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Favorite Light Pollution Filter for EAA?

EAA LP
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#51 ctc86

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Posted 01 March 2016 - 04:54 AM

Thank you Jim! I think I'll go for the Optolong. Doesn't hurt that it's the cheaper of the two. :)

 

Cheers,

Chris



#52 Live_Steam_Mad

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Posted 09 March 2016 - 04:13 PM

Just found what looks to be a treasure trove of actual measured Astro filters curves here ;-

 

http://www.carlostap...ik_cls_ccd.html

 

Previous to this, looking at graphs, seems the Orion Skyglow (Japan not Korea) and the Optolong UHC do what I want the best (H-Beta, OIII, H-Alpha, and IR for galaxies) at a price I can afford. Here, the Optolong is 43 GBP from 365Astronomy, a cracking price I thought (same place I got my Castell 2" OIII).

 

Regards,

 

Alistair G.



#53 Relativist

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Posted 10 March 2016 - 02:26 AM

 

 

 

p.s.  That Celestron RASA filter is huge at 72mm diameter, and super expensive...$500 USD!

 

 

 

I'm not sure, the price per square mm looks good I think, maybe we could cut 3 or 4 smaller filters out of one? From what I've seen with the RASA it's well worth it IMO if you can swing one. Just like anything we do, some of it will be simply a difference in needing to fiddle with white balance, but some of it will be in what ultimately shows contrast. It'd be interesting to compare such multi-spectral filters + OSC vs a Mono + single band filters combined in software.

 

 

So, cost per unit area is in line with other filters, like the Optilong L-Pro. So it's not 'super expensive' after all. I do like these filters as not having to worry about color balance just makes EAA viewing more enjoyable IMO.



#54 jimthompson

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Posted 10 March 2016 - 09:04 AM

 

 

 

 

p.s.  That Celestron RASA filter is huge at 72mm diameter, and super expensive...$500 USD!

 

 

 

I'm not sure, the price per square mm looks good I think, maybe we could cut 3 or 4 smaller filters out of one? From what I've seen with the RASA it's well worth it IMO if you can swing one. Just like anything we do, some of it will be simply a difference in needing to fiddle with white balance, but some of it will be in what ultimately shows contrast. It'd be interesting to compare such multi-spectral filters + OSC vs a Mono + single band filters combined in software.

 

 

So, cost per unit area is in line with other filters, like the Optilong L-Pro. So it's not 'super expensive' after all. I do like these filters as not having to worry about color balance just makes EAA viewing more enjoyable IMO.

 

No Curtis it is not.  You can purchase a 72mm IDAS LPS-P2 or LPS-D1 from Astro Hutech for $329 USD.  That makes the Celestron filter ~50% more expensive with no real difference in performance.  If you were so brave as to purchase the 72mm filter and try to cut it into 1.25" filters the best you could do is cut 5 filters from it, but likely at least one would be broken so let's say 4 filters.  That works out to $499.95/4=$124.99 per filter, plus the cost of a 1.25" filter ring which costs approx. $10, so $134.99 per filter.  You can buy the Optolong L-Pro 1.25" filter for $149...so yes it would be cheaper to buy a big one and cut it up, but it would be even cheaper to do that with the Astro Hutech one.  Personally I don't see the value in this activity since the filter is a poor performer with regards to LP reduction anyway.  I think you are inflating the colour balance issue into something bigger than it really is.  With some practice setting camera or capture device controls white balance is a non-issue.

 

Regards,

 

Jim T.


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#55 Art_NW AZ

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Posted 11 March 2016 - 02:39 PM

Jim T.,

 

Is your excellent karmalimbo website down? I've been trying for three days to get on using previous bookmarks and via links in this topic with no success. Chrome times out with the message "this webpage is unavailable"

 

Thanks, Art



#56 jimthompson

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Posted 11 March 2016 - 03:02 PM

Hi Art,

 

I had somebody else mention it last night.  I have not had a chance to check the server machine which is in my basement.  Sometimes my IP address gets changed by the provider and I have to update my DNS server.  I'll check it tonight when I get home from work.

 

cheers,

 

Jim T.



