 3.5 is in the same field of view.
This could be for a number reasons. First of all, you can check your place in the queue anytime here or by using the #myrequests command. Recent requests are towards the top and older requests are towards the top. To find all of your requested and completed observations, go here and search for your name.
In general, observations are run in order from oldest to newest, but there are several other factors that decide when an observation is run.
The most important factor is the need for clear skies at night! In Halifax, we typically get only 2-3 nights a week with the skies clear enough for part or all of the night. And the clear skies available may not have occurred at the time of night needed to observe your request!
Another factor is the choice of object - if an object is only visible for a short time each night, it competes with other observations observable at the same time. This "competition" isn't always fair! The observations requested by our astronomy students, as they are our "paying customers" with project deadlines have priority (this is usually only a factor from about mid-September to early April each year).
For more details on how the telescope's programs decide when to run an observation, see the next FAQ.
When the telescope is running, the skies are clear and dark, and it is about to run an observation, the first thing it does is scan through the entire observation request queue to see which requests can be observed right now. The factors it checks include:
- that the object's altitude above the horizon at the beginning and anticipated end of the observation is ok
- that the anticipated end of the observation will complete before morning twilight begins
- that the Moon is not too bright and is not too nearby in the sky
From the list of observable requests a "points game" is then played and the observation request with the highest score is run! The factors scored include:
- a benefit for older queue entries - older requests are favoured over more recent requests.
- a benefit for objects with limited visibility - objects that can be observed tonight for a shorter time than others are favoured.
- observer priority - each observer is assigned a priority, either more or less than the default.
- group priority - each observer is a member of a group of observers and each group is assigned a priority, either more or less than the default. This is used to give groups of observers (eg. our students and those working on special projects) greater priority.
- observation priority - each observation request can be given a priority, either more or less than the default.
- same or opposite side of pier - our telescope is attached to what is known as a German Equatorial mounting. That means that it has to "flip" sides when moving from the east-to-west or west-to-east sides of the sky. This takes a few minutes so a penalty is applied if needed.
- distance from current telescope position - objects near where the telescope is currently pointed are favoured.
Optical filters are just that - they "filter" out some colours of light that would otherwise reach the telescope's camera. However, they are named by the colour of light that they actually let through (their "pass band") - for example, "RED" lets through mostly red light and blocks everything else. The "LUM" filter (which means luminance) lets through most of the visible spectrum and is the default filter - it lets through the most light and should be used unless you have a particular reason to use another filter.
All available filters are listed on our tech info page, but some general information that you many find helpful is found below.
There are three logical groups of filters:
- LUM, RED, GRN, BLU: These are a set of filters intended to create a colour image by combining in post-processing either RED, GRN, and BLU filters or RED, GRN, BLU, and LUM filters. As you may have guessed "RED" has a mostly red pass band, "GRN" a green pass band, and "BLU" a blue pass band. These filters are intended for "pretty pictures", not science observations.
- B, V, R, I: These are a set of filters developed in the 1950s by professional astronomers to measure star brightnesses using photo-multiplier tubes and photographic plates. They are part of the Johnson-Cousins photometric system that have pass bands chosen to help classify stars. They are still in common use today as they allow observations to be standardized between multiple observatories and compared to historical published data. They have pass bands that are roughly (in order) blue, green (visual), red, and near infrared, but are not as efficient as the similar RED, GRN, and BLU filters, so they should not be used unless you are taking scientific observations.
- OIII, HA: These are narrowband filters that only pass a very specific colour emitted by certain gaseous nebulae. OIII (Oxygen-III) passes the green light originating from doubly-ionized oxygen atoms. HA (Hydrogen-Alpha) passes the red light originating from atomic hydrogen gas. Stars, galaxies, star clusters, and reflection nebulae emit broadband light and are generally not good uses for these filters as a small fraction of the object's light reaches the camera. Emission nebulae, planetary nebulae and supernova remnants emit most of their light in specific colours which include Oxygen-III and Hydrogen-Alpha, so generally are great uses for these filters. The big advantage is that the filters let through most of the light emitted by the nebulae but reduce greatly the background light pollution (which creates noise).