October 27, 2003

Last Revised October 11, 2007

These instructions are also divided into two parts. The first part is a "quick start" and will be sufficient for most people to get going. The second part is a item by item description of each data field and button function.

The calculator opens up with the default values for a CP4500 camera and a Swarovski ATS80HD scope. The default scope magnification is set at 20x. The default camera focal length is set at 18mm. And the default f-number is set at f4. These default settings are somewhat arbitrary and are the variables that will probably want to change.

For instance, if you are trying to determine the maximum possible 35mm equivalent focal length for your scope and camera, you would change the "Scope Magnification" and the "Camera Focal Length" before calculating the digiscoping data. Given a specific camera and scope, these are the two values you will most likely be changing. If you also wanted to see what effect the camera aperture would have on a variety of factors, you might also enter in a different "Camera f-number".

All of the data fields will allow you to enter in information for different cameras and scopes. These can be real or imagined. For instance if you wanted to see what advantages a 100mm aperture scope would have over your 80mm, you could simply enter in the new aperture value and calculate the results. But I suggest reading the detailed info on each data field and calculation button before doing this type of experimentation.

Be careful when entering values to be sure that you have completely overwritten the previous values. It is very easy to find that you simply appended your new value to the old value. You can easily find yourself with a ridiculous "Scope Magnifaction" of "2030" if you are not careful.

A value of "NaN" in field indicates an error of some sort. It literally means that the data generated is "Not a Number". Odds are that there either is no data entered in a field or that some non-numeric character was erroneously entered somewhere.

If you want to do some "what ifs" for camera design, simply select from the list of sensor sizes. The CCD diagonal, equivalent focal length and camera FOV will be calculated when a different sensor size is chosen. You can type in different horizontal and vertical pixel values and then recalculate using the Calculate FOV and Calculate CCD buttons.

You can vary the camera's f-stop and see how this effect the limits of resolution due to diffraction by clicking the Calculate Digiscoping Data button. You can compare those numbers to the lp/mm limits imposed by the sensor.

I have used George's spreadsheet for over a year and have modified my copy to suit my own purposes. One of the limitations of George's spreadsheet was that you had to have Excel(tm) in order to use it and this was the main reason I made this online Javascript calculator. But I have since found www.openoffice.org which is free software that runs George's spreadsheet very well. There are still advantages to an online resource (like you don't have to do anything if it is updated and this one now has preset values to choose), so I'll keep this going.

This first version of the online calculator gives the core calculations that I found myself using most frequently. It also has an additional feature that allows you to estimate camera to subject distances fairly accurately if you have a good idea of the size of the subject in your image. I initially added this feature to my copy of George's spreadsheet for the "gee whiz" factor. But later I found that it was useful in determining that atmospheric disturbances tend to affect digiscoped images at distances greater than about 50 meters. So getting a good feel for subject to camera distances can be useful and this may actually help that process along a bit.

This calculator gives resolution values in line pairs per millimeter (lp/mm) to make it easier to compare theoretical results with actual results obtained from test targets. Specifically, I use Norman Koren's test targets that can be downloaded from http://www.normankoren.com/Tutorials/MTF5.html - another excellent site (though focused on photography in general and not digiscoping in particular). It was also found that George's original resolution calculations were off by a factor of two (something pointed out by someone on the Yahoo Digiscoping Birds forum). This online calculator does not have this error. However, it is interesting to note that the erroneous values that George's original spreadsheet gave were very good predictors of actual digiscoping performance. This is strong evidence that we are not dealing with an optical combination that is diffraction limited when we digiscope. In the meantime, be aware that it is unlikely that you will actually get image resolution near the theoretical limit. For now, it seems safe to assume that a digiscoping rig will deliver between 50% and 75% of the theoretical resolution calculated.

Version 1.1 (12/06/03) added buttons for presets and made the calculator a bit more compact vertically.

Version 1.2 (12/13/03) removed ugly buttons and replaced them with select lists. Added more presets for a greater variety of scopes. Magnifications for scopes are fairly arbitrary since many eyepieces are available. I had a difficult time locating focal lengths for Nikon and Kowa scopes. Feel free to email me with good technical specifications for any cameras or scopes that you would like to have me include in the list.

Version 1.3 (01/03/04) More entries on select lists added since 12/13/03. Added info for camera minimum zoom values. Also added calculations to provide camera FOV values for these minimum and maximums. Added some quick instructions and detailed descriptions of the calculator fields and functions. Removed the instructions embedded in the calculator interface and modified the naming of some of the buttons and fields for better formatting.

Version 1.4 (01/14/04) Added the 50% MTF and 10% MTF fields. Camera notes and more cameras were added in a later version of 1.3, but I'll mention their addition here. I also adjusted the wavelength of light used in calculating resolution limits from 0.00055 to 0.0005 to be consistent with the calculations on Norman Koren's site. The difference is small, but there seemed to be enough pointers suggesting that the 0.00055 value was a bit more pessimistic than it should be. Its does not shift the values much, but it does shift them.

Version 1.4a-d (05/21/04) Have added various scope and camera values over that past few months as they seemed appropriate.

Version 1.4e (07/17/04) More camera and scope values added including a "No Scope" option to simplify camera only calculations. Added a sensor size pulldown and data. Megapixels are now calculated when a camera is selected or when the CCD calculation is made. These modifications should help with camera "what if" calculations.

Version 1.4f (08/28/04) More cameras added and the sensor selector was modified to fully calculate equivalent focal lengths, CCD diagonal and pixel pitch when selected.

Version 1.4g (01/01/05) Added the Nikon D70 and Kyocera SL400R as well as two TeleVue scopes, the Celestron C5 and the Astro-Physics Traveler. Added a note about the 50% MTF value when considering obstructed catadioptric scopes. Dropped the old camera table.

Version 1.4h (01/21/05) Modified calculation for Digiscoping FOV. The previous calculation was very slightly off due to the use of an incorrect and overly simle formula.

Version 1.4i(03/17/05) Added three cameras. The Sony DSC-W1, Sigma SD10 and Olympus E300.

Version 1.4k(12/08/05) Added more cameras. Canon 5D, Nikon D200, Sony DSC-R1

Version 1.4l(11/12/06) Added Zeiss Camera-Eyepiece DC4, Sony DSLR-A100, Nikon D80, Canon EOS 400D, Sigma SD-14

Version 1.4M(05/05/07) Added William Optics ZenithStar 66 SD APO telescope.

Version 1.4N(05/09/07) Added Nikon P5000 10Mp Compact digital camera.

Version 1.4P(10/11/07) Added Nikon P5100, FujiFilm F30, F31fp, F20 and E900. Also updated the Nikon DSLRs to reflect the newer cameras added. These are now grouped under the same camera when the cameras have the same sensor size and pixel count. Added new Leica Televid 65 and 82 scopes as well as the Swarovski ATS65HD.

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If you have questions, comments, suggestions etc. please feel free to write me at:jay@studio522.com

Copyright © 2003 by Jay Turberville