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Digitized Sky Survey Overview

Digitized Sky Surveys are online services from which gray scale (black and white) and color images may be retrieved for various locations in the sky. The services are available from different institutions around the world and each institution delivers images from a web server. The availablility of these servers varies. It is not unusual for a server to be very slow in delivering images (or simply not delivering at all) from time to time.

The images themselves were acquired through different optical filters, for example the DSS-2 survey was conducted through red, blue and infrared filters. Furthermore, different surveys cover varying amounts of the sky.

New in version 9, images are saved in JPEG format and require less storage than (deprecated) GIF images.

Servers & Surveys

Since each server contains a varying set of images from surveys, users must determine where to go for a given image. A good general rule is that the DSS-2 Red survey at STScI has better image scale and the greatest coverage of the sky; therefore it is a good server and survey to set as your default DSS source.

Server and survey are selected in Preferences. DSS images are displayed in various documents with abbreviated status codes that describe the server and survey from which the image is taken. The table below describes this information.

STScI surveys
Survey Name In Preferences	Status codes displayed with images	Description
DSS-1 Blue	poss1_blue	First generation DSS using blue plates. Covers sky north of -30° declination. Image scale is typically 1.7 arcsconds per pixel.
DSS-1 Red	poss1_red	First generation DSS using red plates. Covers sky north of -30° declination. Image scale is typically 1.7 arcsconds per pixel.
DSS-2 Blue	poss2ukstu_blue	Second generation DSS using blue plates & filters. Covers about half of the sky with more data yet to be processed. Image scale is typically 1.0 arcsconds per pixel.
DSS-2 Red	poss2ukstu_red	Second generation DSS using red plates & filters. Covers nearly all of the sky with a little more data yet to be processed. Image scale is typically 1.0 arcsconds per pixel.
DSS-2 IR	poss2ukstu_ir	Second generation DSS using infrared plates & filters. Covers about a third of the sky with more data yet to be processed. Image scale is typically 1.0 arcsconds per pixel.
SkyView surveys
Survey Name In Preferences	Status codes displayed with images	Description
DSS-1 Blue	DSS1B	First generation DSS using blue plates. Covers sky north of -30° declination. Image scale is typically 1.7 arcsconds per pixel.
DSS-1 Red	DSS1R	First generation DSS using red plates. Covers sky north of -30° declination. Image scale is typically 1.7 arcsconds per pixel.
DSS-2 Blue	DSS2B	Second generation DSS using blue plates & filters. Covers about half of the sky with more data yet to be processed. Image scale is typically 1.0 arcsconds per pixel.
DSS-2 Red	DSS2R	Second generation DSS using red plates & filters. Covers nearly all of the sky with a little more data yet to be processed. Image scale is typically 1.0 arcsconds per pixel.
DSS-2 IR	DSS2IR	Second generation DSS using infrared plates & filters. Covers about a third of the sky with more data yet to be processed. Image scale is typically 1.0 arcsconds per pixel.
DSS-1	DSSOld	Original DSS using mostly red plates & filters. Covers all sky. Image scale is typically 1.7 arcsconds per pixel.
SkyServer survey
Survey Name In Preferences	Status codes displayed with images	Description
SDSS Color	sdsscolor	Sloan DSS data release 18. Limited sky coverage; image scale varies.
CDS survey
Survey Name In Preferences	Status codes displayed with images	Description
DSS-2 Color	dsscolor	Second generation DSS using Hierarchical Progressive Survey (HiPS) technology to generate images. Covers sky north of -30° declination; image scale varies.

Image Size

DSS service can deliver images of any size up to 90 arcminutes square. Deep-Sky Planner can automatically set the image size according to the size of an object, but the user should understand that image sizes above 30 arcminutes are noticably slower to download and display. When an image size has been set automatically and it is not the desired size, users can use the Observing Plan Object Editor (i.e., Plan Editor) to set the size manually for plan documents.

How Deep-Sky Planner Treats DSS Images - DSS Links

DSS images are contained in files that are delivered from web servers. They can be downloaded and stored in a file on your local hard drive and then loaded into a window for viewing, or they can be downloaded and loaded directly into a window (not stored locally). Deep-Sky Planner creates a link for a DSS image that defines the web server, survey and image size. Deep-Sky Planner can detect whether an image for a specific position in the sky is stored locally or needs to be downloaded online. This is indicated in both the Plan Editor and in the DSS object detail tab page in the plan document. Whether you choose to use images stored online or stored locally depends on a number of factors.

Online storage is suitable if you have Internet access at your observing site and you have little hard drive space available on your computer. It is also recommended when you need images for asteroids, comets and planets since these objects move constantly. Accessing DSS images from online storage is slower than accessing them from local storage. To use the online storage strategy:

1.Manually download the image into the DSS detail tab page for the desired object, or

2.Automatically download the image into the DSS detail tab each time an object is selected in the plan document. This strategy is employed by checking the Fetch Requested DSS Image If Not Stored Locally option on the Localize tab of the plan document window.

Local storage is a must if you do not have Internet service at your observing site. It is also recommended when you need images of deep-sky or stellar objects. As stated above, image files can require a large amount of storage space on your computer, but displaying these files is nearly instant. To use the local storage strategy:

1.Download and store the images by selecting Get DSS Images on the General tab of the plan document window, or with either the Plan Editor, or in the DSS detail tab page for each selected object. You can perform simple processing on an image while you view it.

2.If you want to copy images from one computer to another, use the Backup and Restore features and make sure you include (check) DSS Images in each operation.