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So I want to image Venus in UV and IR

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#1 gfstallin

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Posted 12 December 2018 - 04:00 AM

I've ignored our sister planet for far too long. When I was 9, after careful observation through my Sears catalog-sourced Tasco 11TR, I determined that Venus was "boring." While my intellectual interest in Venus has always been intact, it is not something I've bothered with visually since Milli Vanilli, Debbie Gibson, and Mötley Crüe were pumping out hits. 

 

Now in 2018, I figure three decades is enough time to revisit my initial assessment. Maybe it had been a rash decision, made when the planet was too low as seen my from my grandmother's front sidewalk, or from our own front yard. Maybe I had reached that assessment because I did not have monochrome and color cameras and a laptop that could potentially tease out details in UV and IR. Actually, come to think of it, that is probably the reason for my initial assessment. 

 

So, my cameras are a ASI224MC and a QHY5L-II-M. Word on the interwebs is I can manage with these two cameras, particularly the QHY5L-II-M in UV and the ASI224MC in IR. I know I'm missing some hardware. Namely I'll need a UV-pass filter. I have my eye on the Astrodon UVenus filter for that. For IR, I have the 850nm IR-pass filter by ZWO. The only filters I've ever used for planetary are UV/IR cut filters, so I'll assume the frame rates I'm used to will not apply. I have an ADC which my colorblind eyes are not really good at adjusting.

 

Will these two suffice? Are there other filters you would suggest? 

 

Another question I have is in the barlow arena. Right now I have an old, trusty Celestron Ultima shorty, and Celestron X-Cel 2x and 3x barlows. I've read that the barlows will actually block UV, so mine might not work. Am I mistaken here? If not, are there barlows that will not block UV? Do I even need a barlow for Venus even though I might be undersampling without one? I'll be using F/10 SCTs. 

 

Does anybody have any tips and pointers, hardware-related or otherwise? Are there best practices you've developed over time that you can share? 

 

When it comes to imaging Venus, in terms of experience, I'm still that 9-year-old with a Tasco 11TR, so any advice you have will be most welcome. I'm all ears. 


Edited by gfstallin, 12 December 2018 - 04:02 AM.

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#2 sg6

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Posted 12 December 2018 - 04:47 AM

What if anything is sat just in front of the camera sensor?

They usually put something there to protect the sensor surface and that could be an IR/UV cut filter. Ideally it would be a simple plain optical window - flat glass.

 

But if they have a IR/UV cut then it will upset any pass filter you add in.

 

Only ask as it is one of the odd things that gets forgotten or not considered.


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#3 happylimpet

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Posted 12 December 2018 - 04:48 AM

Haha I took a similar path to you, also starting with a 4" tasco reflector, so I understand your position! Milli Vanilli, sheeshhh..

 

Yup, the 224mc is great for IR though rubbish for UV, and i suspect the other cam (mono is it?) will be fine for UV.

 

Yup, you may wish to try unbarlowed for UV, though I use a barlow and still get a lot of light through. Venus is bright, of course. Seeing is usually pretty bad in the UV so barlowing may not be critical.

 

Contrast should be very high in the UV so features should be obvious. In the IR, its much more subtle, though you should get a very sharp image at very high signal/noise, so you can put it through registax and tease out the fine details.

 

Just crack on. Get several minutes of high FPS, well exposed data, as you would for any planet. Do Venus in the day if you can - easier with morning apparations as you just sit on it after dawn.


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#4 gfstallin

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Posted 12 December 2018 - 02:54 PM

Haha I took a similar path to you, also starting with a 4" tasco reflector, so I understand your position! Milli Vanilli, sheeshhh..

 

Yup, the 224mc is great for IR though rubbish for UV, and i suspect the other cam (mono is it?) will be fine for UV.

 

Yup, you may wish to try unbarlowed for UV, though I use a barlow and still get a lot of light through. Venus is bright, of course. Seeing is usually pretty bad in the UV so barlowing may not be critical.

 

Contrast should be very high in the UV so features should be obvious. In the IR, its much more subtle, though you should get a very sharp image at very high signal/noise, so you can put it through registax and tease out the fine details.

 

Just crack on. Get several minutes of high FPS, well exposed data, as you would for any planet. Do Venus in the day if you can - easier with morning apparations as you just sit on it after dawn.

Thank you! The information about contrast in UV is good to know. Also, while it makes complete sense intellectually that seeing in UV would be horrible, it could have led to some disappointment in practice; I would have thought I was doing something wrong. My first morning out I'll probably try with and without barlow anyway and see how it goes. Like you mention, if light isn't getting through, then my barlows are blocking it. 

