best camera

maplearn

I have a Celestron C14 Edge with a .7 reducer and am presently using a ASI2600 which is not a good match, the stars are bloated. What is the best camera you would recommend including new releases? Take mostly nebulae and galaxies in a Bortle 4.5 sky.

Thanks

w7ay

maplearn Celestron C14 Edge with a .7 reducer... using a ASI2600

That combination gives you a Dawes limit of 0.33 arcsec, with the resolution of 0.28 arc sec per pixel.

So, under ideal seeing (very high mountain top, when "seeing" is way better than 0.5"), it is not that poor a match.

However, on an average night at sea level, when "seeing" is 2 arcsec, the stars can bloat to 15 pixels! (and during sub par nights, even worse).

The difference between bad seeing and good seeing is smaller when using a small refractor, since the Dawes limit of the small aperture scopes are probably already around 2 arcsec, and the short focal lengths also produce smaller spot diagrams.

So, what you are observing is likely "seeing" related, instead of camera related.

On an average night, the image from the large scope may not actually be visibly sharper than the image from a much smaller scope, but on exceptional nights, you will sweep the floor with the smaller scope.

Given what you have, unless you mostly image on a very high mountain top, the stars will bloat way larger than the pixel size pretty much all of the time. You can simply scale down the image (or use Binning to save image transfer time). But something like the QHY410C, with its 6 µm pixels may be better for most nights (or, simply Bin down the ASI2600, since you already have that camera; at a small loss of sensitivity vs the QHY410 since CMOS binning is done non-coherently).

Now, with lucky imaging for planets (where you are countering "seeing" by using very short exposures) it is a different story -- you have probably the best hobby grade telescope there is for that purpose. But you will need a different camera too; the frame rate from a ASI2600 is too slow.

Chen

maplearn

w7ay

maplearn

Thanks for your detailed reply. I tried four bin settings, see the attached photos, did not make much difference, the sky was bortle 4.5. (magnitude arcsec squared 20.6) Pictures are raw unprocessed, no flats, darks or bias as samples. Bit overexposed, stacked 10 at 180 seconds. Tried 120 seconds but not much difference. Stars all bloated. Several people suggested the 294MC Pro. Thoughts?

w7ay

maplearn Several people suggested the 294MC Pro

The images you posted confirms that when binned, the stars don't look any more bloated. That is why I suggested simply to Bin the sensor, so the downloads are faster (i.e., there is as much information in the binned images as there is in the unbinned one).

I personally wouldn't touch an IMX294 sensor from ZWO with a 10 foot pole; if you want that extra 20% in pixel size, get an IMX294 sensor from someone else. In any case, the "regular" pixel size is barely larger (20%) than your ASI2600 (4.6 µm vs 3.8 µm). Your bloat is a factor of 1500%, not some small percentages.

You just have to live with the fact (assuming that your average "seeing" is around 2") that a 14" OTA is simply not going to be give you much higher resolution than a good 106 mm refractor. I.e., when scaled to the same image scale, they will both show the same star size (just as all the stars in your images show pretty much the same size after you bin them).

The limiting factor is the 2-arcsec "seeing" which is bloating your star to 2 arcsec, which represents some 15 pixels on your camera. If you want to just take images of galaxies, an 8" SCT is more than adequate in terms of how much sharpness you can get from an OTA at sea level. If you want to do lucky imaging, then by all means use a 14" or 30" OTAs -- different tools for different jobs when it comes to imaging.

Now, you can still use the 14" profitably for visual, since the human eye-brain is very good at doing "lucky imaging." Your brain will see and register something that is momentarily sharp.

Although this white paper is about focusing accuracy, you can also see what "seeing" does to a large OTA (especially the simulated spot diagrams for 1" seeing vs 2" seeing at the end -- and remember that your image scale has seeing at about 15x the diffraction limit where the simulations show just a factor of 1x to 2x of seeing vs diffraction limit):

https://www.innovationsforesight.com/support/celestron-edgehd-back-focus-tolerance/

Once the 2 arcsec "seeing" sinks in, it will all be clear. A 14" is the wrong OTA just to take images of galaxies and hoping for sharper stars and details in the galaxy arms. It won't happen. In this case "bigger," is no better than "smaller." It is not a worse tool, but it is no better than a much smaller and light weight setup. (Observatories use adaptive optics to get around "seeing," but that is in a different price category than ZWO products.)

Again, the 14" OTA is perfect for "lucky imaging" with planets. But until sensor read noise becomes much smaller, galaxies are just too dim to play lucky imaging games.

Chen

maplearn

So basically you’re saying that there is no way I can reduce the bloating with the C14? The good news I guess is that I didn’t go out and by another camera!

w7ay

maplearn So basically you’re saying that there is no way I can reduce the bloating with the C14?

Correct.

With a 2" seeing, your stars (and thus image resolution) are going to be no smaller than 2" even though the Dawes limit of the OTA is better than 0.3". You can then go check how many pixels the 2" corresponds to; you can use this tool:

https://astronomy.tools/calculators/field_of_view/

That (in pixels) would be the FWHM of a star.

Perhaps your dealer is willing to do a 1:1 swap for a C8 HD. It will be win-win for both of you. You will end up with resolutions that are no worse, but don't have to haul that much weight. Either that, or start playing with planetary "lucky imaging," where the extra aperture can be put to good use.

Chen