I can not get the FWHM of this system better than about 13. The OTA is a Meade 16" f/8 sct. Daws limit is .29". The camera is the Aluma 814 with 3.69 micron pixels. The guiding RMS error, using PhD guide software is between .5" and .6". Seeing is a bit of a guess but this is a Bortle class 4 site. The night was very clear but just assume a seeing of 4". Image scaling, I used the formula 206 * Pixel size of 3.69 microns / focal length of 3240mm. This calculates to an image scaling of .23". Using the FWHM formula of sqrt(seeing^2+daws limit^2+guide RMS error^2+image scale^2) that is sqrt(4^2+.29^2+.6^2+.23^2) = 4.06 expected FWHM at best focus. Obviously seeing dominates this calculation. Attached is an image at the best focus I can get with this system. The focuser is a WR30 from Nite crawler. As you will see my best FWHM is around 15. Any ideas what my problem might be?
That's a very long focal length for a small pixel camera. Yes the image scale is 206 * 3.69 um / 3251 mm = 0.23 arc-seconds per pixel. Your FWHM in that image is about 3.5 arc-seconds. That would be "mediocre" seeing at my observing site. (Average is 3.0, excellent is 2.5.) You should be able to get better images... but not a lot better. The star images aren't quite round; it looks like coma. You may need to adjust your instrument's collimation. That will help a bit. The main problem, though, is that the focal length is too long for your pixel size. You need about half that image scale. Using a focal reducer, or binning 2x2, would be in order.
Some possibilities in no particular order: a) is the scope an ACF or an SCT ? most of the Meade f/8 are ACF, not SCT. The ACF generally have a better flat field b) scope collimation - the stars aren't round, they seem to have some coma c) lack of a flat field - do you have a focal reducer / field flatenner of some kind? d) guiding e) main mirror flop can cause shifts around the sky - it might have been fine in one orientation, and not another.
Ok gents thanks for responding as this isn't exactly a problem with the camera. Collimation, yes I notice the elongation of the stars. I have ordered an artificial star for collimation, $110. Maybe I will need the Hoteck laser collimator system at $450. No focal reducer or flat field reducer but I have been looking at at focal reducer for other reasons not for a mismatch in focal length to pixel size of the camera. FWHM you say 3.5" why do I measure 15 plus with the Maxim tool? I must have something set wrong. How do you see 3.5 when I see 14-15 in the information window? When you say I need 1/2 the image scale I assume you mean the 0.23" needs to be more like 0.46". I think I can get a .7 focal reducer and with 2x2 binning then I should be where I need to be, right?
If you can get a 0.7X focal reducer, you'll probably be in reasonable shape. I try to go for a pixel size that is 1/3 of the best seeing FWHM. That provides optimum resolution. You can set the Information window to show FWHM in arc-seconds rather than pixels. It's useful for checking your seeing. You want to turn on the two check boxes as shown below (make sure your focal length is set correctly in File / Settings if you switch to using a focal reducer). You can toggle Display in Arcsec on and off as needed to see in pixels or in arc seconds. https://diffractionlimited.com/help/maximdl/HID_VIEW_INFORMATION.htm
LOL I beat you to it by just a few minutes. Yup found the same and set them and I get the same readings you do. I see the pixel scale is as I calculated at .23". So number work. Thanks so much for the help. Will these setting help in the auto focusing in the observatory window?
Seeing, is there a way to measure seeing with any numeric accuracy? Seeing is clearly the tall tent pole in the FWHM calculation. In fact all other variable are almost negligible in the set I gave you. Seeing is the only one I am guessing at. Here in the southern Black Hills of SD the sky is very clear. Wind at ground level at night is calm. On many moonless night the Milky way illuminates the path to the observatory. Town light from the east rise to 45 degrees noticeable but to the south and west I bet the sky's are at Bortel class 3 to 2 compared to 4 to the east. On another note I am looking into a much faster OTA as a second system. I guess the Aluma 814 would be better suited for that OTA than the 16" f/8. I am looking at this astrograph: Officina Stellare 400mm f/3.8 RiFAst Astrograph - Standard Glass - Alu (optcorp.com) What camera would recommend for either the f/8 or this f/3. I don't see me selling the Aluma 814 as I would loose to much money.
Seeing is complex. It has nothing to do with the clarity of the skies or light pollution. You can sometimes have very poor transparency and excellent seeing at the same time. FWHM is the simplest way to measure it. There are instruments that are specifically designed to measure seeing directly (e.g. DIMM monitors). I published a handy reference chart in this article: https://diffractionlimited.com/how-to-choose-an-astronomical-imaging-camera/ Here's the chart. Along the top is your pixel size. Down the left is your focal length. Just look up your telescope and camera's parameters and it reads out your arc-seconds per pixel. You want that number to be about 1/3 of your typical seeing disk FWHM. My local seeing is typically 2.5 to 3.5 arc seconds. If I want to get the best detail on a good night I'd choose something around 2.5 / 3 = 0.8 arc-seconds. If I was using an Aluma 814 I'd probably want a focal length of around 1,000 mm to take full advantage of my good nights. I might push it as high as 1,500 mm. If you are on top of Mauna Kea you might get seeing under 1 arc-second, in which case you'd want to go for more resolution. Some friends of mine live on a mountain and often get seeing in the 1.5 arc-second rage - they definitely want to use more focal length than I do!
Doug thanks for this info. So my set up at bin 1x1 put me well in the red. Binning at 2x2 looks to put me just out of the red. However, at 1x1 doesn't that just mean I am wasting pixels and not really hurting the image quality? A friend said when I bin 2x2 I need to let the software know this. Isn't that information in the .fits header? Otherwise I don't know how to let the software know I am using a bin other that 1x1. Collin my OTA is an ACF as you suspect.
In MaxIm, Camera Control Window, Expose tab. Look for X binning, Y binning. You just set it there to 2 x 2 or 1x1 and it does the rest.
