Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

When does undersampling become too much?

astrophotography beginner ccd imaging
  • Please log in to reply
13 replies to this topic

#1 ArandomPilot

ArandomPilot

    Lift Off

  • *****
  • topic starter
  • Posts: 19
  • Joined: 27 Feb 2020
  • Loc: Winter Park, Colorado

Posted 21 September 2020 - 12:56 PM

I know that under sampling is a problem in astrophotography, yes I have used bintel astronomy calculator, but I don't think this tells the whole story, more of just a rule of thumb. My camera is an old SBIG ST-2000XM and the pixel size is 7.4um. How far down can I go before undersampling can't be fixed with post processing. The farthest I can go down thus far 560mm according to Bintle. Can I go down further or should I stay where I am at?

 

Thanks,

Gabe



#2 Peregrinatum

Peregrinatum

    Surveyor 1

  • *****
  • Posts: 1,520
  • Joined: 27 Dec 2018
  • Loc: South Central Valley, Ca

Posted 21 September 2020 - 01:02 PM

when your stars start to look "blocky" and drizzle integration doesn't fix it



#3 aatdalton

aatdalton

    Mariner 2

  • *****
  • Posts: 233
  • Joined: 05 Feb 2019

Posted 21 September 2020 - 01:17 PM

Honestly I think of undersampling as a pretty personal taste thing. I guess it's different if you're trying to get sharp DSOs at a "normal" focal length but there are many people (including myself) who enjoy shooting super wide with everything from the Rokinon 135mm f/2 lens to classic 50mm lenses to ultra-wide 14mm lenses. Any of those setups are almost guaranteed to be horribly undersampled but you basically accept that and move along knowing it's a different end product.


  • bobzeq25, Gipht and wrnchhead like this

#4 bobzeq25

bobzeq25

    ISS

  • *****
  • Posts: 21,477
  • Joined: 27 Oct 2014

Posted 21 September 2020 - 01:29 PM

Never.  Images of the Milky Way with wide angle camera lenses can have image scales of 20 or more.

 

It all depends on what you're trying to do.  IMHO people worry too much about this.  I image anywhere from 1.0 to 2.7, more with camera lenses.

 

You might get less resolution, depending on a host of factors like seeing, tracking, etc.  You will get better signal to noise ratio.


Edited by bobzeq25, 21 September 2020 - 01:31 PM.

  • Madratter likes this

#5 SilverLitz

SilverLitz

    Apollo

  • -----
  • Posts: 1,182
  • Joined: 17 Feb 2018
  • Loc: Louisville, KY

Posted 21 September 2020 - 02:19 PM

Under/over sampling is dependent on your seeing conditions.  What I have read puts this under/over point somewhere in the 3-4x range.  There are other factors that would further limit your effective resolution limit, as well.  I think an easy starting point is seeing/3.  For me, my typical seeing is 1.5", so going as fine as 0.5"/px will be beneficial for resolution.  I have proven that 0.3”/px (moderately oversampled) shows more detail than 0.9”/px (moderately undersampled) on M13 for my system and local conditions.  For those with 3” typical seeing, the cut-off would be more like 1”/px.

 

But there is a SNR trade-off (all things being equal) to finer image scales.  It is generally inadvisable to push it to oversampling, as you will get a lower SNR for no resolution advantage.  Also, sampling does not come into consideration when imaging large targets, as FoV considerations dominate getting the entire target on the sensor.  Sampling is a concern when imaging a small target that covers a small portion of sensor, as extra resolution is beneficial for cropping in tight.

 

Following is more info than you requesting, but I am including some more background info for other who may be reading this:

 

For imaging, the main concerns are Field of View (FoV) for composition, image scale for detail of small targets, and Signal to Noise Ratio (SNR).  For large targets, FoV normally the dominates image scale as the critical factor, as these targets will generally require large FoV to fit on the sensor and this necessitates undersampling (e.g. using shorter FL leading to coarser image scales).  For small targets, image scale becomes more important in order to resolve the detail on a highly magnified subject that composes a small part of the sensor.

 

FoV is determined by the combination of the scope's focal length (FL) and the sensor's sensor size.  You can get larger FoV by a shorter FL and/or larger sensor size.

Image Scale (arcsec/px) is determined by the combination of the scope's FL and the sensor's pixel size.  You can get finer image scale (smaller number, more resolution) by a longer FL and/or smaller pixel size.

