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USING FILTERS FOR LUNAR/PLANETARY OBSERVATION

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#1 BillP

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Posted 29 September 2020 - 12:00 PM

For the vast majority of my astronomical observing life, over 50 years, I have never used filters of any kind for planets or otherwise. But after half a century of reading the extraordinary claims by manufacturers and observing organizations alike about the many benefits of the various color filters on planets, I finally decided to give them a try myself to determine if what is written about them is more fact or is more fiction and hyperbole handed down over time.

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#2 Augustus

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Posted 29 September 2020 - 12:06 PM

IMO, a photographic FL-D filter works the same as a Baader Contrast Booster or Neodymium filter at a fraction of the cost.


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#3 vdog

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Posted 29 September 2020 - 12:35 PM

Thank you for your work and reporting on this.  Much appreciated.


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#4 REC

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Posted 29 September 2020 - 01:26 PM

Excellent report Bill! I have a C102 that is an Achro and use a Baader M&SG filter a lot and a Baader Fringe Killer. I had the Contrast booster, but sold it. Now I'm sorry after reading your reviews. I also have the 80A filter that I will give it a try on Jupiter next time. 

 

Thanks again for your report. Denis K. would be proud!



#5 atin4210

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Posted 29 September 2020 - 06:21 PM

Thanks for the very informative report!



#6 BillP

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Posted 30 September 2020 - 12:04 PM

IMO, a photographic FL-D filter works the same as a Baader Contrast Booster or Neodymium filter at a fraction of the cost.

An FL-D filter, as I understand it, is simply a minus-Green filter, which is what a Magenta filter is also.  I have two magenta filters of differing transmissions and the lighter one works the best.  Yes it does produce somewhat similar results to the CB in my testing, but the color it imparted was IMO a little too unnatural for my tastes.  My lightest magenta, the VERNONscope #30 also has less transmission than the CB, so it dimmed the planet more.  It did darken the maria features more than the CB but in that it also lost a lot of the subtle edge details of the maria.  So between that and the excessive dimming and the odd hue it added to the planet I did not prefer it over what the CB was showing as while not providing as stark of a view, it let more details through.  So for all those reasons I preferred the CB over a minus-green filter.  Also important is that I typically observe Mars at exit pupils of 0.5-0.75mm so all my observations for what filters did was relevant for that exit pupil range.

 

ps - thanks for pointing out about FL-D filters as #30 magenta filters for astronomy are hard to come by, so nice to know there are other sources for minus-green filters!!


Edited by BillP, 30 September 2020 - 12:50 PM.


#7 sunnyday

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Posted 30 September 2020 - 08:03 PM

thank you very much, a real monk's job.
even though my english is not good, i could easily understand what you were talking about.
this is very important when talking to people who do not necessarily have all the scientific background.



#8 Ohmless

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Posted 30 September 2020 - 10:00 PM

great study.  did you happen to notice any difference in filter selection when using a smaller aperture?  kudos!



#9 BillP

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Posted 30 September 2020 - 10:18 PM

I did this evaluation only in the 4" scope.  I would expect the results to be mostly extensible to smaller or larger apertures as long as the exit pupil range is not too far off from what I used, 0.75mm to 0.5mm.


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#10 vineyard

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Posted 01 October 2020 - 03:35 AM

This is a really helpful report - thank you!  What do you think combining a Baader moon & skyglow neodymium with a contrast booster would do?  Cheers.



#11 BillP

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Posted 01 October 2020 - 09:19 AM

When I try to predict what a filter will do, I am usually wrong!  I have not tried combining those two so will have to try.


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#12 curronh

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Posted 01 October 2020 - 05:01 PM

Well done! Thanks for documenting this. I will be using you information this fall.

#13 Napp

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Posted 01 October 2020 - 08:03 PM

Thank you for all this work.  Your results are very helpful.  I have experimented with some of your results on a 10 inch DOB and gotten some very good results. I stored your paper in a folder with David Knisely's papers on nebula filters.


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#14 Dano2.0

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Posted 02 October 2020 - 11:23 AM

This is great, well done Bill! Thank you for taking the time to produce this, its very informative.  



#15 Ed D

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Posted 04 October 2020 - 06:58 PM

Thank you for another great article, Bill.

 

Ed D



#16 LDW47

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Posted 07 October 2020 - 04:07 PM

You didn’t try a variable polarizer just as a point of interest did you, I don’t think I saw it but I might have missed your reference ? I have used it on Venus at twilite or just after and it helped to cut out the glare so that the crescent planet was better defined, similarly the moon but its color is way off but there is clearer definition.



