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Old 30-03-2017, 08:10 PM
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SimmoW (SIMON)
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'Chook-off' Camera Comparison Thread

Ok folks, here it is, quite a lot of work to star align, linear fit (to try to match the backgrounds), blink, crop and then resave the pics to a manageable level (all originally derived from FITS files).

After seeing my own 13hr Running Chicken image and being frustrated from a lack of O3 detail/white wispy bits in the chicken's rear compared to Andy's 30hr image, I decided to ask my friend Uri for a single Ha sub that he had recently taken of the IC2944 region, using his QSI683 camera. Then he gave me his other 2 filter images, and so this project has gained legs!

So Andy has provided me with a set of all his 3 subs (30 mins), Uri has his 15 min subs and another astro friend Nick gave me an 8 minute Ha sub from Heathcote using his FLI ML16200 camera.

Please follow this link to view the comparison video and then download the aligned images for your own analysis. Very interested in your opinions!

After doing the analysis, I believe the following:

1.My friend Uri is achieving outstanding results with his QSI and Stellarvue 130 setup!

2. In fact his expertise with PA seems to assist him to match or exceed the FLI camera results.

3. Takahashi refractor quality is most definitely matched by Stellarvue.

4. My own ASI camera punches surprisingly above its price bracket. About $2k versus $4.7k for the QSI and double that again (or more) for the FLI. I expected to see much less wispy details in Ha than in Andy's sub, but most of the signal is there.

5. However at these 'long' exposures for my cam (15 mins), the noise is becoming nasty, and I also notice the dynamic range is worse.

6. Various other issues can substantially affect image quality other than basic camera quality. In my case, focusing and possibly a dirty mirror is contributing to my gigantic star shapes. I am now researching an electronic focuser for my Tak reflector. Now that I can collimate the tak, a good clean will be in store.

7. As a result, I'm certainly keeping my camera, at least until I find that the improved focusing doesn't help. It's a great stepping stone.

8. Extra to this test, I have received a stacked image of 15 subs from Uri, I see some slight improvements but still no magical wispy signal that explains my missing data in the chicken's guts.

One friend has already suggested that the stretching of the images in this test is not equal. I can't see how I can do that without changing the brightness of the individual pics. I did do a Linear Fit in PI before doing an auto stretch. Would be interested in any suggestions as to make this comparison even more objective.

It is very easy to become anal about this. Eg. nick's pic is taken in dark sky territory, maybe Uri's pic lucked out re. seeing, different scopes used, different mounts used, etc. etc.

Let the debate begin, and tearing to shreds of my terrible comparison!?
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Old 30-03-2017, 09:24 PM
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Atmos (Colin)
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I would say that one of the "issues" is that the ASI1600 is best used with shorter exposures. If nothing else this comes down to the full well capacity. You're shooting at F/3.3 with 15 minutes exposures with a camera that has a pretty small well depth. The 16200 sensor has a well depth of 39,000e-. The 8300 has a well depth of 25,500e- while the ASI1600 has a well depth of ~8,200e- at Gain 75 or 4,100e- at Gain 139. At the other end the 16803 has 100,000e-!
I have been imaging at F/5.21 at Gain 139 and I find that anything over 400s with 3nm filters in Ha isn't useful from Melbourne. Would no doubt be different up at Heathcote.

The cost of a camera shouldn't really be used as comparison of its actual quality, that more comes down to the chip used and manufacturer. It is more to do with a match up between the chip and telescope. For instance, a QHY16200 is cheaper and will outperform a FLI-11002 in virtually every respect. Considerably lower read noise and a higher QE; the pixels are a bit smaller but that helps with resolution.

With your telescope, you're imaging at quite a quick focal ratio so focus and collimation are very sensitive to being slightly off. From what I have seen, the Epsilon 130ED is also not as perfect with the tiny pixels of the ASI1600, this causes a "softness".
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Old 03-04-2017, 03:20 PM
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rustigsmed (Russell)
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hi simmo,

here is 2hrs Ha (7nm) 20 min subs, qhy22
https://www.flickr.com/photos/803366...blic/lightbox/
its obviously flickr and stacked not sure if it will help. i'll see if I can go back and dig out a single sub from the vault if you think it will help?

cheers

russ
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Old 03-04-2017, 04:55 PM
glend (Glen)
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Using 15 min subs with the ASI1600 is the wrong approach as Colin has suggested. Take more shorter subs to take advantage of the low noise and max well depth at around Gain 76. Each camera will have its sweet spot. Its not an apples to apples comparison. In my experience the ASI shines when shooting narrowband subs up to max of 5 min.
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Old 04-04-2017, 02:13 PM
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Yes Russ, send me on a single sub, since it's a single sub comparison.Thanks! Will look at the linked image later.

Glen, I have tested the cam thoroughly, could never get the level of detail at 5 or 10 mins compared to 15. Stacking sets of 5 min subs never achieves the detail I see in a single 15min sub. Well for nb anyway.

I've pretty much worked out that it's the allocation of the filters to RGB that is the key, and decent focus of course. I have never seen any 5min images with this cam that get me excited, other than Ray's of course!

So in June I'll be ordering the robofocus. But by then I'll also have my 10 inch humming (knock on wood!). Then maybe the sub lengths will have to reduce.
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Old 04-04-2017, 04:01 PM
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Sub length should have far more to do with the focal ratio than aperture, following that theory they should need to be longer as you'd be going higher than F/3.3 (F/4?).
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Old 05-04-2017, 08:10 AM
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Oh dear Colin, we shouldn't go down that path! Let's save time and link to other discussions! https://stargazerslounge.com/topic/2...ging-worth-it/
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Old 05-04-2017, 08:40 AM
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Quote:
Originally Posted by SimmoW View Post
Oh dear Colin, we shouldn't go down that path! Let's save time and link to other discussions! https://stargazerslounge.com/topic/2...ging-worth-it/
Skimmed over the thread, I do agree with the F/Ratio myth to some extent. Take Ray's SW 10" F/4 with ASI1600. It will work exactly the same as a 10" F/8 as long as the camera has 7.6 micron pixels (given the same QE and read noise). Aperture remains the same as does the pixel scale (~0.78"/pixel).

If aperture alone determined exposure length then a 14mm lens would take hours to get above read noise. It is all about image scale against aperture. Sub exposure time is determined by image scale and sky brightness.

A 50mm lens at F/5 and a DSLR will have the same exposure time as it used on a F/5 10" newt if used at the same location and sky brightness) The difference between the two will be resolution and contrast.
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