ASI ZWO 174

[jambouk]

This camera is most often advertised as being for lunar and solar imaging, with very few retailing sites mentioning of planetary imaging. Is there a reason for this? I’d have thought with the VERY high frame rate capability it would be ideal for the brighter planets. 

[AbsolutelyN]

The 174 has quite big pixels 5.86 vs much smaller 2.9 for the 290. Smaller pixels seem better for planets. Bigger pixels generally better for deep sky - 174 has slightly bigger pixels than a full frame canon 6d. 

[jambouk]

Thanks.

With my C14 and the 290 the sampling is 0.15”/pix at f/11 and max frame rate at 640x480 of 184fps; with the 174 and a 2x PowerMate the sampling is 0.15”/pix at f/22 and a max frame rate at 640x480 of 309fps.

How does the pixel size actually make a difference in these situations? 

[vlaiv]

Pixel size here does not play very important part - except the fact that you needed to use x2 PowerMate which contains 4 glass elements and at least 4 glass/air surfaces more.

It could lead to few percent light loss - very small impact on SNR.

What does play a part here is

1) possible FPS

2) read noise

3) QE

Now, lets consider how much FPS do we need for planetary in the first place? Although there are cameras capable of 500+ fps, would we use it like that when imaging planets? It turns out that for most conditions (scopes, sky conditions) for amateur astronomers, coherence time is something like 5-6ms and up to 10ms if you are lucky.

This translates into 200fps - 166fps and 100fps if you are lucky. It is not camera FPS that is limiting, it is exposure length. ASI290 with its 184fps will have rather small frame loss with 5ms exposure and none with 6ms or higher.

300fps of ASI174 is overkill for planetary imaging. Not so for solar and lunar.

With planetary imaging you want short enough exposure to freeze the seeing - but not shorter than that to get good SNR per sub because planets are not as bright. With Solar and Lunar - one gets plenty of light and exposures down to 2ms are possible without too much impact on SNR. High FPS here counts.

Read noise. This one is simple - lower read noise, the better. With long exposure imaging you can control impact of read noise by using suitable exposure length. With lucky imaging you can't do that because you aim for very short exposure. Each exposure brings in one dose of read noise - it is very obvious that lower read noise is beneficial here.

+ for ASI290 with read noise ~1e vs read noise of ASI174 ~3.5e

QE, well this is self explanatory thing - ASI174 has about 77% vs ASI290 and its 80%. Small advantage for ASI290. Add those 4 glass / air surfaces of Powermate and this difference increased a bit more.

For planetary - ASI290 wins even if it has lower fps.

How about for Solar Ha?

Here ASI174 wins. Solar Ha is usually recorded at F/15-F/30 because of the way Ha filters work. It is much easier to get to optimal sampling rate with large pixels here then it is with small pixels.

Optimum sampling rate with 5.86um pixels in Ha wavelength is F/20.75

Optimum sampling rate with 2.9um pixels in Ha wavelength is F/10.25

It is much easier to aim for F/20 with Ha scopes than it is for F/10 - one needs focal reducer.

Another option is to use x2 binning on ASI290. Since it is software binning - that raises read noise by factor of x2 - it is no longer ~1e, now it will be ~2e - much closer to 3.5e

Then there is matter of sensor size - ASI174 can cover more of the disk because of larger surface of the chip and finally - when there is enough signal, you don't need to worry that much about

1. Exposure length (fps is utilized)

2. Read noise - because there is now other dominant noise source - photon shot noise from strong signal

3. QE of sensor - because you have enough signal

In Solar Ha, it makes more sense to go with ASI174

[jambouk]

Thanks, very informative reply.