#57 DarkRise

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Posted 18 March 2016 - 09:38 AM

Hi Art,

 

I had somebody else mention it last night.  I have not had a chance to check the server machine which is in my basement.  Sometimes my IP address gets changed by the provider and I have to update my DNS server.  I'll check it tonight when I get home from work.

 

cheers,

 

Jim T.

 

Just wondering if the following filter is good for EAA application in  mag 5-5.1 skies with a achromatic lense:

 

http://agenaastro.co...low-filter.html



#58 jimthompson

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Posted 18 March 2016 - 10:31 AM

 


 

Just wondering if the following filter is good for EAA application in  mag 5-5.1 skies with a achromatic lense:

 

http://agenaastro.co...low-filter.html

 

 

Hi DarkRise,

 

The answer to your question is:  for light pollution reduction, no.  Although this filter is advertised as a "mild" light pollution filter, in reality it performs very poorly in that role.  For the same level of filter darkness (technically referred to as "luminous transmissivity"), ie. for the same impact on your exposure time, you would get much better performance from a wide bandpass filter, of which there are many (Astronomik CLS, Meade Wideband, Lumicon Deepsky, Orion Skyglow Broadband, Antares ALP, Optolong CLS). 

 

The Moon & Skyglow filter sold by Optolong and many other suppliers (Baader included), is very effective at color and contrast enhancement when you are observing the Moon or planets.  I use the Baader version often when observing the Moon to enhance the colour differences in the mare, and I use it pretty much always when observing planets.

 

In the role of violet fringe reduction on an achromatic refractor, the Moon & Skyglow will also perform poorly.  It has very good response to the blue part of the spectrum so violet fringing will not be strongly affected.  Note however that all the wideband filters I mention above cut out much of the blue end of the spectrum, and so by their nature help with violet fringing in achromats.

 

cheers,

 

Jim T.



#59 DarkRise

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Posted 18 March 2016 - 10:37 AM

 

 


 

Just wondering if the following filter is good for EAA application in  mag 5-5.1 skies with a achromatic lense:

 

http://agenaastro.co...low-filter.html

 

 

Hi DarkRise,

 

The answer to your question is:  for light pollution reduction, no.  Although this filter is advertised as a "mild" light pollution filter, in reality it performs very poorly in that role.  For the same level of filter darkness (technically referred to as "luminous transmissivity"), ie. for the same impact on your exposure time, you would get much better performance from a wide bandpass filter, of which there are many (Astronomik CLS, Meade Wideband, Lumicon Deepsky, Orion Skyglow Broadband, Antares ALP, Optolong CLS). 

 

The Moon & Skyglow filter sold by Optolong and many other suppliers (Baader included), is very effective at color and contrast enhancement when you are observing the Moon or planets.  I use the Baader version often when observing the Moon to enhance the colour differences in the mare, and I use it pretty much always when observing planets.

 

In the role of violet fringe reduction on an achromatic refractor, the Moon & Skyglow will also perform poorly.  It has very good response to the blue part of the spectrum so violet fringing will not be strongly affected.  Note however that all the wideband filters I mention above cut out much of the blue end of the spectrum, and so by their nature help with violet fringing in achromats.

 

cheers,

 

Jim T.

 

Would you recommend the Optolong CLS or CLS CCD? The later is more expensive. I think the only difference is the IR-cut in the CCD version.

 

Thanks



#60 DarkRise

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Posted 18 March 2016 - 11:29 AM

Also, I have a 2"  Arcturus UHC filter. Just wondering if the UHC filter is too aggressive for my mag 5-5.1 sky. I' m using an astro modified Olympus e-m10 with my  f2.8 300mm lense + speedbooster for a f-ratio of f2.

 

This for widefield viewing. I also have an xterminator that I use with my C11.

 

Thanks



#61 jimthompson

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Posted 18 March 2016 - 12:29 PM

It is interesting that you mention that you have an Arcturus UHC filter.  Based on the spectral response posted online from the retailer, this filter should perform very well; very comparable to the Astronomik UHC or Optolong UHC.  I don't have one of these filters in hand so I cannot corroborate its supposed performance.  Have you used this filter with your setup?  If so, how does it perform?  It would be slightly "darker" than a wideband filter (ie. longer exposures required) but would give better LP reduction.

