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Filter Comparison For Imaging Broad Spectrum Objects In Light Pollution Areas


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Introduction

The primary purpose of imaging with a filter in the city is to mitigate light pollution and help suppress the noise so that the signal of the target object becomes easier to identify.  This is rather easily achieved for nebula type objects that emit at very specific wavelengths but much more challenging to accomplish for full-spectrum sources such as galaxies and globular clusters.  Cutting down on city glare is becoming even more challenging as city lighting transitions from Mercury type lighting that emits at defined wavelengths to LED lighting that generally emits broad spectrum lighting.

 

Raw files along with processed images can be downloaded here:

http://www.artcentrics.com/02_Astronomy/Studies/FiltersBroadband/Filters_Compare-BroadBand.htm

 

The purpose of this study is to determine what the best filter (if any) is for imaging broad-band objects (Galaxies, Globular Clusters, Open Clusters ect.) in the big city between the four filters presented here.

 

Imaging Session Details

  • Location: Chandler, Arizona
  • Bortle Scale: 7
  • Date: 2019.03.10
  • Conditions: No moon in sky, clear skies
  • Imaging Window: 2:00am – 5:33am
  • Hardware Configuration: | C-11 | SC Corrector | {Filter} | QHY128c |

 

The Target

  • Object: NGC5907
  • Visual Magnitude: +10.16
  • Apparent Size: 11.3 x 1.8 arcmin
  • Object Type: Edge-on Spiral Galaxy
  • Constellation: Draco
  • Target selected is in the northern section of sky.  Imaging secession was timed so that it was near the meridian when imaging to minimalize atmospheric effects between filters.

 

Imaging Process

Take 3 images, 5 minute exposures for a total of 15min integration time with each of the following configuration:

| C-11 | Corrector | {Identified Filter Below}  | Camera |

 

Filter

Time Range

Altitude Range

Comments

None

01:58 – 02:14

55.0° - 57.0°

 

Baader Moon

02:39 – 02:54

59.9° - 61.5°

 

OPT Triad

03:19 – 03:35

63.8° - 65.0°

 

Astronomik CLS-CCD

04:20 – 04:36

67.0° - 67.1°

 

Astronomik CLS-UHD

05:18 – 05:33

65.7° -   64.8°

 

 

 

The Filters

Filter

Specifications

Chart

Baader

Moon and Skyglow Filter

Moon and Light Pollution filter

 

Primary Usage:

·         Visual: Moon, Planets, Deepsky

·         Astrophotography: Planets, Moon

 

List Price: $132.00

 

 

Astronomik CLS-CCD

       95% transmission at 486nm (H-beta)

       95% transmission at 496nm (O III )

       95% transmission at 501nm (O III )

       97% transmission at 656nm (H alpha)

       1st Passband 450nm to 520nm

       2nd passband 640nm to 690nm

 

List Price: $200

 

 

 

Astronomik CLS-UHC

·         97% transmission at 486nm (H-beta)

·         97% transmission at 496nm (OIII)

·         97% transmission at 501nm (OIII)

·         97% transmission at 656nm (H alpha)

  

List Price: $200

 

 

 

OPT Triad

·         FilterType: Triband

·         Manufacturer OPT

·         Wavelengths H alpha/656.3/3nm

·         Wavelengths OIII/H beta/ Center 493/18nm

 

List Price: $775

 

 

 

 

The Hardware

Hardware

Manufactor and Model

Specifications

Telescope

Celestron C-11

·         Type: Schmidt-Cassegrain

·         Aperture: 279.4 mm (11")

·         Focal Length: 2,800 mm (110")

·         Focal Ratio: f/10

·         Secondary Mirror Obs: 95mm = 12%

·         Weight: 27.5 lbs

Mount

iOptron CEM60EC

·         Type: Center Balanced Equatorial Mount (CEM)

·         Payload: 60lbs

·         Controller: Go2Nova 8407+

·         Mount Weight: 27 lbs

·         Resolution: 0.06 arc seconds

·         GPS: Internal 32-channel GPS

·         Autoguide Port: ST-4

 

Focal Reducer/ Corrector

Starizona SCT Corrector (LF)

·         Focal Length: 2,000mm

·         Focal Ratio: f/7.2

·         Image Circle: 42mm

·         Mechanical Backfocus: 132mm from top of mounting threads

 

Camera

QHYCCD QHY128c

·         Sensor Type: Color CMOS

·         Read Noise: 1e- to 4e-

·         Pixel Size: 5.97 um

·         Well Capacity: 74ke-

·         Bit Depth: 14 bit

·         Dynamic Range: 14 stops

·         Sensor: Sony  Exmor IMX1238

·         Dark Current: 0.0006e-/p/s @ -15C

·         Pixel Scale: 0.581 arcsec/pixel

·         Image Size: 58.6 x 39.5 arcmin

 

 

Analysis

The purpose of utilizing filters when imaging is to eliminate unwanted signal from the image resulting in an increased signal.  As a result the Signal to Noise ratio should be improved.  As a result when multiple images are stacked together the target object is more prominent than what it would appear without the filter. 

