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685nm filter question

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#1 mdaulton33

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Posted 21 July 2021 - 02:23 PM

Hi All,

 

I'm using the Televue PVS 14 setup on my 12" f5 dob. I have several 2" filters - 7nm and 3.5nm h-alpha. I've seen some post where some are using a 685nm IR filter....can someone give me some insite on when that filter is useful....

Thanks

Marshall



#2 Mazerski

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Posted 21 July 2021 - 02:47 PM

The 610, 642 and 685nm IR filters block out excess light so the view you get has good contrast between sky and star stuff (open clusters, globulars and galaxies).

 

In high light pollution the 610 is not useful as it lets too much light in high polluted skies.

The 642 allows some nebulosity and the 685 knocks out most nebulosity with the darker background.

 

The filters you mention in post are for emission nebula. 
Example: the 642nm IR on M42 looks great - you see more stars than with the Ha filter and you still get a lot of nebulosity. In handheld mode with say a 135mm f/2.8 lens, the Milky Way clouds in the area of the Eagle down thru Lagoon area look great (still with 642). 
 

It depends on how much light pollution you have. 


Edited by Mazerski, 21 July 2021 - 03:28 PM.

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#3 PEterW

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Posted 21 July 2021 - 04:20 PM

For city dwellers 685nm let’s you see a sky full of stars like we shouldn’t be able to see. The 642nm gets some followers for less polluted skies. These are cheap compared to the narrowband ones, so no reason not to get one.

Peter

#4 PEterW

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Posted 21 July 2021 - 04:31 PM

For city dwellers 685nm let’s you see a sky full of stars like we shouldn’t be able to see. The 642nm gets some followers for less polluted skies. These are cheap compared to the narrowband ones, so no reason not to get one.

Peter
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#5 ButterFly

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Posted 21 July 2021 - 04:41 PM

The Baader 685 and the Lumicon Night Sky H-alpha are parfocalish.  They make a good pairing under my fairly darker skies.  The Astronomik 642, unlike the Baader and Lumicon above, is actually a band pass filter from around 640-850nm.  I like all three.

 

Even under very dark skies, my Baader 610 barely gets used.  The only reasonable use for that is if one wishes to blind a puma with 500nm light, and not you or your device.

 

The 685 is a nebula killer, so it's great for clusters/reflection portions of HII regions.


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#6 Dale Eason

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Posted 21 July 2021 - 06:55 PM

In my heavy city light pollution ( I can only see Mag 2.5 stars  most of the time) I use the 685 for galaxies, open clusters and Globs or just to scan the sky hand held.   On good nights I can actually see the clouds of the summer milky way starting near Sagittarius,

 

For a point of reference with my 10 F3 telescope and the 685 I could even see Carolin's rose NGC7789.   Not well but many dim tinny stars enough to see it was definitely a cluster.  Then when I went to a darker site where I could see Mag 3 without help NGC7789 was a spectacular sprinkle of tiny stars with the same equipment.

 

Dale


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#7 Jeff Morgan

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Posted 22 July 2021 - 04:09 AM

The Astronomik 642, unlike the Baader and Lumicon above, is actually a band pass filter from around 640-850nm. 

 

That Astronomic sounds interesting.



#8 ButterFly

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Posted 25 July 2021 - 10:55 PM

That Astronomic sounds interesting.

It's great in my backyard, and almost always in my filter wheel for 1x use.



#9 mdaulton33

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Posted 01 August 2021 - 07:26 AM

Thanks for the replies..

So - if I understand - the IR filters are more for galaxies and globulars - and not used stacked with with Ha filters?



#10 Mazerski

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Posted 01 August 2021 - 08:55 AM

Right - no stacking. In addition to galaxies and globulars... 

 

I live on east Coast - skies are really messed up but with 135mm f/2.8 lens attached to PVS7 (that’s 5x in hand-held Mode) with 642nm IR filter, the Milky Way clouds in Sagittarius look great, and I can see nebulous blobs for the Lagoon and Omega nebulas. 


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#11 ButterFly

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Posted 01 August 2021 - 09:38 AM

H-alpha + 685 = EBI

 

Draw the spectra to see this.  One multiples the transmission spectrum of one by the other when they are stacked.



#12 Jeff Morgan

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Posted 01 August 2021 - 10:51 AM

Another aspect of the 685 comes to mind ...

 

On the Dielectric Diagonal thread there was some question as to whether or not the telescope objective was passing the extra spectrum the BBHS diagonal was capable of.

 

If the objective was not passing much above 700nm, then using a 685 long pass should result in quite a dim view. Since it does not, it would seem lenses and mirrors a not "gatekeepers" on the 700-900nm range.



#13 ButterFly

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Posted 01 August 2021 - 04:39 PM

Another aspect of the 685 comes to mind ...

 

On the Dielectric Diagonal thread there was some question as to whether or not the telescope objective was passing the extra spectrum the BBHS diagonal was capable of.

 

If the objective was not passing much above 700nm, then using a 685 long pass should result in quite a dim view. Since it does not, it would seem lenses and mirrors a not "gatekeepers" on the 700-900nm range.

Inconclusive by itself.

That's what led me getting the Lumicon and 685 in the first place.  I wasn't aware that the 642 is a band pass (the shiny mirror finish was no match for my idiocy) with a cut-off around 850.  The diode in my remote is above that cutoff.  What passes through it is the reflection of the diode's light on the body of the remote.  The remote's plastic body is actually transparent in near-IR.

 

The sky background is not much different between the Lumicon and the 685, but the nebulae are obviously much smaller.  It's really too close.  It's 40nms against the hundreds they both pass.  What muddies the waters is that the sky background is not much different between the 642 and 685, under my skies.  The spectral response of my particular tube is unknown, as is the spectrum of my sky.  A panel of various LEDs at different wavelengths, or a tunable laser, is much easier.

 

Comparing 685 alone, with 685 + 642 (with its cutoff) is something I haven't tried yet.  I've dealt with the refactors by not caring anymore and just accepting it.  The dob is the issue because I can't just take off the secondary to compare images.  It's secondary is dielectric aluminum and its primary is overcoated aluminum.  I've been using the 67PP attached to the wheel anyway, so I'll try sliding in the 642 with the slide.  There should be a break in the monsoon tonight.

 

Nebula killer is fun, so the 685 is a keeper.  The Lumicon is parfocal with it, so that's also a keeper.  The 642 is just a great filter, so that's also a keeper.



#14 careysub

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Posted 01 August 2021 - 06:22 PM


 

The Baader 685 and the Lumicon Night Sky H-alpha are parfocalish.  They make a good pairing under my fairly darker skies.  The Astronomik 642, unlike the Baader and Lumicon above, is actually a band pass filter from around 640-850nm.  I like all three.

 

Even under very dark skies, my Baader 610 barely gets used.  The only reasonable use for that is if one wishes to blind a puma with 500nm light, and not you or your device.

 

The 685 is a nebula killer, so it's great for clusters/reflection portions of HII regions.

The Optolong 650 nm is similar to the 642 nm (I expect, I have the first but not the second). Still lets H-alpha through.




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