For mono it's 4x signal 2x noise for a bin2. For color that depends what binning is used - normal 2x2 bin 2 will destroy the color information, while color bin2 is actually 4 green 2 blue and 2 red so much more pixels and it's not always implemented/supported.

I still didn't decided which mono to pick - 178 or 174. 178 at bin2 will be similar to 174 to some extent, although smaller, and you can't get more than bin2 in FC. If you don't have like slower than f/16 telescope then 178 wouldn't have to be binned and will be better performer than 174 (bit lower noise, dark frame looks better too). I'll have f/19 DK as planetary telescope so bin 2x would be always used.

As for DS imaging 174 big pixels are usable with normal DS imaging telescopes, while very small 178 pixels would work nicely with very short focal lengths (lenses and some very small refractors) - on one side it's a lot of pixels on the other not many photons.

I'm pretty sure that the CMOS cameras from ZWO only support software binning which means that a 2 x 2 bin will only give you a 2X gain in signal to noise. If the CMOS cameras could actually do a hardware bin then a 2 x 2 bin on the ASI178 would actually produce a 4X gain in signal to noise and you might then be fairly close to the unbinned ASI174.

However, because of the smaller pixels on the ASI178 I don't think you will ever get an equivalent sensitivity to the ASI174 so I suspect that even with binning you will need to expose longer with the ASI178, and then even more so without binning.

Without binning the differences between the two sensors could be very substantial, maybe a factor of 4X in equivalent exposure.

However, the ASI178 could be well suited for VERY fast, short focal length systems. Perhaps something like the HyperStar on a Celestron SCT (around f-stop 2.0 and with a short enough focal length to take advantage of the sampling offered by the ASI178's 2.4um pixels).

However, I think for medium focal length and long focal length systems the ASI174 is the camera you want for DSO photography.

I've seen some very good DSO images that were taken with the Sony IMX174, however almost everything I've seen in DSOs that has been taken with an ASI178MC has looked fairly poor (just not enough sensitivity or maybe just not very good technique by the user -- can't yet tell).

However, I'd certainly be interested in seeing some good DSO images from the ASI178MM -- if it can actually be done.

james1ca wrote:

However, because of the smaller pixels on the ASI178 I don't think you will ever get an equivalent sensitivity to the ASI174 so I suspect that even with binning you will need to expose longer with the ASI178, and then even more so without binning.

That's just difference from resolution. Note that 178 has lower read noise than 174 and even double the noise is bit below 174 read noise.

james1ca wrote:

Without binning the differences between the two sensors could be very substantial, maybe a factor of 4X in equivalent exposure.

Different resolution.

The signal (photons collected per unit time) will be directly proportional to the area of the pixels. Thus, if you square each pixel you can estimate the signal collected with each pixel given a unit of exposure.

ASI174MM = 5.86um x 5.86um = 34.2

ASI178MM = 2.4um x 2.4um = 5.76

That's a ratio of 34.2/ 5.76 or 5.95 : 1

That's kind of a big gap even given the better read noise values on the ASI178MM. I'm pretty sure that a 2 x 2 software binning won't make up that difference, so the ASI178 is always going to be at some disadvantage as far as the needed exposure time.

this is an interesting topic

after binning 2, ASI178MM performs as a camera like this:

FW

pixel size: 4.8um

resolution

read noise.4e - 2.7e

ASI174MM:

FW.4Ke

pixel size: 5.6um

resolution*1280

read noise: 6e - 3.5e

seems better than 174 from principle， and there is no random horozitonal lines

I don't know if there is anything wrong with my caculation

I think it comes down to whether you either need or want the smaller pixel sampling offered by the ASI178. Sure, you can bin the ASI178 to make up for its lower sensitivity but after you do that you are left with both a lower pixel count and smaller field that what you'd get with the ASI174. Going from the native 6M pixels on the ASI178 down to only about 1.5M pixels after the binning is a pretty big compromise on what you can do with the final image.

With the monochrome version of the sensor you could just bin the colors for an LRGB composite which I guess wouldn't be that bad of a setup. But, what do you do when you simply want narrow band? Do you bin that also and end up with both a small field and a low pixel count?

As to whether a binned ASI178 would produce images that were as good as an unbinned ASI174 I think there are too many differences between the sensors/cameras to know one way or the other. I suspect the answer will only come from results produced in the field on a variety of different subjects.

In any case, until someone can show me a DSO image that was captured by the ASI178 that is as good as some that I've seen from the ASI174 I will remain somewhat skeptical about the DSO capabilities of the ASI178.

For planetary both cameras would work on their optimal f/ratios. But not for DS imaging.

rkayakr wrote:

Thanks for the responses. I am intrigued by the idea of using the ASI178mm for planetary at 1x and for DSO at 2x binning. What I would really like is a back side illuminated, 14 bit ADC, low noise, low amp glow sensor with 4 mm pixels. Maybe next year.

And a good size sensor too.

Backside illumination for small pixels is more of an necessity than a big performance factor like in some more professional CCDs or GSense CMOS sensors with big pixels.

I have a cooled mono 178 on order. I plan to use it with a 0.5x reducer that gives me a super fast F2.8 imaging train for the same resolution as I would have had with a 174 at F5.5 for about the same FOV. So I will be nearly 2 stops faster with a 178MM-Cooled.

My comparison was:

100 x 67 arc minutes at 1.94arcsec/pixel at F2.8 for the 178 vs

77 x 48arc minutes at 2.37 arc sec/pixel at F5.5 for a 174.

That will be my DSO setup with the camera.

When I do planet imaging - I don't need any barlows with the 8-inch SCT at F10 (0.24 arcsec/pixel) - just do a ROI field and I am away. I get 0.24 arcsec/pixel with that

Given the lower read noise, less other noise as mentioned by Sam (low amp glow, lack of random lines etc) I am hoping I will be ahead with my setup than with a 174. A 4 x faster imaging chain I am hoping will make up for the smaller pixels.

PS: I don't expect too much centre wavelenth shift troubles with narrowband imaging and more due to the high speed - I plan on the reducer going on the nosepiece of the camera - post the filterwheel. So the light incident on filters is F5.5. I have tried this configuration with a ICX618 based mono camera and it definitely works.

Generic 0.5x focal reducers make field quality go down very quickly. The faster the f-ratio / field defects the stronger the field quality seems to be. So f/2.8 with that focal reducer won't be what you may expect ;)

This is what happens when you use 0.5x focal reducer with f/5 Newtonian: https://www.flickr.com/photos/riklaunim/4534312374/in/album-72157623453725623/lightbox/ (Meade DSI III Pro - 2/3" sensor, reducer on a short nosepiece giving less than 0.5x reduction).

BTW ASI178MM ordered ;)