Observing Venus with filters.

[Guest]

Friday night I was observing Venus. Always dazzles with it's brilliance. I could see that it was like an upside down half moon with regards to it's phase. Interestingly, at one point some clouds drifted across the field of view. At this point I could see a sharply defined half phase. Although a bit dimmer. I guess that the cloud had acted to filter the dazzling light. It has got me thinking about how I could recreate this effect with filter accessories. Perhaps an adjustable polarising filter? What filters do others use for Venus. Has anyone else experienced this effect with clouds and Venus? 

[John]

Filters and Venus are covered in this recent thread. Might be worth a read:

Sometimes thin cloud provides an excellent natural filtration service  

[Owmuchonomy]

John's reference to "sometimes" is more like "frequently" for me! 🤣 In all seriousness, attenuation of bright objects due to thin cloud cover can greatly enhance the view.  We had the same when the local Probus group came up to LTO recently and Venus was greatly enhanced by the thin veil of cloud.  Same applies to ND filters and the Moon.  Haven't tried one on Venus yet. 

[Knighty2112]

I’ve found my GSO 15A yellow filter works very well when viewing Venus. Also, when slight cloud cover goes in front it improves the views too. Tried quite a few filters on Venus, but 15A seems to work the best, at least for my eyes.

[mikeDnight]

A variable polarizing filter would certainly help when it comes to observing Venus.  And as regards thin cloud, I find that it often helps me in positioning the subtle cloud top detail. Last night was clear for a while, but clouds did eventually encroach, but as they did, there were moments of excellent definition as the clouds dimmed the view. So although the sketch claims no filter was used, there was briefly a natural light cloud filter, as there often is during my observations of Venus.

[Pete Presland]

Excellent sketch and report Mike.

[Rusted]

As we are on the subject:  I could see "a" in your drawing, Mike.

I went back over to the observatory to tidy up after a day of solar imaging between cloud.

Venus was still above the trees to the west. So I took off the H-a filter stack and fitted an eyepiece.

I was using my iStar 6" f/10, 1500mm f/l, H-alpha objective in a home made OTA.
Mounted on my massive, home made Goto GEM, in my home made, raised dome.

With the D-ERF, full aperture filter in place, Venus was razor sharp but deep, blood red.
A hungry and bloody, half moon! So I added a TS star diagonal and 2x Shorty Barlow.

More viewing comfort and eye relief as I worked my way up though my collection of Meade 4000s.
I reached the 9.7mm and Venus was still holding sharp at ~300x.
None of the usual glow and glare typical of white light viewing.

I was [almost] sure I could see soft surface markings and a wavy terminator.
BTW: I still had the 2" Beloptik KG3 UV/IR and Baader 35nm H-a filters in place too.
You can never have too many filters.

Edited March 19, 2020 by Rusted

[mikeDnight]

On 19/03/2020 at 20:45, Rusted said:

As we are on the subject:  I could see "a" in your drawing, Mike.

I went back over to the observatory to tidy up after a day of solar imaging between cloud.

Venus was still above the trees to the west. So I took off the H-a filter stack and fitted an eyepiece.

I was using my iStar 6" f/10, 1500mm f/l, H-alpha objective in a home made OTA.
Mounted on my massive, home made Goto GEM, in my home made, raised dome.

With the D-ERF, full aperture filter in place, Venus was razor sharp but deep, blood red.
A hungry and bloody, half moon! So I added a TS star diagonal and 2x Shorty Barlow.

More viewing comfort and eye relief as I worked my way up though my collection of Meade 4000s.
I reached the 9.7mm and Venus was still holding sharp at ~300x.
None of the usual glow and glare typical of white light viewing.

I was [almost] sure I could see soft surface markings and a wavy terminator.
BTW: I still had the 2" Beloptik KG3 UV/IR and Baader 35nm H-a filters in place too.
You can never have too many filters.

That's great! Not easy by any means but becomes more obvious with careful observation. 

I've noticed that as light cloud crosses the field of view, I often get a slightly improved view of the cloud detail. I know the late Peter Grego was an advocate of the variable polarizer for viewing cloud top detail, so I was just googling variable polarizer's and came across this. It seems a VPF can help in revealing subtle detail, but what caught my attention was the attached image, because this image is pretty much how I see the cloud top's without a filter, only not quite as dark, but I see the mottling and bright regions without much trouble usually.

There was something else that I learned last night while watching a program about light. UV light gets dispersed by earth's atmosphere as the planet nears the horizon, and I hardly ever view Venus below 30° due to a hill and high tree line to my south west and west. So, in theory the UV spectrum won't be as badly affected as it would be when the planet is closer to the horizon. And generally the sky background is brighter when the planet is higher, so Venus isn't as glaring, which in theory may be at least part of the reason I see cloud detail relatively easily, similar to the attached pic, but without a filter.