136 replies to this topic

[Starman1]

And the Semi Apo is the gentlest of the Minus violet filters.

The Contrast booster not only kills the purple, but also much of the blue.

The Moon & SkyGlow filter does have a tiny bit of cutoff at the purple end of the spectrum.

You might call it the gentlest of all. 

[russell23]

The names "Fringe Killer" and "Semi-APO" are misleading.   Neither filter "kills" the purple fringe or makes the achromat like an APO.   What both filters do is reduce the visible purple fringing ~50%.   The 50% reduction in visible CA does allow for improved performance at higher magnifications.  Stars will be sharper at and the achromat will be capable of being used at higher magnifications than without the filter.

 

The true "fringe killer" is the Baader 495 Longpass filter and this is the one I recommend for a large achromat.   It completely eliminates the purple fringing and allows the large achromat to be used at higher magnifications.

 

Dave

[Starman1]

Dave,

I agree.  But people who feel the 485nm cutoff of the Contrast Booster is too extreme are also likely to find the 495nm cutoff of the LongPass filter too extreme.

One good possibility is to use a narrowband or O-III filter on nebulae (these should eliminate the violet fringe) and a LongPass filter on stars.

[russell23]

Dave,

I agree.  But people who feel the 485nm cutoff of the Contrast Booster is too extreme are also likely to find the 495nm cutoff of the LongPass filter too extreme.

One good possibility is to use a narrowband or O-III filter on nebulae (these should eliminate the violet fringe) and a LongPass filter on stars.

 

I guess it depends upon your intended target and your goal.  The transmission curves for the contrast booster and 495 Longpass filters can be found on Agena's website:

 

http://agenaastro.co...lanetarium.html

 

Here is how I read the transmission curve graphs provided:

 

495 Longpass:

 

475 --> 5%  to 495nm -->85% -->540nm 95% --> 700nm 95%

 

Contrast Booster:

 

475 0% --> 495 90%

495 90% --> 530 25%

530 25% --> 550 92%

550 92% --> 575 0% --> 595 0%

595 0% --> 610 95%

610-700  90% +/- 10%

 

The transmission curves show a lot deeper cuts with the contrast booster than the 495 Longpass.  Maybe the contrast booster lets a little more through in the 475-495 range - though from the transmission curves it is not clear to me that there is much difference there. 

 

I used the 495 Longpass for several years permanently on the star diagonal of my Vixen 140 Neo-achro.   Once in a while I would compare emission and planetary nebula with and without the filter and did not see any dramatic difference.  In fact with the improved ability to get a sharp focus using the filter I found it easier to pull fine structure out of nebula with the 495 LP filter than with no filter.

 

Certainly you can go with an OIII filter if you really want to target nebula specifically, but the 495 LP filter has a broad usage and eliminates the purple fringing completely.   I see the Fringe Killer and Semi-APO filters as a half measure because they only reduce the purple fringing ~50%.  

 

I think people get too wrapped up worrying about the "color shift" of any of these filters.  I used to worry about that myself but when I started using the 495 LP with my large achromat I realized that the improvement in performance at high magnification was incredible and the color shift was only noticed on the brightest stars, moon, and planets.  And the bright stars were a beautiful golden color with the filter which is nice to look at anyway. 

 

Dave

[glend]

It also depends on the focal length of your achro refractor. A scope like my 127mm iStar R30 F12 does not need much, if any, CA improvement, so the 'gentler' Semi-Apo is all I ever put on it. Depending on the target I can get away with no filter at all.  The R30 glass goes a long way to pushing an f12 into f15 CA territory. I can't uderstand why people would want a f5/f6 achro but alot of them are being made it seems.  You can't really image with them, so the short f ratio just gives you a wider field of view.  Might as well buy a f5 reflector as that will give you true colour rendition.

Edited by glend, 15 February 2016 - 04:48 PM.

[CollinofAlabama]

I owned a Fringe Killer filter once which I used with an old Orion 100mm F/6 refractor (Sidgwick-Conrady value of 1.5 -- terrible!)  I found the color rendition of Jupiter, though not so "purple hazed", unacceptible, and preferred Jove-avec-violet.  I don't care for Jupiter, the King of the Gods, Optimus Maximus himself, the beautiful cream orb with all his glorious clouds bands rendered a cartoonish Jolly Green Giant.  I could care less how "technically detailed" the view might be.  It was abhorrent, a Πισς Christ for Jovian disciples.  Today, my C102GT (really a 97mm clear aperture, 1016mm focal length achromat with a SC value of 2.74) gets by with a cheap Vivtar 81B 2" filter and 1.25" Baader MSG combo for bright objects, and that's all I need, especially for such a long, small aperture achro.

