25 replies to this topic

[camman]

 Hi Guys  I am after some advice what is the better filter to image venus in UV 

Astrodon u venus Filter or Baader Johnson Bessel U The Bessel U shows lower UV transmission but higher overall transmission due to the extended red. Its also a third cheaper

https://astrograph.n...Bessel-U-Filter

 

https://www.firstlig...ic-filters.html

Edited by camman, 14 February 2022 - 05:22 AM.

[Tapio]

Hm, those are photometric filters.

How about this filter ? It has better transmission too.

https://www.firstlig...ary-filter.html

[Tulloch]

This article from Christophe Pellier might help you decide.

https://www.planetar...ters-for-venus/

[camman]

Hm, those are photometric filters.

How about this filter ? It has better transmission too.

https://www.firstlig...ary-filter.html

Tried that its going back. It produces a ghost image. Seriouse IR leak

[Tapio]

Interesting information.

Didn't know that you could use a photometric filter for Venus.

Once I had Baader UVenus filter but sold it due to lack of use.

[camman]

This article from Christophe Pellier might help you decide.

https://www.planetar...ters-for-venus/

Yes great report. Thanks. Am i correct that the Baader Bessel photometric U Here https://www.firstlig...ic-filters.html

 

Is the same as the baader Johnson U in that report ?

[camman]

Interesting information.

Didn't know that you could use a photometric filter for Venus.

Once I had Baader UVenus filter but sold it due to lack of use.

I know what you mean i did similair i previously had The Astrodon

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[Tulloch]

Yes great report. Thanks. Am i correct that the Baader Bessel photometric U Here https://www.firstlig...ic-filters.html

 

Is the same as the baader Johnson U in that report ?

You'll need to ask Christophe about that, he posts here regularly...

https://www.cloudyni...t-blue-filters/

[camman]

You'll need to ask Christophe about that, he posts here regularly...

https://www.cloudyni...t-blue-filters/

Ok thanks. hopefully he will look on the post

[CPellier]

Hi guys, I sent a reply to Neil but here are more infos:

- In practice, Bessel = Johnson. Those are filters that (more or less..) stick to the photometric Johnson U band (even the Chroma, despite its custom shape). 

- Wavelengths shorter than 350 nm are usually not gathered by our imaging systems, because many of our optical components will not transmit the very short wavelengths (correcting plates, barlows, AR windows of cameras, some ADC's...)

This is the advantage of the Johnson/Bessel filters, they transmit the 380-400 nm range that many other filters won't, so these latter are not going to perform better "in the field". Using such filters with better profit would ask for special optical trains such as fused silica barlow lenses (or Siebert UV barlows), perfectly reflecting telescope with no lenses or plates, and some really good cameras without there protective windows!

As described on my article linked by Andrew above, currently look for (in that order): Chroma Bessel U, Astrodon Johnson U, Baader Johnson U, then if none of them is found, Astrodon UVenus or sloan u' (I think it is the same filter btw).

The new Chroma sloan u' looks to cut at 390 nm and so should be roughly equivalent to the Astrodon Johnson U.

Edited by CPellier, 15 February 2022 - 04:17 AM.

[camman]

Hi guys, I sent a reply to Neil but here are more infos:

- In practice, Bessel = Johnson. Those are filters that (more or less..) stick to the photometric Johnson U band (even the Chroma, despite its custom shape). 

- Wavelengths shorter than 350 nm are usually not gathered by our imaging systems, because many of our optical components will not transmit the very short wavelengths (correcting plates, barlows, AR windows of cameras, some ADC's...)

This is the advantage of the Johnson/Bessel filters, they transmit the 380-400 nm range that many other filters won't, so these latter are not going to perform better "in the field". Using such filters with better profit would ask for special optical trains such as fused silica barlow lenses (or Siebert UV barlows), perfectly reflecting telescope with no lenses or plates, and some really good cameras without there protective windows!

As described on my article linked by Andrew above, currently look for (in that order): Chroma Bessel U, Astrodon Johnson U, Baader Johnson U, then if none of them is found, Astrodon UVenus or sloan u' (I think it is the same filter btw).