#17 Dave Mitsky

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Posted 07 October 2020 - 05:39 PM

For the most part, I only use color filters when observing Mars, those being primarily Wratten #21 (orange) and #80A (blue) filters and a Brandon CCM30 (magenta) filter.  I don't care for the various red filters very much even with large apertures. 

The Brandon magenta filter is by far my favorite.  It's unfortunate that the CCM30 has to be used with an adapter tube and that other filters can't be stacked with it due to the Brandon design.

I sometimes use a #80A filter for observing the GRS.



#18 BillP

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Posted 07 October 2020 - 07:53 PM

You didn’t try a variable polarizer just as a point of interest did you, I don’t think I saw it but I might have missed your reference ? I have used it on Venus at twilite or just after and it helped to cut out the glare so that the crescent planet was better defined, similarly the moon but its color is way off but there is clearer definition.

I did, but did not like it.  I felt it reduced the resolution.  I also used a variety of neutral density filters alone to dim it sufficiently.  In both of those cases I felt the combo I listed worked best for me.


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#19 LDW47

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Posted 07 October 2020 - 08:05 PM

I have various colored filters of which you have mentioned but I use mostly the 2 yellows #,s 8 & 12 and the newly acquired yellow-green. I should go back and give the others another go based on your comments. I really don’t view the moon and planets all that often, I am a dso guy for the most part.



#20 DeWynter

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Posted 08 October 2020 - 02:36 AM

Thank you very much, Bill, for the great report!

I've got a bunch of colour filters, but found that M&SG and Baader CB are giving the best results.

 

Did you try to stack CB and #12 / Baader Yellow for Moon or just CB and #8?



#21 BillP

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Posted 08 October 2020 - 02:10 PM

I did, it added more of a yellow cast than I like, so preferred the #8.  There is a real interplay between performance improvement and aesthetics that each person needs to find.  So no one answer to what is best, especially when stacking as personal preferences can sometimes be more important than detail performance. 


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#22 BillP

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Posted 08 October 2020 - 04:04 PM

I forgot to mention, IMO if one does not have an specialty filters the #12 Yellow by itself did a great job at bringing out features on Mars, especially darkening the maria.  So for color filters it did quite well all by itself.



#23 Starman1

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Posted 10 October 2020 - 11:43 AM

This is a really helpful report - thank you!  What do you think combining a Baader moon & skyglow neodymium with a contrast booster would do?  Cheers.

The Baader Moon & Sky Glow Filter and the Baader Contrast Booster are both on neodymium-oxide glass.

From ~490nm and longer, they are identical in response.

The Contrast Booster adds a strong minus violet component to the filter, so its response drops off sharply below 475nm.  The Moon & Sky Glow filter has high transmission down to 400nm.

Hence, stacking them would result in what would essentially be a Baader Contrast Booster filter with a lower transmission at all wavelengths above 475nm.

 

Would you want that?  It would dim Mars more and still give approximately the same spectral response as the CB, but at a lower level.

But, it would also damage image sharpness, as wavefront error is dependent on the two sides of the glass being perfectly parallel, which you could not guarantee

when two filters are stacked together.  This is why, in my opinion, a variable polarizing filter is never as sharp as a neutral density filter of the same transmission.

 

I don't think you would want to stack these together.


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#24 vineyard

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Posted 10 October 2020 - 02:04 PM

The Baader Moon & Sky Glow Filter and the Baader Contrast Booster are both on neodymium-oxide glass.

From ~490nm and longer, they are identical in response.

The Contrast Booster adds a strong minus violet component to the filter, so its response drops off sharply below 475nm.  The Moon & Sky Glow filter has high transmission down to 400nm.

Hence, stacking them would result in what would essentially be a Baader Contrast Booster filter with a lower transmission at all wavelengths above 475nm.

 

Would you want that?  It would dim Mars more and still give approximately the same spectral response as the CB, but at a lower level.

But, it would also damage image sharpness, as wavefront error is dependent on the two sides of the glass being perfectly parallel, which you could not guarantee

when two filters are stacked together.  This is why, in my opinion, a variable polarizing filter is never as sharp as a neutral density filter of the same transmission.

 

I don't think you would want to stack these together.

That is incredibly helpful Don, thank you!



#25 humma kavula

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Posted 12 October 2020 - 09:24 AM

outstanding work bill. thank you so much. 




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