 

From the sounds of it, if I had a larger achro with a lower SC value, I'd simply have to endure greater chromatic aberration.  I don't find the CA all that bad, really, in the 102GT, but I've got the filters to clear things up at higher power should I need it.  I guess the bottom line is ED doublets, APO triplets, or newtonians/SCT/MCT's for serious planetary work in any scope above 4", unless you have a custom, long-tubed achromat and an observatory to house it in.

Edited by CollinofAlabama, 05 March 2017 - 12:14 AM.

[russell23]

I think the minus violet filters end up being a very personal thing in part due to differences in color sensitivity.   But I really did not like any of the minus violet filters that much for planetary observations.   The best filtered views I had of Jupiter came when I used the Vixen 140 with 107mm or 80mm aperture masks and a #8 light yellow filter.  The color shift to my eyes was negligible and the views were CA free.   

 

I recommend the CA reducing filters when using a larger achromat for deep sky, lunar, and double star observations.  They help achieve sharper star images at higher magnifications which expands the magnification range upward for which the scope is useful.  

[Messyone]

 

 

 

None of these filters were turn the large, fast achromats into excellent planetary scopes.  But they will significantly increase the ability to use these scopes at higher magnifications above 100x.

 

 

For anyone who is used to good quality reflectors and ED/apo refractors, they will probably be disappointed in the color shift and still soft image.  That is the cup half empty attitude.  

 

The cup half full:  They have their drawbacks but they can show you more.. 

 

Jon

 

 

For planetary yes.   I never considered the Vixen 140 filtered soft on the Moon or DSO.  But definitely the achromat I had was not a good enough optic for planets.

 

I would love to have a chance to try an I-star achromat with the 495 LP filter to see if a better quality objective figure would produce different planetary results than what I had with the Vixen 140 - which I suspect was a middling optical figure.

 

Dave

 

Dave,

 

I have just tried the LP 495 in my Istar 6" f12. Removes CA 100%. I could see detail in the cloud bands on Jupiter, a much cleaner and sharper view than no filter.  The yellow shift....got used to it very quickly....a non issue. Best view came with a Morpheus 9mm giving 200x with .75mm exit pupil. 

 

Over the years I have tried a few minus v filters such as WO minus v, Baader semi APO, Fringe Killer alone and combined with the Moon and Skyglow a #8 yellow ....none are as good as the 495. 

 

Matt

[Starman1]

Too bad you didn't have the Baader Contrast booster, which has the highest cut off of Baader's minus violet filters.

However, like all the "notch" filters, there are some other wavelengths removed, unlike the 495nm Longpass.

[Wford]

I got the best results with the 495 Longpass filter

Sent from my SM-A510F using Tapatalk

[russell23]

Matt and Wayne,

 

Thank you for sharing.   It is helpful to have additional people confirm that the 495 Longpass completely removes the purple fringing.  The only time I recommend a #8 or the Fringe Killer is when the prospective filter user is concerned about the stronger color shift.   It really is a choice between a strongly yellow shifted image with complete removal of purple fringing vs. a closer to neutral color with ~50% of the purple fringing remaining.

 

Dave

[Messyone]

One day I'll have to try the Contrast Booster. 

 

As I said earlier I was really surprised how the yellow shift was a non issue with the 495. I'm not sure I'll leave it in permanently, but for Jupiter and possibly Saturn, it does an amazing job at shapening up the image. Without the 495 I had to look hard to see some of the details in ol' Jups cloud bands...the 495 is a fine filter for Achros is all I can say, which is lucky as all I have Refractor wise is long fl Achros......should have bought one a few years ago 

 

Matt

[nashvillebill]

Interesting stuff!  I too have the C102 f/10 and yes there is some minor purple CA looking at Venus.  I don't know if I'm ready to start procuring more filters, though.

 

I already have a #15 Yellow as well as an Orion Moon filter, would either of those help with CA on bright planets?

[Sarkikos]

I have a C102 f/9.8.  I tried the 495 Longpass when viewing Jupiter.  To my eyes, the yellow shift was too drastic.  Whatever improvement there was in sharpening the image was not worth the yellow tone of the planet.  I don't like a yellow Jupiter.

 

Overall, I like the Baader Neodymium (Moon & Sky Glow) best as a general planet filter, and for Jupiter in particular, no matter what the telescope.  I've also tried the Baader Contrast Booster, Semi-Apo, Fringe Killer and VR-1.

 

Mike

Edited by Sarkikos, 10 March 2017 - 01:38 PM.

[Starman1]

The Baader M&SG filter has the least violet removal of all the Baader filters, so would naturally impart the least spectral change.

[Sarkikos]

Maybe that's why I like it ... or at least one of the reasons why.

 

Mike

Edited by Sarkikos, 10 March 2017 - 02:21 PM.