Software used for Stacking and image analysis is PixInsight.

 

Raw Files Statistics

Here we took the each of the tree raw files generated for each configuration and performed analysis on the files utilizing the SubframeSelector tool in PixInsight.  As expected the filters appear to have helped raising the SNRWeight value in most instances.  While I’m not certain of the equation governing the Signal to Noise Ratio Weight (SNRWeight) in Pixinsight, I think we can determine the relative signal change since the Noise value is readily available.

 

Index

 Filter

SNRWeight (average)

FWHM

(avg)

Noise

(avg)

*Signal

Comments

1-3

None

1.06

7.28

312.8

331.6

 

4-6

Baader Moon

1.38

6.11

131.3

181.2

Seems to be quite good

7-9

OPT Triad

1.12

5.81

26.6

29.8

 

10-12

Astronomik CLS-CCD

1.05

7.49

78.1

82.0

 

13-15

Astronomik CLS-UHD

1.45

0.34

31.0

45.0

Seems to be quite good

 

 



Stacked Files Statistics

Utilizing PixInsight each of the three raw files were processed using the process flow indicated below.  In addition, we also processed the group of filtered files together.  All sets of images were processed identically.

Image Process flow:

|Image Calibration | Cosmetic Correction | Debayer | Star Alignment | Image Integration |

 

Index

 Filter

SNRWeight

FWHM

Noise

*Signal

Comments

1

None

155.5

7.19

17.72

2,755

 

2

Baader Moon

81.0

5.77

9.67

783

 

3

OPT Triad

5.00

5.83

2.62

13.0

 

4

Astronomik CLS-CCD

27.1

7.39

5.13

138

 

5

Astronomik CLS-UHD

14.8

6.09

3.73

55.0

 

6

All filters Stacked (exclude None)

128.2

6.49

2.06

264

This just for fun, not in consideration

 

 

 

Processed photos

Each of the images were processed identically in PixInsight for the following steps (In order of operation):

| Dynamic Crop | Dynamic Background Extraction | Automatic Background Extraction |

| Background Neutralization | Photometric Color Calibration |

| Multiscale Linear Transform-Luminance | Multiscale Linear Transform-Chrominance |

 

Images were then duplicated and broken up into two identical sets to have final stretch performed in two differing processes:

 

  1. Batch Stretch - Performed stretch on the “All-Filters Combined” image and then this stretch was applied to the rest of the images so the complete set all received identical stretch.
  2. Custom Stretch – Stretch each image based on background noise where I tried to drop the background noise to just under detectible amounts for each image.

Final Results are on the following two pages…

 

Batch Stretch

01 – No Filter

02 - Baader Moon

 

03 – OPT Triad

04 – Astronmik CLS-CCD

05 – Astronmik CLS-UHD

06 – All Filters Combined


Individual Stretch

01 – No Filter

02 - Baader Moon

 

03 – OPT Triad

04 – Astronmik CLS-CCD

05 – Astronmik CLS-UHD

06 – All Filters Combined


Conclusions

Statistical analysis of the Raw files leads us to conclude that the Baader Moon and the Astronomik CLS-UHD filters provide the best Signal to Noise Ratio.  Yet when images have been stacked, the None and Baader Moon come in as showing the best SNR.  This data seems to be in conflict since process used stack the photos was identical for all stack photos.

 

Inspection of the final stretch images is difficult and subjective.  For the images where identically stretch is performed on all images the Astronmik CLS-CCD filter seems to be able to provide the best contract between the galaxy and the background.  Images with Individual stretch are by nature of the stretching process much more difficult to judge, but to me it seems that the Astromik CLS-CCD and Astronmik CLS-UHD filter images appear to be the better images (I am not overly confident in this).

 

We are trying to minimalize noise while keeping as much signal as possible.  My final conclusion is that the Astronmik CLS-CCD or UHD filters may be the best filter for imaging broad spectrum objects such as galaxies and globular clusters in the city measuring in with a Bortle Scale 7 while the Baader Moon filter should be considered in Darker sites that measure at Bortle scale 4.  Real world examples seem to bear this out.

 

Some real world imaging examples are provided below:

 

Whale and Hockey Stick Galaxies

Massacre Ground Trailhead, AZ (Bortle 4)

Baader Skyglow filter

31 frames @ 5min = 155min integration time

 

M-106 Galaxy Group

Chandler, AZ (Bortle 7)

Astronomik UHC filter

17 frames @ 5min = 85min integration time

 

Splinter Galaxy (NGC 5907)

Chandler, AZ (Bortle 7)

Astronomik UHC filter

30 frames @ 5min = 150 min integration time

 


  • DonBoy, jimthompson, rekokich and 2 others like this


12 Comments

Timely article for me.  Because of health concerns I had to give up my own observatory in Benson, Arizona (40 miles east of Tucson) and move back in to town.  I still have my own observatory, but I am only blocks from the UofA campus.  I wrote the two articles entitled "My Other Telescope is an 8.4 Meter" so I have lots of experience at the macro level but am still learning the basics in my own project.  I just made the switch from DLSR to ASI178MM monochrome with filter wheel and have to choose which light pollution filter to put in my optical train.  I had decided upon the CLS-CCD but only based on anecdotal information.  Your analysis validated my choice, and now I feel much more confident that I've made the right choice.  Thanks for the hard work and much needed objective data.