 

I would not choose the -CCD version of any filter.  They have a built in UV/IR cut filter, and there are often times that you do not want to block infrared such as when observing galaxies.  Compared to the Optolong UHC, the Optolong CLS is not as good a performer...I would choose to get the Optolong UHC, but you already have an Arcturus UHC which is supposed to be roughly the same (if we trust the data from the retailer).

 

Regards,

 

Jim T.



#62 DarkRise

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Posted 18 March 2016 - 01:35 PM

It is interesting that you mention that you have an Arcturus UHC filter.  Based on the spectral response posted online from the retailer, this filter should perform very well; very comparable to the Astronomik UHC or Optolong UHC.  I don't have one of these filters in hand so I cannot corroborate its supposed performance.  Have you used this filter with your setup?  If so, how does it perform?  It would be slightly "darker" than a wideband filter (ie. longer exposures required) but would give better LP reduction.

 

I would not choose the -CCD version of any filter.  They have a built in UV/IR cut filter, and there are often times that you do not want to block infrared such as when observing galaxies.  Compared to the Optolong UHC, the Optolong CLS is not as good a performer...I would choose to get the Optolong UHC, but you already have an Arcturus UHC which is supposed to be roughly the same (if we trust the data from the retailer).

 

Regards,

 

Jim T.

No, not with the e-m10. The problem is that I'll have to permanently modify the lense filter holder so it can hold a 2 inches filter. Like that:

 

http://gleeman.blogs...-filter-on.html


Edited by DarkRise, 18 March 2016 - 01:35 PM.


#63 StarCurious

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Posted 28 March 2016 - 05:53 PM

 

...

Can you hazard a guess on how the advent of the bright, white LED street light conversions in many cities might affect the place of the various types of filters on your chart - or in general? Although the fixtures can be better designed for shielding, the dramatic increase in brightness, often with heavy degrees of blue end of the spectrum, is a big change. I have no idea what percentage street lighting makes up for overall city pollution. Any idea how this change is affecting filter performances? Narrowbands survive better? Broadbands suffer??...

 

Hi Jon,

 

Your question is a good one, and one that is becoming progressively more important as it seems that LED lighting has really become popular in new developments, and to a lesser but growing extent in existing municipalities.  I don't think anyone is really sure what the end result is going to be.  In theory with the more efficient lighting (LED's) comes more clever use of the light...full cut-off street lights, less light directed to the sky, etc.  However light intensity levels have increased, and as you mentioned the spectrum of the LED light is different than traditional outdoor lighting. 

 

I searched around online this afternoon and found what I could on outdoor LED lighting spectra.  The information readily available is not great, but seems to all consistently show the same general spectra: a broad band emission centered around 550nm (green) and a narrower band emission at 450nm (blue).  I've attached a graph that compares the typical LED spectrum with other sources of light pollution.  It appears that LED lighting is very similar to older man-made lighting except for the spike at 450nm.  This spike is what gives these lights the characteristic bright cool appearance.  If the spectrum I found can be trusted, the spike seems to be on the far side of Hbeta and O-III wavelengths, suggesting that our LP filters should still work okay.  Similarly the broader emission centered on 550nm seems to sit nicely between O-III and Halpha, so again we may be okay to keep using the LP filters we have.  The impact of LED lights would be more on broadband filters than narrowband.  I also would guess that we will have more difficulty observing reflection nebulae with LED lighting around since these nebulae have a strong blue component.

 

The problem of light pollution is not a simple one.  You have to consider not only the conditions locally (in your neighbourhood) but also the conditions around the city as a whole.  LP filters will be most effective dealing with the overall glow that the city casts on the sky.  In that regard I think outdoor LED lights are not prevalent enough to make a difference yet.  Locally though, such as street lights outside your house, I can see them being more of a problem.  My suggestion is to try to observe from a location that hides your telescope from local lights.  I have my scope setup in the backyard fairly close to the house so that the house blocks the street lights.  I also have a fair number of trees around the perimeter of my yard that effectively blocks other local outdoor lighting in the area.  This leaves only the general sky glow to deal with using filters.

 

cheers,

 

Jim T.

 

Jim,

 

Thanks for this very informative thread.

