BRIEF Telescope, Camera, and Filter Factors To Consider Boyce Astro: http://www.boyce-astro.org 1
Overview: This class is intended to help you in choosing how to image your candidate Double Star. For this class, you will have the advantage of automated imaging. However, you will still play a significant role in determining how these images are taken: Telescope selection, exposure length, and filter type. Boyce Astro: http://www.boyce-astro.org 2
Presently, your images will be acquired through itelescope. This is a network of telescopes located in New Mexico, Spain, and Australia. Below are some images of the scopes available to you. Boyce Astro: http://www.boyce-astro.org 3
You will order the images and select all the parameters through the Boyce-Astro website: www.boyce-astro.org. The image to the right is the form that you will fill out in order to order your Double Star images. To do this, you will need to understand, and know the following: Particular Telescope needed WDS designation of your Double Star WDS Coordinates in RA/Dec Filter to be used Exposure length Time of exposure So, how do you gather this information? Go to the next slide, and let s get started. Boyce Astro: http://www.boyce-astro.org 4
Choosing a Telescope: itelescope has three locations from which images may be taken: Spain, New Mexico, and Australia. Spain and New Mexico are best for stars with a Declination between -20 through +90 Australia is good for +20 through -90. NOTE: Reference the Online Class tutorial Class How To Select Stars for Observation PowerPoint for a lesson on determining the time of year, and time to image your particular Double Star Each of these sites contain a variety of telescopes, which we will highlight in a minute. Each telescope contains a specific camera. The cameras can be broken down into two categories: One Shot Color & Black and White. One Shot Color: These telescopes take color images in a single exposure. For most scientific work, One Shot Color cameras are not useful due to the way they collect data. HOWEVER, for Double Stars, One Shot Color cameras work just fine without significant degradation in data. Black and White: These cameras capture data through the use of filters, described on the next slide. Boyce Astro: http://www.boyce-astro.org 5
Choosing a Filter: All starlight that is visible to our cameras, and your eye, is a combination of seven different colors: Red, Orange, Yellow, Green, Blue, Indigo, and Violet. Boyce Astro: http://www.boyce-astro.org 6
Choosing a Filter: For a given star, its temperature determines its color: the hotter, the bluer, the colder, the redder. However, with Black and White cameras (most scientific astronomical cameras are Black and White), below (right) is the image that is received: Boyce Astro: http://www.boyce-astro.org 7
Choosing a Filter: In the image, although Black and White, you are receiving all the light as indicated on the left. This is ok for most images, but in some cases, to be discussed on later slides, can create problems for Double Stars where they are relatively close to each other and quite bright (ex. a low magnitude number). In this case, we can use filters to remove some of the light in order to obtain useful images. Boyce Astro: http://www.boyce-astro.org 8
Choosing a Filter: The image below on the left, represents all light from the star. From a filter perspective, this is what you achieve with a Luminance Filter. The image on the bottom right, shows what a filter can do: Cut out all bands of light except for a particularly desired band. This will take a potentially over-exposed image and make it useful for data collection. Boyce Astro: http://www.boyce-astro.org 9
Choosing a Filter: The image below on the left, is a particular Double Star system imaged through a Luminance Filter (allowing all light bands through). The one on the right, is the same Double Star imaged through a Red Filter (allowing ONLY the red light to pass through). Note the difference? Luminance Filter Boyce Astro: http://www.boyce-astro.org 10 Red Filter
Choosing a Filter: Here is the same Double Star imaged through three different filters: Luminance, Red, and Green. Luminance Filter Red Filter Green Filter Boyce Astro: http://www.boyce-astro.org Red Filter
Choosing a Filer: The available filters to choose from with itelescope are: Luminance* Red* Green Blue Hydrogen Alpha* Silicon (SIII) Oxygen (OII) The recommended filters for Double Stars are indicated with a *. Boyce Astro: http://www.boyce-astro.org
Choosing a Telescope: The next step in determining a suitable telescope is to look at the resolution of the telescope/camera combination. For Double Stars, the lowest resolution is the best as we often try to image Double Stars that are close to each other. This means that the lower the amount of space (arc-seconds) covered by a single pixel, the better. Using the New Mexico telescope/camera combinations as an example, here is an example of the available resolutions: Resolutions of 2.0 arc-secs/pixel and lower are desirable. Boyce Astro: http://www.boyce-astro.org
Choosing a Telescope: Here are available Telescope/camera combinations and their resolutions: Boyce Astro: http://www.boyce-astro.org
Choosing Exposure Length: What follows is a very fundamental (read: Extremely simplistic) overview of how an astronomical image is acquired. An astronomical camera is called a CCD: Charged Coupled Device. At the core of each CCD is basically a light sensitive grid where each image is captured by allowing individual photons of light from the star to be collected in buckets. The collection of photons in the buckets results in an image. Boyce Astro: http://www.boyce-astro.org 15
Choosing Exposure Length: Each bucket s job is to capture photons (light) from the each star. The brighter the star, the more photons. The fainter the star, the less photons. Additionally, the longer the exposure the more photons are allowed to be captured by the buckets. Sounds good right? Maybe. Boyce Astro: http://www.boyce-astro.org 16
Choosing Exposure Length: Let s take an example of a single star with average brightness on a CCD grid. In this case, a single bucket has collected the photons. Boyce Astro: http://www.boyce-astro.org 17
Choosing Exposure Length: The longer the exposure, and/or, the brighter the star, these buckets can fill up with photons. Once full, the photons will bleed over to adjacent buckets. This is OK if the stars are sufficiently far apart. However, if they are too close, or the primary star is significantly brighter than the secondary, then problems arise. Boyce Astro: http://www.boyce-astro.org 18
Choosing Exposure Length: In the example below, we see two stars on the right. With an increase in exposure time we see the image on the left, where the buckets are full and begin to bleed into one another. This is not good for Double Star measurement as you can no longer tell where the center of each star is located, and thus cannot obtain a proper measurement. Boyce Astro: http://www.boyce-astro.org 19
Choosing Exposure Length: So, how do you avoid the image on the left, and always obtain the image on the right? Answer: Balance between exposure time and Filter choice. Remember from above that the filters can cut out portions of light allowing longer exposures without over saturating the CCD buckets. A Luminance filter, given a bright star (ex. Magnitude 7 or lower) can quickly saturate the CCD buckets. However, the same star with a Filter can make the left image look like the right one. In short, this is the balance between exposure time and filter choice that you must determine. Your instructor can assist you in this quest. Boyce Astro: http://www.boyce-astro.org 20
Questions? Boyce Astro: http://www.boyce-astro.org 21