The new Chroma sloan u' looks to cut at 390 nm and so should be roughly equivalent to the Astrodon Johnson U.

Hi Christophe your comments about AR Windows. Barlows (Essential for my Newtonian ) And ideally using a ADC ( I have the ZWO ADC ) One can see the logic in your recommendations. I did wonder about removing the AR  protect window of my ZWO 178m Camera.

 

But without also getting a different  ADC . And a fused silica uncoated barlow lens. Something at this stage i am not prepared to do. The choice and my budget means only one filter i think. The Baader Bessel U Photometric.

The  Astrodon sloan U2 photometric filter i have seen secondhand is tempting, I got good results with the Astrodon U Venus filter ( As you say you think they are the same thing ? ) And no barlow being needed for my 8" (actually 7.3 stopped down) Classical Cassegrain

 

I did use a barlow with my 10" Newtonian and the Astrodon U Venus. But not a ZWO ADC. I am unsure of any AR Coated window on the old 618 DMK cameras or any UV Reducing coatings it may or may not have had on that older camera. So my situation and Equipment has changed somewhat since i got reasonable results with the Astrodon U Venus Filter.

As you say it kind of seems safer to buy the Baader Bessel U

Decisions ?

[CPellier]

Hi Greg, I did removed the AR window of all my cameras. It does add a bit more UV.

I have not the proof that the Astrodon UVenus and u' are the same filters, but I looked closely to their passbands and if they are not the same, the difference would be no more than 2-3 nm in width, so nothing. It would sound logical that Astrodon re-used the UVenus as a u' to save costs...

Yes I would still recommand the Baader Johnson U (not their UV Venus). Apart of the fact that many things are going to absorb UV shorter than 350 nm (except at prime focus of the cassegrain without ADC), we may still take into account the camera that is going to be more sensitive at 380-400 than before 350  (even if the IMX178 looks better than other), and our own atmosphere, that is poorly transparent between 300/350.

I did made UV Venus images with the first gen of the Baader Johnson U, it was very good...

[camman]

Hi Greg, I did removed the AR window of all my cameras. It does add a bit more UV.

I have not the proof that the Astrodon UVenus and u' are the same filters, but I looked closely to their passbands and if they are not the same, the difference would be no more than 2-3 nm in width, so nothing. It would sound logical that Astrodon re-used the UVenus as a u' to save costs...

Yes I would still recommand the Baader Johnson U (not their UV Venus). Apart of the fact that many things are going to absorb UV shorter than 350 nm (except at prime focus of the cassegrain without ADC), we may still take into account the camera that is going to be more sensitive at 380-400 than before 350  (even if the IMX178 looks better than other), and our own atmosphere, that is poorly transparent between 300/350.

I did made UV Venus images with the first gen of the Baader Johnson U, it was very good...

Yes Christophe you helped me see it makes sense to get the Baarder. I originally purchased the Optolong U Venus which had a big IR Leak. i quickly took a photo off my screen. 

Ive sent that back. So Now i will purchase the Baader Johnson U. Your advice has been very Helpful. And very much appreciated. 

Best Regards Neil.

 

 

Seriouse problem with Optolong U Venus. Massive IR leak

[CPellier]

Indeed...this is a leak ! This is regrettable as that filter is not exactly a cheap one...

[banjo1000]

Are you sure your double image is your primary image? It looks to me like you are looking at the diffracted image which has bounced off your pixels, up to your coverslip and then back to your sensor off the front and rear surfaces of your coverslip.  We have to contend with these double-bounce images if we use sensors with coverslips.  It is instructive to put Venus in the center of your detector and grossly overexpose it.  Without filters, you will see a cross pattern from diffraction. If you add a band pass filter such as a UV, IR or narrow band filter, the streak resolves into images of Venus.  

[camman]

Are you sure your double image is your primary image? It looks to m nextday.e like you are looking at the diffracted image which has bounced off your pixels, up to your coverslip and then back to your sensor off the front and rear surfaces of your coverslip.  We have to contend with these double-bounce images if we use sensors with coverslips.  It is instructive to put Venus in the center of your detector and grossly overexpose it.  Without filters, you will see a cross pattern from diffraction. If you add a band pass filter such as a UV, IR or narrow band filter, the streak resolves into images of Venus.  

