Which QHY camera for DSO?

[Stub Mandrel]

7 minutes ago, Adam J said:

I agree, but I do think that something odd if going on in those images you processed. This is what I got when I tried it just now with super pixel without my normal process of separating the channels in ISIS.

This is the BLUE channel from 18 x stacked subs.

This is the red channel from a SINGLE RED sub.

Its incomparable. I dont know how you have ended up with so much signal in your blue and green channels on the images you posted. I would hate to think that the signal from a single blue sub would have looked like if that was the stack of 18....

This is cooled and full spectrum 550D.

I think my images posted earlier in this thread are just too noisy for meaningful comparisons, despite the blue being much more stretched than the red. Your images above are closer to my super-pixel + cooled ones (just checked, 14 5-minute subs). Red:

Blue, which might look as noisy as yours if stretched to match the red one:

 

[Adam J]

4 minutes ago, Stub Mandrel said:

I think my images posted earlier in this thread are just too noisy for meaningful comparisons, despite the blue being much more stretched than the red. Your images above are closer to my super-pixel + cooled ones. Red:

Blue, which might look as noisy as yours if stretched to match the red one:

 

The other thing is that the 550D is lower noise once cooled than the 450D when cooled, I know because I have cooled both. I chose the 550D in the end for that reason and because you are left with higher resolution when imaging in H-a, 12mp vs 18mp. On the other hand the 450D is much easier to mod and cool in the first place. It should make a huge difference if you can get the sensor down to about 0c. You should be using long exposures once cooled, 10 to 30 mins in fact. Hope it works out for you.

Probably best we let this get back on topic now though.

[Orion1]

What are the advantages and disadvantages of small and big sensors with short focal length 200-400mm?

[ollypenrice]

8 minutes ago, Orion1 said:

What are the advantages and disadvantages of small and big sensors with short focal length 200-400mm?

There are no 'disadvantages' with larger sensors but there are demands made by them. The optics need to be able to cover them. Here you check the image circle of the optics to see that it is at least as large as the diagonal of the chip. Larger sensors also require the mechanical construction of the system to be more tilt-free. Sometimes beginners think that a smaller sensor 'gets you closer to the target' but this in entirely false. In a given set of optics you get a larger and more detailed image by having smaller pixels so that the object's image is projected onto more of them.

Olly

[wimvb]

17 minutes ago, Orion1 said:

What are the advantages and disadvantages of small and big sensors with short focal length 200-400mm?

Field of view. If the pixel size is the same, the small sensor behaves like the large sensor cropped. The large sensor covers more of the light circle of the lens. So you may have more star distortions and vignetting in the corners.

If the pixels are of different size, there's a difference in resolution (arcsecs per pixel).

Finally, if the number of pixels differs, there will be a difference in file size.

(Olly beat me to it)

[ollypenrice]

12 hours ago, Stub Mandrel said:

Olly, THERE IS NO DEBAYERING IN THOSE IMAGES.

The blue channel has about 10% the QE of the red channel for Ha according to the source below, so is shooting at 800 ISO, the blue channel is working at about 100 ISO.

The green channel has probably only about 2-3% according to this (I suspect a bit more in practice), so green is somewhere down below 50 ISO.

But they do both get a signal.

It is this 'bleed through' that helps us judge different subtle shades. That bump in the red under blue is very important - it's why we perceive purple (red+blue) as very close to violet. Note there are a couple of tricky areas on the curve where the camera has poorer discrimination than the eye as R:G:B ratios are duplicated (or at least approximated).

But in a game of signal-to-noise do you want  the very low Ha signal passed by the G and B of the Bayer Matrix or would you be better off without it? As I said at the outset I've never shot Ha through a Bayer matrix so I've no experience on which to draw.

Olly

 

[Orion1]

1 hour ago, ollypenrice said:

There are no 'disadvantages' with larger sensors but there are demands made by them. The optics need to be able to cover them. Here you check the image circle of the optics to see that it is at least as large as the diagonal of the chip. Larger sensors also require the mechanical construction of the system to be more tilt-free. Sometimes beginners think that a smaller sensor 'gets you closer to the target' but this in entirely false. In a given set of optics you get a larger and more detailed image by having smaller pixels so that the object's image is projected onto more of them.

Olly

Are you saying it better to have a smaller sensor (1in. or m43) over a large (aps-c) sensor with shorter focal lengths (200-400mm)

I’ve always wondering why so many of the high end astro cameras have small sensor with relative low MP count...

[Adam J]

39 minutes ago, Orion1 said:

Are you saying it better to have a smaller sensor (1in. or m43) over a large (aps-c) sensor with shorter focal lengths (200-400mm)

I’ve always wondering why so many of the high end astro cameras have small sensor with relative low MP count...

No he is saying its harder due to tilt with a fast f-ratio making it harder still, but to be honest with you most scopes will support an APS-C sized sensor and your lens is designed to do that too. I think that all the larger QHY168c you mentioned in your original post comes with a tilt adjuster anyway. So i would not get overly worried about it, it is something you will have to get to grips with eventually anyway. In general a larger sensor is always desirable its just full frame sensors that make stuff very difficult. You should be ok with APS-C and below.

The low MP count is because larger physical pixels are (as a general rule only) better in asto imaging as they gather more light and you end up with a higher signal to noise ratio in your final image. Often you will here people talking about something called binning which is essentially taking 4 x pixels and turning them into one even larger pixel. The reason that raw sensor resolution is not a big thing in astro imaging is that the atmosphere is always moving and blurs the images slightly so its most often the case that more smaller pixels will not get you any actual increase in detail on the target.

Larger sensors will just give you a larger field of view but, the optimal pixel size will depend on the focal length of the scope you are using. Shorter focal length benefit from smaller pixels in general....but only to a point. In the end most of the time you will find you want pixels larger than 4um hence the low pixel counts.

[ollypenrice]

46 minutes ago, Orion1 said:

Are you saying it better to have a smaller sensor (1in. or m43) over a large (aps-c) sensor with shorter focal lengths (200-400mm)

I’ve always wondering why so many of the high end astro cameras have small sensor with relative low MP count...

No, the opposite. I would always like the largest possible sensor. 'Possible' means several things:

- that I can afford

- that my optics can cover

-that my mechanical components can support

- for which I can afford the filters.

The reason you see small chip CCD cameras so often is simply because these are the chips available to the camera manufacturers. Big ones are also available but they are very expensive, the filters likewise, and there is no full frame CCD chip available with small pixels, more's the pity. In astronomy most people want to match their pixel size to their focal length. They don't necessarily want the smallest possible pixels. At long focal length larger ones are to be preferred. At short FL small ones are favourite. The CMOS cameras are starting to provide alternatives though.

Olly

Edit: sorry, crossed with Adam there. But we agree. 

[Stub Mandrel]

2 hours ago, ollypenrice said:

But in a game of signal-to-noise do you want  the very low Ha signal passed by the G and B of the Bayer Matrix or would you be better off without it?

That is the question!

With a modded, cooled camera where two green pixels are catching 38% of the photons that red gets (2 x 19%, see graph I posted earlier with green showing 20% of red at 650nm) the answer may well be yes. That's 1.4 times as many photons but noise increases by 1.7 (square root of 3).

It may also depend on other factors, not least the subject.

 

(Edit - following the astrosurf graph 650nm gives 90% at red, 17% at green, 6% blue), using all four pixels gives 144% of red signal alone, noise increases by root 4 or 200%, so while adding green may be worth it, adding blue may not. Clearly this is going to be very camera dependent as even different cameras from the same manufacturer can have very different response curves.

[Orion1]

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