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Rodd

Reducer Myth: Some data.

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On 10/15/2017 at 15:26, Filroden said:

the TOA also spreads them over 2.3x as much of the sensor

Err sorry to come a bit late, some mistake here IMHO. 2.3x is the magnification factor (ratio of 2 focals) between the two OTAs, not the spreading factor : Since you're spreading "solid angles" on a surface, you're actually spreading over 2.3² = 5.29x of the sensor.

So the output exposure level ratio should rather be 1.66/5.29 = 31.4%. Or the reverse: the Televue is 3.19x faster (again, at given pixel level). Sounds stangely big, now; But remember our eye's perception of brightness is not linear, and so are our screens too, so check real ADU levels. Oh and the cherry on the cake :), that's what you guess directly from the focal ratios, following the common rule: 7.7²/4.3² = 3.2 ! (same value, apart rounding approximations here and there)

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3 minutes ago, rotatux said:

Err sorry to come a bit late, some mistake here IMHO. 2.3x is the magnification factor (ratio of 2 focals) between the two OTAs, not the spreading factor : Since you're spreading "solid angles" on a surface, you're actually spreading over 2.3² = 5.29x of the sensor.

So the output exposure level ratio should rather be 1.66/5.29 = 31.4%. Or the reverse: the Televue is 3.19x faster (again, at given pixel level). Sounds stangely big, now; But remember our eye's perception of brightness is not linear, and so are our screens too, so check real ADU levels. Oh and the cherry on the cake :), that's what you guess directly from the focal ratios, following the common rule: 7.7²/4.3² = 3.2 ! (same value, apart rounding approximations here and there)

This is where theorey and reality diverge I think.  Based on your statement, the TV should be able to capture in 10 minutes what teh TOA takes 30 minutes to capture.  Trust me when i say that a 10 min Ha sub with the TV does not have the SNR of a 30 min TOA sub.  Not even close.  I still take 30 min Ha subs with teh TV

Rodd

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11 minutes ago, Rodd said:

Trust me when i say that a 10 min Ha sub with the TV does not have the SNR of a 30 min TOA sub.  Not even close.

Trust you :). I was talking of pixel levels (ADU). The SNR goes with the square root (of 3.19x), so to catch the same SNR would mean to compare 10mn TV with ~18mn TOA. So of course agree 30mn TOA *is* much better.

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1 minute ago, rotatux said:

Trust you :). I was talking of pixel levels (ADU). The SNR goes with the square root (of 3.19x), so to catch the same SNR would mean to compare 10mn TV with ~18mn TOA. So of course agree 30mn TOA *is* much better.

Oh--so 30 min TOA would be a about a 17min TV (not sure that's right--just an estimate)--has to be close.  That is useful--thanks!

Rodd

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8 hours ago, Stub Mandrel said:

It does depend on the exact operation. If there is no shift in alignment then fewer pixels will be involved.

I think in most imaging programs a straight integer:1 resample will just fuse the appropriate pixels unless you have anti-aliasing switched on.

Anti-aliasing will give a smoother result by using more pixels but will also smooth the noise so I suspect (but can't prove) the S:N ratio may not be affected.

This can actually be "measured" somewhat, just create image with pure gaussian noise of certain sigma

1. measure standard deviation on image it should be equal to sigma.

2. make a copy and bin that image 2x2 and measure stdev again - it should be half of original sigma

3. make a copy and scale down x2 and measure stdev - if it differs significantly from sigma/2 then scaling algorithm is introducing additional correlation between pixels

I've just done this example in ImageJ - Gaussian noise with sigma 9 - measured stdev 9.0138, bin x2 stdev: 4.4973, scaled down x2 (bicubic) stdev: 3.642, scaled down x2 (bilinear) stdev: 2.8237

As you see both bilinear and bicubic resampling affects noise distribution by introducing correlation between resulting pixels of scaled down image and "lessening" the noise - it is actually smoothing image similar to blurring so detail is affected more than with normal bin x2.

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13 hours ago, Rodd said:

According to the FOV calculator the pixel scale with the FSQ at F3 would be 3.5.  For widefield shots I thought this would be fine.  I have seen widefield shots at this scale that are amazing (Olly's come to mind).  A smaller pixel camera would be welcomed--but then I would loose FOV (Unless I went with the ASI 1600mmcool--but I am not interested in CMOS imaging style at this point (a plethora of subs).  I find 100 or so too many as it is.    But--here's teh thing--unless it will be SIGNIFICANTLY faster, its not worth the $7K it will cost to set it up.

Rodd

As you know, my solution is to run two F5 FSQ106Ns in parallel to make an effective F4 at 3.5"PP. If there were a camera model I could live with that had the full frame chip and smaller pixels I'd have it, but I don't know of one.

I have seen both the Riccardi Honders and Epsilon in action here and I know that they are not for me, but that doesn't mean they might not be for you. Collimation and tilt are ultra critical so some time will be lost sorting them out and the open tube will require more maintenance. For most people this wouldn't be a problem but I have about 15 telescopes on site in six observatories and I am NOT looking for more maintenance!!! :BangHead: I need 'easy.'

You can buy 2 old 106N instruments for less than the price of one new one and at least you know for sure that you are going to halve your imaging time over using just one. 

Olly

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21 minutes ago, ollypenrice said:

As you know, my solution is to run two F5 FSQ106Ns in parallel to make an effective F4 at 3.5"PP. If there were a camera model I could live with that had the full frame chip and smaller pixels I'd have it, but I don't know of one.

I have seen both the Riccardi Honders and Epsilon in action here and I know that they are not for me, but that doesn't mean they might not be for you. Collimation and tilt are ultra critical so some time will be lost sorting them out and the open tube will require more maintenance. For most people this wouldn't be a problem but I have about 15 telescopes on site in six observatories and I am NOT looking for more maintenance!!! :BangHead: I need 'easy.'

You can buy 2 old 106N instruments for less than the price of one new one and at least you know for sure that you are going to halve your imaging time over using just one. 

Olly

True....but I would need a mount as well as mine can't handle 2, so that kind of eliminates the option.  I agree about the scopes as well--refractor for me.  Besides the Epsilon would have about the same pixel scale I believe.  I have pretty much convinced myself that a pixel scale of 3.5as/p would be OK for widefield.  Its just the time that I am still wondering about.  My gut tells me that the difference between F3 and F4.3 will not satisfy me.  

Rodd

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3 hours ago, Rodd said:

True....but I would need a mount as well as mine can't handle 2, so that kind of eliminates the option.  I agree about the scopes as well--refractor for me.  Besides the Epsilon would have about the same pixel scale I believe.  I have pretty much convinced myself that a pixel scale of 3.5as/p would be OK for widefield.  Its just the time that I am still wondering about.  My gut tells me that the difference between F3 and F4.3 will not satisfy me.  

Rodd

I'll be glad to send you a calibrated linear stack of data at 3.5"PP for you to look and see if it might be for you. It would need to be a TIFF so if you PM me an email address I cand send it to you.

Olly

Edited by ollypenrice

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