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Testing new IR filters for the IMX462 - Jupiter and Saturn

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#1 CPellier

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Posted 19 June 2021 - 04:55 AM

Hi all,

The gas giants have still been hard to observe from my home recently (only at a late dawn), but on June 14th I finally met good conditions. 

Last winter I found an ASI462MC and I spent some time to review the filters that can be used to exploit the huge IR sensitivity of the camera. It is not a legend, since I have been able to image Jupiter in CH4 at 20 fps in binning 1x !!

My choices so far are:

- The Astronomik IR740 as the main longpass filter, to reach the highest resolution and SNR (the Baader 685 can be used as well, but I prefer the 740 for Jupiter at least)

- The Sloan z' will catch all the monochrome part of the sensor. The Astronomik 807 would be slightly better here, but I'm interesting in trying photometric calibration. These two filters are a better choice than the IR850 commonly recommanded here.

- The usual CH4/890,

- And the Y filter, whose purpose will be to exploit the very end of the chip sensitivity, with more efficiency that my good old RG1000. The Y filter corresponds to an intermediate photometric band in an atmospheric window between two H2O bands ; it is also well centered on another CH4 band, that one at 1 micron, and also on the thermal emission band of the venusian surface :) 

Here is a graphic that displays the transmission of the filters along with the albedo of Jupiter (that of Saturn is very close, with only deeper CH4 bands).

Attached Thumbnails

  • filters_planetary_IR_Jupiter.png

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#2 CPellier

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Posted 19 June 2021 - 04:55 AM

And a complete set of images for comparison. The z' is slightly sensitive to the CH4/890 band, and shows a higher contrast at large scale (it also produces a highly steady image, but the resolution is of course reduced in comparison from visible light). The Y is quite close to the CH4/890, and is quite easy to use with the 462, much more than the RG1000 with the 290MM.

Attached Thumbnails

  • ICZY_astrosurf.png

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#3 CPellier

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Posted 19 June 2021 - 05:03 AM

These are the individual images.

Attached Thumbnails

  • j2021-06-14_03-43_ch4_cp.png
  • j2021-06-14_03-57_Y_cp.png
  • j2021-06-14_04-02_z_cp.png
  • j2021-06-14_04-10_ir_cp.png

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#4 CPellier

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Posted 19 June 2021 - 05:04 AM

And Saturn

Attached Thumbnails

  • S2021-06-14_03-00_Y_cp.png
  • S2021-06-14_03-16_z_cp.png
  • S2021-06-14_03-27_ir_cp.png

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#5 Ittaku

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Posted 19 June 2021 - 05:11 PM

Nice images. For IR, I have the Astronomik 642, 742, the 807, and the ZWO CH4, and I have the ASI462MC as well. I find myself routinely using the 742 for near IR the most (the 642 is good with my mono camera on bad seeing days as an alternative to colour). How do you manage the mono mixing for 742 given the reduced sensitivity in green and blue? I'm just using 1/3 from each but I'm not sure what is the "correct" mixing.



#6 nfotis

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Posted 19 June 2021 - 06:04 PM

Nice comparison. I feel that the IR filters (both the 850nm and the 742nm) offer the highest detail on Jupiter.

Saturn is much harder to see differences.

How would you combine the colour capture with the IR pass capture?

 

N.F.



#7 CPellier

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Posted 20 June 2021 - 01:41 AM

Nice images. For IR, I have the Astronomik 642, 742, the 807, and the ZWO CH4, and I have the ASI462MC as well. I find myself routinely using the 742 for near IR the most (the 642 is good with my mono camera on bad seeing days as an alternative to colour). How do you manage the mono mixing for 742 given the reduced sensitivity in green and blue? I'm just using 1/3 from each but I'm not sure what is the "correct" mixing.

I lower the red intensity in order to remove the red excess; the image takes on a rather blue/mauve hue. I don't think there is something that we could call "correct mixing"... this is rater empirical ! 

 

Nice comparison. I feel that the IR filters (both the 850nm and the 742nm) offer the highest detail on Jupiter.

Saturn is much harder to see differences.

How would you combine the colour capture with the IR pass capture?

 

N.F.

I would not :) IR luminances produces artifical colors, wrong details and reduce the resolution. The small details are more contrasted on Jupiter in IR as you says, but this comes at the expense of the optical resolution of the telescope, that is severly reduced after 800 nm. 


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