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Filters & Video Astronomy presentation - tonight @ 9:30pm EDT

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#1 jimthompson

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Posted 19 August 2014 - 12:29 PM

G'day all,

 

After some words of support from friend and fellow video astronomer Stephan Lalonde, I have decided it would be worth it to give the presentation on "Filters & Video Astronomy" that I prepared for the 10th Annual Video Star Party but did not get a chance to present.  It so happens that I should be free this evening (Aug. 19th) to present it on NSN, though a little later say around 9:30pm EDT.  If you are interested in seeing this introductory presentation on filters and their use in video astronomy, please feel free to tune in tonight to the NSN channel:  AbbeyRoadObservatory.

 

http://www.nightskie...RoadObservatory

 

cheers!

 

Jim Thompson



#2 Mikefp

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Posted 19 August 2014 - 07:38 PM

Thanks and will watch.



#3 Relativist

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Posted 20 August 2014 - 01:35 AM

Great presentation Jim! Would you mind posting a link?

 

I did get to ask some questions but didn't get to hear all of your answers, only some. What I'm wondering is if you've used a filter wheel and if so if you've had any issues with light leakage. Also, given the RBG histogram and other settings on the Mallincam & Miloslick (as a case study) are there typical settings that can be used to approximate the views of some of the popular deep sky filters?

 

...Curtis



#4 jimthompson

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Posted 20 August 2014 - 09:41 AM

Hi Curtis,

 

The link to my presentation slides is:

 

http://karmalimbo.co...my_July2014.pdf

 

I have tried using a filter wheel but only when doing Lunar and Planetary work.  I have not tried using one for deepsky.  I instead have simply removed the camera from the scope and changed filters manually.  Your concern over light leakage is probably a valid one for deepsky since the exposure times are generally quite long.  Light leakage will depend on the design of the particular filter wheel you are using, and whether or not you are able to implement additional precautions like no local lighting and or an extra layer of dark material drapped overtop.  If you have a filter wheel meant for astrophotography I would not expect there to be a problem.

 

Regarding using the histogram tool in your camera control software to adjust the RGB of the image, I am not sure what your question is.  If you are asking whether or not you can adjust the histogram to cut out wavelengths of light that we don't want to see (ie. light pollution), the answer is NO.  What your camera sees and what you see are two entirely different things.  The adjustment you make to the histogram is meerly shifting around the image data coming from the camera, changing how it is displayed between the three colour channels of your monitor.  This has no connection to the different wavelengths of light that are being perceived by your camera.  There are many layers of translation in between.  The CCD sensor sends a string of voltage values one for each pixel to the image processor for further conversion into an NTSC analog video stream (chroma + luminance) that then get's digitized by your capture device and translated into a digital video stream (RGB).  So right at the start the sensor itself is already integrating all the wavelengths of incoming light into one intensity value per pixel.  The only way to separate out the unwanted part of this signal is to filter it out before it gets to the camera.

 

Now if you are asking if one can figure out the white balance correction needed (using the histogram) to correct for a particular deepsky filter, then the answer is YES.  This correction will however change somewhat with seeing conditions, exposure time and gain setting.

 

Best Regards,

 

Jim T.



#5 A. Viegas

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Posted 20 August 2014 - 12:18 PM

Curtis, I use a nautilus 7 position USB filter wheel and I have never noticed any light leakage even going 3 mins in Ha. Prior to the nautilus I had an Orion 5 position wheel and I similarly never noticed any light leakage. The bigger problem with filter wheels is refocusing. Some filters require very little adjustment. Others much more. I sometimes focus with Bahtinov mask on each of the filters. Then write down the focus offsets on my shoestrIng FCUSB program.

Al

#6 DonBoy

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Posted 20 August 2014 - 12:24 PM

Jim,

 

Really enjoyed your presentation.  Thank you.

 

I'd like to ask you a question about an experience I had the other night with an Astronomic IR cut filter.

 

I was using my MallinCam SSIc to view clusters and bright planetary nebulaes.  The equipment setup was my C8 @ f3.3.  I was using a Optec NexGen 3.3 FR which attaches to the Cmount of the camera.  The Optec NexGen comes in two parts; the lens assembly and a camera specified spacer.   I had adhered the IR filter in the spacer area right near the Cmount threads so it was very close to the camera.  What happened was I was seeing an inordinate amount of warm pixels.  I then took the IR filter out and the warm pixels totally dissappeared.

 

My question is could this IR cut filter have reflected back the IR heat coming from the sensor back on itself and thus caused the temperature on the sensor to appreciably rise and yield warm pixels?  


Edited by DonBoy, 20 August 2014 - 12:24 PM.


#7 jimthompson

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Posted 20 August 2014 - 03:05 PM

Hi Don,

 

I am glad you enjoyed my presentation.  The answer to your question about seeing more warm pixels when you had your IR cut on is:  yes it is possible.  All of the IR cut filters made for astronomical use that I have encountered are "interference" type filters, just like deepsky filters.  They block IR by selectively reflecting it away.  So it is indeed possible that the heat that was coming from the sensor was reflecting off the IR cut filter back at itself, thus warming up the chip and producing more warm pixels.  It is hard to tell if this was all that was happening though, as the presence of the filter very close to the sensor may also have somehow affected the way that heat was being rejected by the sensor in general.  There could have also been some sort of interplay between the filter and the glass window that seals the sensor chamber which would be quite close to the filter in your case.  It could also be a photoelectric effect where stray photons are bouncing around in there, but this would be evident more from an image with more random noise and not a fixed pattern of warm pixels.  Do you remember if the location of the warm pixels was fixed from refresh to refresh?

 

Best Regards,

 

Jim T.



#8 DonBoy

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Posted 20 August 2014 - 04:17 PM

 

Do you remember if the location of the warm pixels was fixed from refresh to refresh?

 

The warm pixels were in the same place throughout the captures.

 

The Optec focal reducer is pretty massive and is a 2" design so there is a lot of metal to draw heat away, as it did when no IR cut filter was inplace.  I've used this same SSIc doing white light solar without a focal reducer, and used it with the same IR cut filter but never noticed an adverse effects.


Edited by DonBoy, 20 August 2014 - 04:57 PM.


#9 jimthompson

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Posted 21 August 2014 - 12:10 PM

The difference may be the fact that you were using the camera for long exposures.  When using it for solar system imaging, your exposure times were probably too low to see the effect.  This is generally true for other types of optical distortions such as coma or vignetting.  They may well exist but you will tend not to see them when your subject of observation is bright (ie. solar system objects).

 

Best Regards,

 

Jim T.



#10 Relativist

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Posted 21 August 2014 - 01:04 PM

Don, great job with the SSIc there :-)

Do happen to have an estimate of the temperature you'd say the sensor was at?

Also approximately how far away was the sensor from the filter?

Lastly, I'm assuming it's a 1.25" filter.

#11 DonBoy

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Posted 21 August 2014 - 03:15 PM

Curtis,

 

I have no idea about the temperature of the sensor.  

 

The 1.25" filter was approximatelly 35mm away from the sensor.



#12 Relativist

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Posted 24 August 2014 - 08:37 PM

By the way did someone record this, and if so is it posted online?



#13 geminijk

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Posted 25 August 2014 - 02:37 PM

AWESOME..nice job Jim. 

 

John







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