When trying to decide on the optimal DSLR and telescope combination, it is important to know how much of the sky will be included in the field of view. Furthermore, it is important to match the focal length of the telescope with the imaging sensor for obtaining the best results.
There is a handy (and free) calculator available that will make your job of matching telescope to digital SLR very easy! Blast on over to Ray Shore’s site for his Calculator for DSLR Astrophotography page. Note that the link opens in a separate window so that the calculator can be conveniently referenced. This article provides the basic concept and instructions for using Ray’s DSLR Astrophotography calculator.
Entering the Data
The first step is to enter the DSLR and telescope data. A handy table is provided for the most common DSLR’s used for astrophotography:
The data in this table is used to populate the following fields:
- Camera Sensor Width (mm)
- Camera Sensor Height (mm)
- Camera Maximum Resolution- Width (pixels)
Just find your camera in the list and use its data in the respective fields shown below.
If your camera is not listed, refer to the digital SLR user manual or do a Google search for the information. It is generally easy to find. Be sure that your camera sensor width and height are expressed in millimeters.
Note about camera maximum resolution: this is the number of pixels “wide” in the highest resolution setting.
Next, you must provide the telescope focal length. Note that this must be the focal length in millimeters.
When all four fields are loaded with the specific data for DSLR and telescope, click the “Calculate” button. The next sections explains the results.
Arc Minutes of Sky- Will it Fit in the FOV?
It is important to know if an object will fit within the borders of the final image. The amount of sky that will be framed in your astrophoto is determined by the imaging sensor size of your DSLR (in millimeters) and the focal length of your telescope (in millimeters). Let’s use the following example to see how well certain objects fit within the field of view.
We will use the data for the Canon 300D. First load the data as shown below.
Click the calculate button to get these results:
The first two fields indicate the amount of sky that can be imaged with the Canon 300D and a 600mm focal length telescope. Thus, an object (e.g. nebula) that is less than 130 arc minutes wide and 86.5 arc minutes high should fit in the image taken by this DSLR/telescope combination.
Now compare this to the list of Messier objects listed below the calculator.
As you will see, all 110 Messier objects (except M31) will fit within the field of view! But if you double the telescope focal length (i.e., 1200mm), then you have a problem fitting M42 within the field of view as well.
Arc Seconds Per Pixel- Optimum Sampling
This is another important parameter to keep in mind when matching a particular DSLR to a telescope. When it comes to astrophotography, the best results depend on the pixel size of the DSLR sensor matching the resolution of the telescope. This relates to the concept of sampling. Sampling refers to the number of sensor pixels that are used to represent the detail in a scene (or specific object such as a star). Or put another way, sampling is the ability of the camera to accurately reproduce (digitally) what is seen through the telescope. The general rule of thumb is approximately 2 arc seconds of sky per pixel. There are two issues to avoid:
Oversampling– too many pixels are being used to reproduce the detail.
Undersampling– not enough pixels are being used to reproduce the detail.
Back to the example above. The arc seconds/pixel from a Canon 300D/600mm telescope combination is 2.5. This appears to be a good combination for optimal sampling!
This is a parameter that is not used too often in amateur astronomy. But sometimes you may want to know how many times an object is magnified in the astrophoto. The calculator provides this information based on the details that were entered. Following the example through, the Canon 300D and 600mm focal length telescope produced an image of an object that was magnified 22 times.
This concludes the tutorial for using the DSLR Astrophotography Calculator from Ray Shore’s website. If you have any questions or comments, please feel free to contact Ray via email at firstname.lastname@example.org. Or, use the comments section below.
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