 

Recently all the street lights in my neighbourhood have been replaced with LED's.  Based on your graphs in post #41, for galaxies with EAA, I think Lumicon Deepsky stacked with Baader 495nm Longpass may work as the former blocks 550nm, and the latter blocks 450nm, the 2 peaks from LED lighting. I already have these 2 filters.

 

My other option is to simply purchase and use a 610nm Longpass filter for galaxies, if stacking the Lumicon Deepsky with Baader 495nm Longpass doesn't work. I know there are 685nm IR Pass, even 850nm IR Pass, but I am concerned that they may be too dark and require too long exposure for my modest Celestron 102 SLT aperture and mount, and there won't be enough stars detected for Sharpcap to Live Stack.

 

Which way is better, if it works?  If not, are there better solutions? Thanks in advance.


Edited by StarCurious, 28 March 2016 - 07:34 PM.

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#64 JonNPR

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Posted 28 March 2016 - 09:45 PM

Starcurious, i hope you will report back on your findings with the filters and combinations.

 

Jon 



#65 jimthompson

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Posted 29 March 2016 - 11:35 AM

Jim,

 

 

Thanks for this very informative thread.

 

Recently all the street lights in my neighbourhood have been replaced with LED's.  Based on your graphs in post #41, for galaxies with EAA, I think Lumicon Deepsky stacked with Baader 495nm Longpass may work as the former blocks 550nm, and the latter blocks 450nm, the 2 peaks from LED lighting. I already have these 2 filters.

 

My other option is to simply purchase and use a 610nm Longpass filter for galaxies, if stacking the Lumicon Deepsky with Baader 495nm Longpass doesn't work. I know there are 685nm IR Pass, even 850nm IR Pass, but I am concerned that they may be too dark and require too long exposure for my modest Celestron 102 SLT aperture and mount, and there won't be enough stars detected for Sharpcap to Live Stack.

 

Which way is better, if it works?  If not, are there better solutions? Thanks in advance.

 

 

Hi StarCurious,

 

I guess since you already have these two filters, you may as well try them out.  I suggest you pick a single galaxy target to use during your test, perhaps M-51 as it is well placed high in the sky right now.  Then try a list of filter combos like:  no filter, just Baader 495 high pass, just Lumicon Deepsky, Baader 495 + Lumicon Deepsky.  I like to save screen captures at a series of exposure times (5, 10, 20, 40, etc).  Things I look for are the impact of the filter when using the same exposure, and how much additional exposure is needed to give the same relative brightness.  Finally I look for the increase in contrast between galaxy and background.  This is usually achieved when you increase your exposure until the background starts to get too light and/or you start getting significant amounts of saturation in the target.  If you have a chance to do this test, please let us know how it goes.

 

In the end I expect you will not see a significant improvement by stacking these filters.  The problem is that the 495 high pass will only cut a small part of the Deepsky filter's pass band off on the left side (where the LED light emissions are) but also will attenuate over the entire pass band hurting emissions at all wavelengths.  From the response curve for the 495 pass it looks like this attenuation is even larger in the IR band which is detrimental to viewing galaxies.

 

I have had good success using an IR high pass filter on galaxies.  You can experiment with this at an affordable price using a #29 Dark Red or even better a Lumicon Halpha Night Sky which you can sometimes find used at a reasonable price.  The Baader 610nm high pass is very similar to a #25 Red, so not quite as dark as a #29.  I suggest trying one of these filters and seeing how you do before going even deeper into the IR.  I have used a #29, 650nm pass, 680nm pass and 720nm pass in my tests and the trend is that contrast gets better the longer the cut-off wavelength.  My analysis suggests that for the sensors we are using in these cameras the best contrast is achieve at 850nm cut-off.  The drawback is of course your effective exposure time has to go up accordingly.

 

cheers,

 

Jim T.


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#66 StarCurious

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Posted 29 March 2016 - 04:23 PM

 

Jim,

 

 

Thanks for this very informative thread.

 

Recently all the street lights in my neighbourhood have been replaced with LED's.  Based on your graphs in post #41, for galaxies with EAA, I think Lumicon Deepsky stacked with Baader 495nm Longpass may work as the former blocks 550nm, and the latter blocks 450nm, the 2 peaks from LED lighting. I already have these 2 filters.