[mogur]

Does it make any sense to stack 2 different minus-violet filters, or will the filter with the tighter bandpass be dominant anyway?

[MrJones]

I want to try this one:

 

http://www.chromafil...9_14160087.aspx

[Starman1]

Does it make any sense to stack 2 different minus-violet filters, or will the filter with the tighter bandpass be dominant anyway?

The one with the tighter bandwidth (higher cutoff) will dominate.

[nashvillebill]

A follow-up to my earlier question about using a #15 Yellow filter.  I just got a chance to try it on Venus, which was sitting in a slight violet fog at 111x (9 mm Meade Series 5000 Plossl in a Celestron 102GT f/10)  The yellow filter cleared it up and I could immediately focus more clearly and got a razor-sharp crescent.  However, it was a very deep yellow view overall which certainly would obscure dimmer objects.

 

I believe I will order up a #8 Yellow and try it, for a few bucks it seems like a good experiment.

[deSitter]

The rule here is - for chromatic index <2, forget high power views, they will be colorful. For chromatic index 2-4, the Baader Semi-APO is both neutral colored and effective (very effective) at cutting out the purple haze and produces a dramatic increase in contrast. For chromatic index >4 you don't need a filter, but I still notice color improvement by the Semi-APO with my 3" f/15 classic refractor, although in that case the benefit is canceled by the slight dimming.

 

In short the Semi-APO is one of my most used accessories in my f/9 and above grab/go achromats. Its best use is coupled to a moderately fast achromat that has outstanding spherical correction.

 

-drl

Edited by deSitter, 12 March 2017 - 10:08 AM.

[nashvillebill]

I'd like to get the Semi-APO one day, but at $90+, it's a little pricey, so instead I ordered the $13 #8 Yellow.

[nashvillebill]

Okay, a final follow-up in case someone finds this thread in the future.  I received my #8 Yellow filter and have used it on Venus, Jupiter, Saturn, and the Moon.  I am satisfied with its performance: the slight purple haze around Venus is gone, I observed it last night at 200x magnification and was able to focus quite clearly.  There was a trace of  violet on one limb but barely noticeable.  Again, this was in a Celestron 102GT f/10 achro.

 

No noticeable color shift on the Moon, and I was using 150x magnification on Jupiter and Saturn, again there was a hint of violet on one limb of Jupiter but no certainly "fog" around it.  Even bright stars did not have any violet haze around them.  I did not have a chance to view M42 so I can't pronounce judgement on its use on DSO's.

 

For a mere $13 I am quite satisfied.  I'll keep it screwed into the diagonal.

[Starman1]

On the Moon and planets, a yellow filter will also increase contrast.

A nice, and successful, $13 experiment, it seems.

You will want to remove it on DSOs, however, since it subtracts 17% of the light (83% transmission).

[russell23]

On the Moon and planets, a yellow filter will also increase contrast.

A nice, and successful, $13 experiment, it seems.

You will want to remove it on DSOs, however, since it subtracts 17% of the light (83% transmission).

When I was using #8 LY and the 495 Longpass filters with my Vixen 140NA refractor I kept them on the star diagonal all the time - including for deep sky observations.  While they remove light there are several things that mitigate the impact this light reduction has on the view.

 

1.  The light removed is mostly in the portion of the visible spectrum that is not focused into the image because it is blurred out of the image.

 

2.  With the filter it is possible to get noticeably sharper images.  

 

3.   The filter clears out a background scatter of contrast robbing defocused light across the sky background. 

 

In my experience the combination of these three factors actually means it is possible to see more detail, not less when using filters like the #8 LY or the 495 Longpass with an achromat.   I never found that with the filters there was significant dimming of the image.  In all cases I found that deep sky objects revealed more details with the filters than without.  

 

For dense open and globular star clusters the difference at higher magnifications is very obvious.  The filter sharpens up the star images which means more stars can actually be cleanly resolved into pinpoint stars.  Without the filter many of the fainter stars are too blurred to detect.   In numerous starfields I routinely observe - after using the filter -  I noticed additional stars with the filter that I had not noticed without the filter.   

 

Even nebula can benefit from the filters.  I found with the filter I was able to pick out more fine details once I started using the filter.  

 

Due to all these benefits I kept a 2" filter on my star diagonal at all times when I was using an achromat.  Since switching to the SW120ED I haven't had to worry about a CA filter, but I hang onto the Baader 495 LP.  The SW120 is a great scope to test how much dimming the filters actually dim DSO because it focuses all the light into the airy disk.  Therefore all the light removed by the filter actually was in the image - unlike the situation with an achromat.

 

Achromat's I used the filters with include the SV809D (80mm f/9.4), ST80, and the Vixen 140NA.  Obviously the impact was most dramatic with the Vixen 140, but the filters were beneficial with all of these scopes.