 

Also, it looks like my ad for a UVenus filter is bearing some fruit, so that is one last piece of the puzzle.

 

Filters: Check

Cameras: Check

Managed Expectations for Seeing in UV/IR: Check

Milli Vanilli's Greatest Hits on repeat: Check

 

I'm also super excited for an external focuser my wife ordered for Christmas. With a Rigel Systems DC motor, I think I should be set. 

 

 

George


Edited by gfstallin, 12 December 2018 - 10:57 PM.

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#5 happylimpet

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Posted 12 December 2018 - 07:51 PM

Yeah... an electric focuser is hands down one of the best tools for upping your planetary imaging game. Makes a HUGE difference.

 

Girl, you know its true.


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#6 tjensen

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Posted 13 December 2018 - 03:48 PM

The only other thing to think of is your telescope optics. If they are coated at all, you might lose the UV with them. An alternative to UV is violet... not quite to the UV end of the spectrum, right around 380-450nm.


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#7 gfstallin

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Posted 13 December 2018 - 07:51 PM

The only other thing to think of is your telescope optics. If they are coated at all, you might lose the UV with them. An alternative to UV is violet... not quite to the UV end of the spectrum, right around 380-450nm.

I think I should be okay with a SCT based on images I've seen, though I've read refractors could be an issue. 

 

George



#8 PiotrM

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Posted 14 December 2018 - 02:21 AM

I've read that the barlows will actually block UV, so mine might not work. Am I mistaken here?

May absorb a lot of UV if they have aggresive coatings - and then you would need a fused silica Barlow to avoid it.

I think I should be okay with a SCT based on images I've seen, though I've read refractors could be an issue.

SCT aren't good in UV due to spherochromatism and spherical aberration. They do work, but the image quality will be reduced. Mirror based systems - Newtonian, DK, RC will be superior.

#9 happylimpet

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Posted 17 December 2018 - 10:12 AM

Any luck so far?



#10 rehling

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Posted 17 December 2018 - 01:17 PM

I use the Astrodon UVenus with an ASI1600mm (earlier ASI120) and they easily pull the detail in Venus' clouds. But, some key points, from my experience:

 

1) Yes, a Barlow and UV lead to vastly greater exposure times than is usual for Venus. The Barlow increases the exposure time by about a factor of 7 for me, because the linear increase in size is squared for an increase in exposure times and experimentally, my Barlow only passes about 60% of the UV. That said, it's not bad compared to imaging, say, Uranus in visible colors. BUT

2) Time is often of the essence, because when Venus is high above the horizon, you're imaging in daylight, and I have never seen useful details from imaging Venus in daylight. The day (or twilight) sky is brighter in UV than in the visible colors. And, if you have sunlight hitting your telescope, that'll quickly ruin your seeing due to tube currents.

3) I have had a far easier time imaging before sunrise than after sunset, because the ground is not re-radiating the day's heat. In 2017, I had a routine of imaging Venus right before sunrise (even going a couple of minutes after sunrise) and it was one of the best, spiritual experiences I've had with astronomy. It forces everything into a tight timeline, though, which can create errors, sessions that don't yield usable imagery, etc.

 

4) Near IR (~650nm) is helpful for imaging Venus or Mercury in the day sky, because the day sky is pretty dark in IR and poor seeing impacts the longer wavelengths less. However, if the Sun is hitting the telescope, it'll quickly heat up. Strategies to prevent that include: a) Finding a location where the telescope is in the shade but Venus is nonetheless within view; b) Covering the telescope in something reflective (silver or white); c) Keeping the telescope absolutely out of sunlight before the session begins; d) Hurrying! If you get your picture in the first 2 minutes after you uncover the scope, that's much better than if you wait 10.

5) If you need to find Venus in the daytime, there's a whole set of strategies around that. I've found that very minor shifts in transparency can make the difference between Venus being very easy to find (I have even found it naked-eye and aimed my telescope at it in the daytime exactly the way I would at night) and almost impossible to find (white on white). The problem increases enormously the closer Venus is to the Sun. I have imaged Mercury when it was only 5° from the Sun, but that requires great sky conditions.

6) I've had poor luck imaging cloud top detail in IR, but it is certainly possible.

 

The great thing about Venus is that it is, theoretically, visible almost all the time! Living in a place with severe light pollution and many foggy nights but few cloudy days, I've at times considered making Venus one of my top targets as it neutralizes the difficult factors in my viewing environment. And many of the same factors related to Venus also apply to Mercury, and it's definitely fun to have a daytime session where you image three or four objects while minimizing your exposure to the cold and loss of sleep.


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