 

My other option is to simply purchase and use a 610nm Longpass filter for galaxies, if stacking the Lumicon Deepsky with Baader 495nm Longpass doesn't work. I know there are 685nm IR Pass, even 850nm IR Pass, but I am concerned that they may be too dark and require too long exposure for my modest Celestron 102 SLT aperture and mount, and there won't be enough stars detected for Sharpcap to Live Stack.

 

Which way is better, if it works?  If not, are there better solutions? Thanks in advance.

 

 

Hi StarCurious,

 

I guess since you already have these two filters, you may as well try them out.  I suggest you pick a single galaxy target to use during your test, perhaps M-51 as it is well placed high in the sky right now.  Then try a list of filter combos like:  no filter, just Baader 495 high pass, just Lumicon Deepsky, Baader 495 + Lumicon Deepsky.  I like to save screen captures at a series of exposure times (5, 10, 20, 40, etc).  Things I look for are the impact of the filter when using the same exposure, and how much additional exposure is needed to give the same relative brightness.  Finally I look for the increase in contrast between galaxy and background.  This is usually achieved when you increase your exposure until the background starts to get too light and/or you start getting significant amounts of saturation in the target.  If you have a chance to do this test, please let us know how it goes.

 

In the end I expect you will not see a significant improvement by stacking these filters.  The problem is that the 495 high pass will only cut a small part of the Deepsky filter's pass band off on the left side (where the LED light emissions are) but also will attenuate over the entire pass band hurting emissions at all wavelengths.  From the response curve for the 495 pass it looks like this attenuation is even larger in the IR band which is detrimental to viewing galaxies.

 

I have had good success using an IR high pass filter on galaxies.  You can experiment with this at an affordable price using a #29 Dark Red or even better a Lumicon Halpha Night Sky which you can sometimes find used at a reasonable price.  The Baader 610nm high pass is very similar to a #25 Red, so not quite as dark as a #29.  I suggest trying one of these filters and seeing how you do before going even deeper into the IR.  I have used a #29, 650nm pass, 680nm pass and 720nm pass in my tests and the trend is that contrast gets better the longer the cut-off wavelength.  My analysis suggests that for the sensors we are using in these cameras the best contrast is achieve at 850nm cut-off.  The drawback is of course your effective exposure time has to go up accordingly.

 

cheers,

 

Jim T.

 

Thanks, Jim.

 

I just printed your post, and will test these as soon as possible.



#67 StarCurious

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Posted 29 March 2016 - 04:35 PM

 

Jim,

 

 

Thanks for this very informative thread.

 

Recently all the street lights in my neighbourhood have been replaced with LED's.  Based on your graphs in post #41, for galaxies with EAA, I think Lumicon Deepsky stacked with Baader 495nm Longpass may work as the former blocks 550nm, and the latter blocks 450nm, the 2 peaks from LED lighting. I already have these 2 filters.

 

My other option is to simply purchase and use a 610nm Longpass filter for galaxies, if stacking the Lumicon Deepsky with Baader 495nm Longpass doesn't work. I know there are 685nm IR Pass, even 850nm IR Pass, but I am concerned that they may be too dark and require too long exposure for my modest Celestron 102 SLT aperture and mount, and there won't be enough stars detected for Sharpcap to Live Stack.

 

Which way is better, if it works?  If not, are there better solutions? Thanks in advance.

 

 

 My analysis suggests that for the sensors we are using in these cameras the best contrast is achieve at 850nm cut-off.  The drawback is of course your effective exposure time has to go up accordingly.

 

cheers,

 

Jim T.

 

Hi Jim,

 

Are you able to estimate, guesstimate or give us a sense of how many times or percent longer exposure is needed with an IR 850nm pass, relative to no filter?

 

Since almost everything else is cut off, light pollution won't be a problem. The only question is then whether 3 or more stars can be detected for Sharpcap to align for Live Stack.  Would you be able to test it out when you have a chance?

 

Thanks,

Joseph



#68 StarCurious

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Posted 29 March 2016 - 06:40 PM

This post#5 below and post #1 contain information that may be useful reference for our discussion here, although the thread was in the context of image intensified night vision.

 

http://www.cloudynig...ates/?p=6804832

 

Since I am using a smaller aperture scope (Celestron 102 SLT), my tests would favour the cutting off the shorter wavelengths.



