We are familiar with the expression, “Too much of a good thing”, which leaves you with the thought that excess may do harm. I can attest the expression, “Your mileage may vary” is true because I know my wife can drive much further on a tank of gas than I can.  One other hand “One size fits all” is rarely true and I do not have to mention the things that no longer fit. As an engineer I am very familiar with the idea of “diminishing returns” as it plays a important role in design. Its important not to over achieve in one aspect at the cost of others.

But what do these have to do with astronomy?

The calculation sheet below is available in the Old Photons Observatory Public Google Drive at Calculation Sheet. Calculations are made for both the primary system, SBIG STF-8300M FW8-8300 CCD on the Celestron EdgeHD 11 telescope and secondary system  ASI1600MC Pro CCD on William Optics 90mm telescope.

Both CCDs are the same size at 22mm on their diagonals, however the CCD pixel resolutions are quite different. STF8300/C11 has a scale of 0.57arcsec/pixel where ASI1600/MR90 has a scale of 1.26arcsec/pixel. This is calculated from pixel size divided by focal length. The factor 206 comes from (180/π)*60*60/1000 which is a scale factor that converts radians to degrees, degrees to arc seconds, and meters to mm.

It is difficult to get a grip on how big an arc-second is. One arc-second of angle is approximately the thickness of a dime at a distance of nearly a half kilometer. If that is hard to visualize then perhaps the same dime at arms length is about 500 arc-seconds. Nope,… neither of these does it for me. I guess I should have written that it is difficult to get a grip on how damn small an arc-second is.

Because telescopes have a finite aperture, they have a limited resolving power. The larger the aperture the higher the resolution of the image the telescope can place on the CCD sensor’s surface. Dawes limit is one calculation of this resolution. The C11 has a limited resolution of 0.43arcsec where the MR90 has a limited resolution of 1.33arcsec. These are the performances the telescopes would have if they were perfect and only limited by their finite aperture. This is where the term diffraction limited comes from and your telescope should be diffraction limited.  The central obstruction of the C11 and its collimation will degrade its resolving power, but we can be assured it will never be better than the Dawes limit.

The STF8300/C11 has a scale of 0.57arcsec/pixel with a Dawes limit of 0.43arcsec. By the numbers, the STF8300 is the limiting part of the system because its resolution is poorer than the C11.  The ASI1600/MR90 has a scale of 1.26arcsec/pixels with a Dawes limit of  1.33arcsec. Unlike the primary system, telescope is the limiting part of the system because MR90’s resolution of poorer than that of the ASI1600. The only 3 factors that contribute to these outcomes are pixel size, aperture and focal lengths.

Aperture is a good thing in most minds, and more is better, but pixel size needs to shrink smaller to keep up. In this respect too much of a good thing can leave harm in our pocketbook and wasted untapped telescope potential.

When childrens’ stars twinkle, the astrophotographers are likely in bed. When the stars are twinkling we would say that Seeing is poor. The Pickering Seeing scale is used as a way to quantify seeing.  Because seeing conditions are dominated by atmospheric conditions the clear sky clock, for example, predicts seeing based on current environmental data.

Wikipedia indicates that best conditions result in about 0.4 arcsec seeing. However this is only expected at high-altitude and small island observatories. One or two arcsec seeing is a more appropriate to assume for the rest of us.

This brings us to topics of diminishing returns and variable mileage. The 0.57″/pixel, Dawes limited 0.43″ STF8300/C11 and 1.26″/pixel Dawes limited 1.33″ ASI1600/MR90 systems are both into the diminishing returns area. Only under better than average seeing will the STF8300/C11 show its value. Image quality will depend on nightly variable seeing conditions and so in this regard your mileage will vary.

The calculated field of views are 32’x24′ and 98’x74′. The ASI1600/MR90 will have 3 times the FOV of the STF8300/C11 and it is for this reason both systems available. In astronomy one size does not fit all and a change in equipment is desirable.

Adrien