 

There is no advantage to a finer image scale than determined by Nyquist Critical Sampling, as it results in no extra achievable resolution.  Nyquist Critical Sampling is rate (frequency no higher than, or arcsec/px no smaller than) at which ALL of the embedded information (filtered signal) can be reconstructed.

 

Nyquist Critical Sampling is a fraction of the size of the finest detail (or multiple of the highest frequency component) of the filtered signal.  I refer to this divisor of size or multiplier of frequency as the Nyquist Factor.  Estimates of this Nyquist Factor are between 3 to 4 for 2-D images, and 2 for 1-D waveforms.

 

An estimate of a filtered signal's finest detail is the smallest measured FWHM of any star.  The true signal is filtered (or blurred) by seeing conditions, resolving power of the telescope, and the tracking ability of the mount.

 

Critical Sampling ~ SQRT(Seeing^2 + Dawes^2+ ...) / Nyquist Factor.  (I am not sure whether to add the mounts error in quadrature into the prior equation, or to treat the equation as the limit with a theoretically perfect mount and degrade the estimate, since seeing conditions would also show up in the measurement of the guiding accuracy (e.g. double counting).  This could also be the reason for the Nyquist Factor range.)

 

Seeing is a measure of blurring caused turbulence in the Earth's atmosphere, measured in arcsec.  This is what causes stars to “twinkle”, like the image of a coin on the bottom of a swimming pool jiggles with the ripples on the water surface.

 

Dawes Limit is a measure blurring due to wave diffraction caused by size of the telescope’s aperture edge, measured in arcsec.  The larger the diameter of the scope the sharper the image, and the lower the Dawes Limit.  Dawes Limit = 116 / Aperture Diameter, in mm.



#6 AtmosFearIC

AtmosFearIC

    Apollo

  • -----
  • Posts: 1,235
  • Joined: 10 Dec 2015
  • Loc: Melbourne

Posted 22 September 2020 - 01:36 AM

Over sampling has its limits but under sampling doesn’t. If you’re shooting at 0.2”/pixel and your average seeing is 2” then you’re going to always have fat fuzzy stars, lower than optimal SNR and a small FOV.

If however you’re under sampling it all comes down to a question of whether you’re wanting FOV and image depth OR resolution. Under sampling entirely depends on what your preference is and therefore you cannot under sample too much. As mentioned earlier, it is impossible NOT to “under sample” with a 135mm lens as you’re at 3”/pixel with minuscule 2 micron pixels. Yet no one ever complains about whether they’re sampling optimally.
  • bobzeq25 and SilverLitz like this

#7 Galaxyhunter

Galaxyhunter

    Mercury-Atlas

  • *****
  • Posts: 2,827
  • Joined: 02 Jan 2006
  • Loc: Northern Illinois

Posted 23 September 2020 - 03:58 PM

Over sampling has its limits but under sampling doesn’t. If you’re shooting at 0.2”/pixel and your average seeing is 2” then you’re going to always have fat fuzzy stars, lower than optimal SNR and a small FOV.

If however you’re under sampling it all comes down to a question of whether you’re wanting FOV and image depth OR resolution. Under sampling entirely depends on what your preference is and therefore you cannot under sample too much. As mentioned earlier, it is impossible NOT to “under sample” with a 135mm lens as you’re at 3”/pixel with minuscule 2 micron pixels. Yet no one ever complains about whether they’re sampling optimally.

Is this a critical ( fatal ) error in the native non binning form?  Would software binning cure this?



#8 Stelios

Stelios

    Voyager 1

  • *****
  • Moderators
  • Posts: 10,277
  • Joined: 04 Oct 2003
  • Loc: West Hills, CA

Posted 23 September 2020 - 04:44 PM

Is this a critical ( fatal ) error in the native non binning form?  Would software binning cure this?

Depends what you mean "cure." Resampling 4x1 (what you'd want with 0.2"/px scale) would shrink the pixel count by a factor of 16 and reduce the image size. It would now look better. This is why images look so much nicer on an iPhone screen :)

 

The question however, is why are you wasting money on telescope-mount-camera combination to achieve that 0.2"/px that you can't use? 



#9 AtmosFearIC

AtmosFearIC

    Apollo

  • -----
  • Posts: 1,235
  • Joined: 10 Dec 2015
  • Loc: Melbourne

Posted 23 September 2020 - 06:13 PM

Is this a critical ( fatal ) error in the native non binning form? Would software binning cure this?


Yes, you can bin which will obviously improve your SNR but one of the other issues is field size. I have a QHY183M and a 10” F/10, these paired up give 0.2”/pixel. Now I could do a 3x3 bin to get to a respectable 0.6”/pixel but you’re left with a 2.2mp image and a minuscule FOV.