#69 StarCurious

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Posted 29 March 2016 - 06:47 PM

The Lumicon Deepsky filter blocks everything approximately between 530nm and 630nm.

 

http://www.lumicon.c...erspec_prnt.pdf



#70 ChrisFC

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Posted 30 March 2016 - 02:54 AM

I know is not on topic but it is related ...

For IR v non-IR - is it a hard and fast rule that you want IR for all galaxies but not for all nebulae ? If not, how do you find which objects emit imaging friendly IR ?

#71 StarCurious

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Posted 30 March 2016 - 08:45 AM

Jim,

 

I had false starts last night on "the test", mostly due to my lack of skills in focusing and Sharpcap.  I am still struggling with focusing even though I got my new Orion Accufocus and Bahtinov focus mask - I think this problem was the root cause of the rest of the struggles last night.

 

I did have a plan going in ... I set out to make 4 dark captures, all with 350 gain, for exposures 4s, 8s, 12s, 16s.  Unfortunately, I used my Lumicon Deepsky as a spacer as well in order to achieve 0.5x focal reduction. As a result, I wasn't able to test without Deepsky filter as I ran out of time for refocusing.  I did manage to test Deepsky alone stacked with Baader 495nm Longpass.  It turned out that with either Deepsky or with both Deepsky and 495nm Longpass, I was able to get enough stars to align and stack at 8s and 12s.  I tried 4s and neither combination worked. I did not have time to test 16s.  I wasted a great deal of time in saving to the defaults as FITS stack and only later on did I realize the problem and I had to repeat and "save as viewed" to get the png files. Another mistake was I saved all images under the same object name "M51" when I should have included the name(s) of the filter(s) used.

 

It is clear that I need more learning and practice before I restart this filter testing exercise.

 

FWIW, the parameters used for one image captured are below:

 

[ZWO ASI224MC]
Pan=0
Tilt=0
Output Format=SER file
Binning=2
Capture Area=1304x976
ColourSpace=RGB24
Sensor Temp=4.7
Hardware Binning=On
High Speed Mode=On
Turbo USB=80
Flip Image=Horiz
Frame Rate Limit=Maximum
Gain=350
Exposure (ms)=12
Timestamp Frames=Off
White Bal (B)=59(Auto)
White Bal ®=63(Auto)
Brightness=240
Gamma=78
AutoExpMaxGain=300
AutoExpMaxExp=30
AutoExpMaxBrightness=100
Subtract Dark=\SharpCap Captures\darks\ZWO ASI224MC\RGB24@1304x976\8.0s\gain_350\dark_10_frames_2016-03-30T01_09_05.png
Display Brightness=1
Display Contrast=1
Display Gamma=1
TotalExposure(s)=600
StackedFrames=50

 

I am not 100% sure, but I think it was captured with both Deepsky and 495nm Longpass.  I think I still have focusing problems(?), among others ...

M51 _Stack_50.png


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#72 jimthompson

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Posted 30 March 2016 - 10:04 AM

Hi Jim,

 

 

Are you able to estimate, guesstimate or give us a sense of how many times or percent longer exposure is needed with an IR 850nm pass, relative to no filter?

 

Since almost everything else is cut off, light pollution won't be a problem. The only question is then whether 3 or more stars can be detected for Sharpcap to align for Live Stack.  Would you be able to test it out when you have a chance?

 

Thanks,

Joseph

 

 

Hi Joseph,

 

I did some testing a few years back that I boiled down into a graph, see below.  I don't have an 850nm IR pass filter but one can extrapolate from the graph what the impact on exposure time is.  The %LT for some known IR pass filters, when used with an EXview HAD sensor are:

 

(Lumicon Deepsky = 50.4%)

 

Lumicon Halpha Night Sky = 48.5%

#29 Dark Red = 47.5%

generic 650nm high pass = 42.6%

Baader IR Pass = 40.0%

generic 680nm high pass = 36.7%

generic 720nm high pass = 28.8%

generic 760nm high pass = 21.7%

generic 850nm high pass = 9.7%

 

The curves on the graph are exponential fits to the data:

 

Band Pass Filters             (100 / %LT)^1.32

High Pass Filters              (100 / %LT)^1.96

 

The curve fit would suggest that your exposure time would be about 5x longer than no filter when you use a generic 650nm high pass filter, and 97x longer if you use a generic 850 nm filter.  The 850nm point is a big extrapolation from my measured data so the 97x figure is likely not accurate.  Also, the data in the plot was collected using a camera with ICX418 sensor (non-EXview HAD) which is much less sensitive to IR.  Sounds like there is room for me to do some more testing!  Regardless, I would guess that the exposure time with the 850nm filter is in the 10x to 20x no filter range at least.