Heres another shot straight after with a different filter. Tried the same again the next day. Again switched filters. Again it went away 

Edited by camman, 16 February 2022 - 11:45 AM.

[camman]

Indeed...this is a leak ! This is regrettable as that filter is not exactly a cheap one...

Hard to believe Optolong. Are selling these ?

[John Boudreau]

Hard to believe Optolong. Are selling these ?

Baader had a similar problem maybe 15 or so years ago with their 1.25" UV Venus filter. Oddly, their 2" filter worked fine. 

 

They did fix the problem fairly quickly, IIR.

[camman]

Baader had a similar problem maybe 15 or so years ago with their 1.25" UV Venus filter. Oddly, their 2" filter worked fine. 

 

They did fix the problem fairly quickly, IIR.

Yes i remember that. Could never quite understand why the 2" version worked fine ?

I was told by the UK supplier the filter will be tested and sent back to Optolong. 

[John Boudreau]

Yes i remember that. Could never quite understand why the 2" version worked fine ?

I was told by the UK supplier the filter will be tested and sent back to Optolong. 

As  best as I recall, the two sizes used a different substrate and the 2" one had some natural IR blocking. Not sure if they changed the coatings or changed the substrate for the 1.25" version. I think it was the later.

[camman]

As  best as I recall, the two sizes used a different substrate and the 2" one had some natural IR blocking. Not sure if they changed the coatings or changed the substrate for the 1.25" version. I think it was the later.

Interesting seeing these top companies trying to fine tune their filters to work correctly with Venus. And often getting it wrong. As can be seen with Optolong. And previously Baader. Not exactly lightweights in the filter world. 

[banjo1000]

Camman, here is my version of your double Venus image.  I found this to the "east" of the primary image of Venus. I don't have an easy way to share the video from which I grabbed this frame, but that video slews from this double image to the primary image. I'm not convinced that the sensor coverslip is involved. This might be a bounce off the side of a tube.  Interestingly, notice that the upper image is slightly smaller than the lower.

 

Whatever this image is, I don't think it means that your filter is defective. If you see the double image again in the future, it means you are getting close to the primary, but you aren't on top of it yet!  

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[camman]

Camman, here is my version of your double Venus image.  I found this to the "east" of the primary image of Venus. I don't have an easy way to share the video from which I grabbed this frame, but that video slews from this double image to the primary image. I'm not convinced that the sensor coverslip is involved. This might be a bounce off the side of a tube.  Interestingly, notice that the upper image is slightly smaller than the lower.

 

Whatever this image is, I don't think it means that your filter is defective. If you see the double image again in the future, it means you are getting close to the primary, but you aren't on top of it yet!  

Hi what i don't understand is as soon as i try a different filter w47+bg39  and ug1+ bg39 the problem is gone. I don't remember the focus point being that different either.

 

I also have seen this post with someone getting the same problem. As far as i can tell. I have never ever had a double image like this, in 20 years of imaging ? All manner of scopes and filters ? 

 

https://www.cloudyni...-uvenus-filter/

Edited by camman, 24 February 2022 - 09:31 PM.

[banjo1000]

Camman, I figured out how to share the video I made for you showing the relationship between the double image like yours and the main image.  This video will also be of interest to aspiring dark siders. It shows how a mask can be used to suppress double bounce diffracted images off the pixelated sensor.

https://youtu.be/Y2Dr1kBROwM

[KMH]

Camman, I figured out how to share the video I made for you showing the relationship between the double image like yours and the main image.  This video will also be of interest to aspiring dark siders. It shows how a mask can be used to suppress double bounce diffracted images off the pixelated sensor.

https://youtu.be/Y2Dr1kBROwM

Joe,

 

Thanks for this from an aspiring dark sider.  My first attempt last weekend with a 1000 nm longpass did have quite a few ghost images.  I have a cheap-ish guide camera I may try applying a mask to.  Now I also have a 10 nm bandpass filter to try.

 

Kevin

Edited by KMH, 25 February 2022 - 02:09 PM.