Photo
JTYoder2017
May 07 2019 09:31 PM

Thanks for the feedback.  Glad some else can benefit from the analysis.

 

James

Good stuff.  I trust you don't mind my referencing further information.

 

More useful information, including more quantitative tests, in this thread.  Some of the best imagers on this site (which does not include me <smile>) participated.

 

https://www.cloudyni...-the-answer-is/

 

More information.  Here the issue is sharpness.

 

https://www.cloudyni...adband-targets/

 

Bottom line.  People's opinions on broadband LP filters vary.  Some serious imagers use them, some don't, even in light polluted skies.  Some only use them on emission nebulae, where they work best.  I suggest trying one on various targets, and evaluating the results in your final images, which is what counts.  The workflow used in this test is excellent, it included gradient reduction, which is essential.

 

Which one?  That gets harder.  One impact of broadband LP filters is that they, operating by ripping out chunks of the spectrum, distort colors.  You can adjust things for a reasonable result, but some data is just gone.  The stronger the LP filter, the more it distorts color.  As is so often true in AP, there are tradeoffs.

Perfect timing that I stumbled on this great report. I have been an ATM all these years and started playing with an equatorial mount recently. Then added a GOTO drive to it using OnStep. I now want to attempt imaging. Initially I thought I couldnot because I live on the east coast near bright city like Philadelphia. Today I found this report. Wow, I think I will not give up on imaging if filters can help me get over light pollution. Thank you JTYoder2017 for this.

Photo
FlankerOneTwo
May 10 2019 07:49 PM

Nice work, thanks for the report. Any plans to include the IDAS LPS-D2 and Optolong L-Pro?

Photo
JTYoder2017
May 11 2019 03:33 AM

Thanks

Just testing the equipment I have.  The filters you mention do look like interesting filters.  There is a very nice review on filters including these here.

Photo
jimthompson
May 16 2019 07:01 PM

Your results are consistent with my findings under even higher light pollution levels, the Astronomik UHC provides superior performance.  Very nicely done!

 

Cheers,

 

Jim T.

Very interesting report.  I found similar results with the Astronomik UHC + IR Block combo.  

 

https://www.cloudyni...t/#entry9088790

 

BTW why are you listing the Astronomik UHC as a Astronmik UHD?  Did I miss something?

 

Thanks,

 

Don

Photo
JTYoder2017
May 22 2019 11:02 AM

Typo,  This should be UHC.  thanks for pointing that out.

I've been on a bit of a galaxy hunt lately - I image from suburban Philadelphia under a Bortle 7 or Bortle 8 skies (different sources say different things). I image in monochrome and currently have a dual setup going on. I'm using a Baader Moon & Skyglow in lieu of a luminance filter on my Newt and ZWO 1600 combo and a Chroma LoGlow for luminance on my 8" RC and and QHY183. I don't have hard and fast scientific data, but I definitely feel like I'm getting a better SNR using those filters in place of a luminance filter - the L channel is much easier to process overall than it used to be when using a regular L filter.

Some images I've done this galaxy season with the Newt/ZWO/Baader Moon & Skyglow:
https://www.astrobin.com/395753/
https://www.astrobin.com/402244/
https://www.astrobin.com/408397/

And with the 8"RC/QHY183/Chroma LoGlow:
https://www.astrobin.com/399142/
https://www.astrobin.com/399949/
https://www.astrobin.com/407870/
https://www.astrobin.com/408909/

I'd definitely recommend them - that Baader M&SG should be part of every astroimager's arsenal. 

    • RazvanUnderStars and JTYoder2017 like this
Photo
JTYoder2017
Jun 07 2019 08:51 PM

Those are some mighty fine images you have there!

    • ChrisPA likes this

Hard to really make much comparison as the images are not calibrated or stretched hard enough to really see the difference. To me no filter or M&SG looks best, but again I can't really tell without larger images.

 

I've used all of these (or a similar variant) in the past except the Triad, and I'm always going to advocate for no filter, or a mild one like IDAS or M&SG in really bad LP, as the destruction of color is not worth it.

 

Personally I would completely disagree with your conclusion and would NEVER use a UHC for broadband objects period, anywhere, and probably not a CLS either. I would definitely never use a filter at all for broadband in Bortle 4. I learned that no filter is needed even in Bortle 7: https://www.astrobin.com/314853/ https://www.astrobin.com/377153

 

I look back on the ones I did in Bortle 7 with a filter and it bothers me that the color is off. Looking at the images I've done with no filter I feel much better about them.



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