#10 Galaxyhunter

Galaxyhunter

    Mercury-Atlas

  • *****
  • Posts: 2,827
  • Joined: 02 Jan 2006
  • Loc: Northern Illinois

Posted 23 September 2020 - 06:31 PM

Depends what you mean "cure." Resampling 4x1 (what you'd want with 0.2"/px scale) would shrink the pixel count by a factor of 16 and reduce the image size. It would now look better. This is why images look so much nicer on an iPhone screen smile.gif

 

The question however, is why are you wasting money on telescope-mount-camera combination to achieve that 0.2"/px that you can't use? 

What I mean by "Cure" is will it fix the "Fuzzie Stars".   As far a "Wasting" money on Scope - mount - camera,  I have a 2024 FL OTA.  I have always used a mono CCD.  I'm debating to go with a OSC camera.  People say the CMOS is the wave of the future & CCD is dead. So I have been looking at CMOS cameras & I can't find any with large pixels. To get an image scale of 1.0", I need to find a camera that has a pixel size of 9.82.  The largest one I found so far is 4.78.  I was told that CMOS does not "onboard" bin like CCD's can. It has to be done in software.  Hence the question of binning.

 

The .2" / pix was just the reference from the post I got the quote from.



#11 AtmosFearIC

AtmosFearIC

    Apollo

  • -----
  • Posts: 1,235
  • Joined: 10 Dec 2015
  • Loc: Melbourne

Posted 23 September 2020 - 07:33 PM

What I mean by "Cure" is will it fix the "Fuzzie Stars". As far a "Wasting" money on Scope - mount - camera, I have a 2024 FL OTA. I have always used a mono CCD. I'm debating to go with a OSC camera. People say the CMOS is the wave of the future & CCD is dead. So I have been looking at CMOS cameras & I can't find any with large pixels. To get an image scale of 1.0", I need to find a camera that has a pixel size of 9.82. The largest one I found so far is 4.78. I was told that CMOS does not "onboard" bin like CCD's can. It has to be done in software. Hence the question of binning.

The .2" / pix was just the reference from the post I got the quote from.


Binning will “cure” it as you can drop a star from 9 pixels to 3 pixels across.

You are right in that there isn’t many larger pixel CMOS sensors out there. QHY410C might peak your interest as it’s a 24mp full frame with 5.94 micron pixels.

CCD is on its way out as there isn’t much new R&D going on but there is in CMOS. Going from mono to OSC does have its quirks. I have both mono and OSC and a OSC CMOS is certainly not a direct replacement of a mono CCD.

#12 Galaxyhunter

Galaxyhunter

    Mercury-Atlas

  • *****
  • Posts: 2,827
  • Joined: 02 Jan 2006
  • Loc: Northern Illinois

Posted 23 September 2020 - 07:41 PM

Binning will “cure” it as you can drop a star from 9 pixels to 3 pixels across.

You are right in that there isn’t many larger pixel CMOS sensors out there. QHY410C might peak your interest as it’s a 24mp full frame with 5.94 micron pixels.

CCD is on its way out as there isn’t much new R&D going on but there is in CMOS. Going from mono to OSC does have its quirks. I have both mono and OSC and a OSC CMOS is certainly not a direct replacement of a mono CCD.

Thank you  Sir.



#13 TelescopeGreg

TelescopeGreg

    Vanguard

  • -----
  • Posts: 2,168
  • Joined: 16 Jul 2018
  • Loc: Auburn, California, USA

Posted 23 September 2020 - 08:59 PM

 Now I could do a 3x3 bin to get to a respectable 0.6”/pixel but you’re left with a 2.2mp image and a minuscule FOV.

Binning changes the pixel size, but it does not change the field of view.  That's a function of the physical size of the chip, which does not change.
 



#14 AtmosFearIC

AtmosFearIC

    Apollo

  • -----
  • Posts: 1,235
  • Joined: 10 Dec 2015
  • Loc: Melbourne

Posted 23 September 2020 - 09:57 PM

Binning changes the pixel size, but it does not change the field of view. That's a function of the physical size of the chip, which does not change.


That’s actually my point, if you have a sensor that samples at 0.2”/pixel you have a tiny FOV unless you have a 50+MP sensor. QHY183 with my 10” F/10 gives a 20 arcminute diameter FOV. That expands out to half a degree at 50mp.


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: astrophotography, beginner, ccd, imaging



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics