ASI120mm vs DMK41

[JB80]

In my last thread and one of Steve's we where having a bit of trouble using the barlows at times so I decided to try out and see the difference between using two cameras and if one may be better than the other.

Now of course there are differences with the sensor, pixel size and resolution so there should be some noticeable change between the two.

I didn't really get the time to do as much as I wanted as the wind was picking up so the shots with the DMK were under bouncy conditions but I'm not sure it has made too much difference.

I have tried to keep everything I have done similar, I have shot around 1000 frames for the avi's, processed the best 20% in AS!2 and then tried to apply the same settings in astraimage and photoshop. The DMK41 did have dust bunnies however so that may have introduced some slight artefacts.

The ASI120 was shooting at 32 fps and the DMK at 15.

First up the cameras without a barlow.

The ASI120mm

The DMK 41

I think the ASI120 here is the winner, the overall image scale is more pleasing and there was definitely less noise to process out. Applying the same denoising settings to the DMK image has clearly left speckles.

Now with the x2 barlow

The ASI

The DMK

Here I feel the results are flipped totally, the ASI produces a softer image and a much noisier one. I actually used less noise reduction on the DMK image as it handled that better. There was still more noise than I would of liked though but the result is interesting nonetheless.

So my conclusions are for my set up at least and under the conditions I get is the ASI120mm is the go to camera for straight up shots and my mosaics but if I want to do barlowed work then to use the DMK41.

Other conclusion is maybe I could do with a better quality barlow and possibly even a x1.5 barlow.

However I'm a tad concerned how this may affect my planetary, because of the wind I didn't get a chance last night but my results using the barlow and the asi have been a disaster, no detail at all and this may suggest it's not the best camera for it and my scope. I don't know.

[michael.h.f.wilkinson]

Interesting thread. I think the fairest comparison would be to image at the same image scale per pixel. This has the same number of photons hit each pixel. Differences in QE will clearly show in that case (in favour of the ASI120, I would guess).

[JB80]

You may be right but if I'm honest about it I don't know how to work that out. I guess reduce the capture area of the DMK by a certain amount?

It's also not going to really change the outcome that unless seeing is miraculous the combo of my scope, x2 barlow and ASI are pants as I really wanted to find out what handles the magnification better.

However it shows that natively it's a pretty good combo.

[michael.h.f.wilkinson]

The DMK41 has 4.65 micron square pixels, the ASI120 3.75 micron square. The ASI120MM will receive the same image scale per pixel (which is what counts) with a 1.6x barlow compared to the 2x barlow in the case of the DMK41. Likewise, the DMK41 at prime focus has the same image scale as an ASI120MM when using a focal reducer of 0.8x. By using the same Barlow for both cameras, you are giving the ASI120MM more of the seeing problems (by effectively magnifying 1.24x more), and only 65% of the number of photons. Processing the results in the same way is bound to cause trouble.

Another point is that you use the same number of frames, but with different frame rates. The DMK was running at 15FPS and may have used exposure times as long as 65 ms, whareas the ASI120 was running at 32 FPS, which allows at most 31.25 ms exposure. This could further degrade the image, as fewer photons are available (not just 65% of the DMK41 but just 30% of the number of photons per pixel). You could argue a plus of the ASI 120MM is that you can run it this fast, so you could double the number of frames, and still have captured everything in the same time, freezing seeing more effectively. Alternatively, the exposure time could have been increased to reduce photon noise per frame.

[JB80]

Fair enough, I see your point but I can only work with the equipment I have and that is the end goal to work out what combination is better with what I have at my disposal to try and get a better end result. That is why I have come up with the above.

That said I was also planning on trying the barlow without the lens screwed in to see if this gave a bit less magnification but the wind beat me.

[michael.h.f.wilkinson]

I realise you have to work with the equipment you have, but exposure time or number of frames captured can be varied easily, and may have a profound impact on the outcome

[melsky]

This thread has raised a question, what sort of exposure time do you aim for? I used to use a DMK41 but have switched to a Skyris 445m and when imaging the moon I have always tried to keep the shutter speed up over 1/200 sec otherwise each individual frame will be blurred by the atmosphere but if your stacking does this matter?

Mel

[Peter Drew]

Better to stack "clean " images than blurred ones ?