 

cheers,

 

Jim T.

Attached Thumbnails

  • LT-vs-ExpTime2.png

Edited by jimthompson, 30 March 2016 - 11:15 AM.

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#73 jimthompson

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Posted 30 March 2016 - 11:38 AM

I know is not on topic but it is related ...

For IR v non-IR - is it a hard and fast rule that you want IR for all galaxies but not for all nebulae ? If not, how do you find which objects emit imaging friendly IR ?

 

Hi Chris,

 

Yes, the rule is pretty well defined.  All emission nebulae emit in the same handful of wavelengths (Halpha, O-III, Hbeta) and galaxies are all more-or-less broad spectrum emitters.  When observing in light polluted conditions, the best contrast on emission nebulae comes from cutting out everything but the handful of desired wavelengths (Halpha, O-III, Hbeta), so no IR.  When observing galaxies (and globular clusters for that matter) in light polluted conditions, you get some improvement using LP filters that pass IR but the best contrast comes from using IR pass filters (ie. only IR gets through, no visual band).  If you want to observe both nebulae and galaxies in the same session without changing filters, an acceptable compromise is to use an LP filter that also passes IR such as a Lumicon Deepsky or Astronomik UHC...both very popular amongst video astronomers.  This filter choice does not give the best performance for nebulae or galaxies, but will still give a significant improvement on both.

 

If you are so lucky as to have access to dark skies, the story changes a little.  The best contrast on emission nebulae is still achieved by using an LP filter + IR cut (even better than no filter), but on galaxies you are better off using either no filter or an IR cut...yes I said an IR cut.  This is because under dark skies there is still a fair amount of LP in the IR band (this is why using an IR cut helps on nebulae).  The peak emission from galaxies is in the middle of the visual band so clipping off the IR part actually helps with contrast if there is no LP in the visual band.  The improvement on galaxies using an IR cut under dark skies is small so for most people not worth the effort.

 

cheers,

 

Jim T.


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#74 John.Sankus

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Posted 30 March 2016 - 11:47 AM

So here's a question I have for the experts here - 

 

I have a Lumicon DS filter for my C8 for use in the city.  Should I use this for all DSO?  or take it out when i view certain objects?



#75 jimthompson

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Posted 30 March 2016 - 11:50 AM

...

I am not 100% sure, but I think it was captured with both Deepsky and 495nm Longpass.  I think I still have focusing problems(?), among others ...

attachicon.gifM51 _Stack_50.png

 

Hi Joseph,

 

Yes, it appears that focusing still needs some work.  Don't be discouraged though.  With any new setup it takes time to learn all the sweet spots for focusing, etc.  Once you've gone through the exercise a couple times it will become easier.  For sure use the Bahtinov mask as it will make life much easier.  I make a point of going to one of the brightest stars out at the time as part of my initial mount alignment routine.  While there I get my focus spot on.  Using very narrow filters plus the additional attenuation of the focusing mask can make the signal very low so you need a very bright star to focus with.  Since you are using M51 as your test target, maybe use Arcturus as your focus star as it is relatively nearby in the sky.  It is worth the time to GOTO the star to refocus when you change filters, then GOTO the target object when done.  It seems tedious but will be much easier in the end.

 

You have an interesting complication, that your filter forms part of your spacers to the focal reducer.  This will make it difficult to determine the impact of the filters on exposure time as your f-ratio will be different between configurations.  Is there any way you can set the spacing independent of your filter selection?  Can your filters go on the end of the FR closest to the telescope?  I agree that sometimes due to reflections it is better to put the filter in between the camera and FR, but for your testing it will be much easier to just have the filters on the end of the FR.

 

Keep up the good work, and good luck!

 

cheers,

 

Jim T.


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