Optimising contrast when observing the planets

[Giles_B]

Another year, another planetary observing season - I've managed to get out a couple of times this month in the small hours for an hour or so. I've worked out I can manage about an hour between 1am-3am and still work okay the next day if I get an early night and don't have too early a start.

As with the previous two years, I'm off to an enthusiastic start, but I'm plagued by the muddiness and lack of detail in the views I'm getting. DSO views are fine, although they lack contrast when I observe in my Bortle 5 garden. They are better when I get to the countryside. But planets... Last night I felt I could make out Jupiter's banding but not much else. After 45 minutes at the eyepiece I could barely make out the Cassini gap on Saturn, just a faint hint of the gap coming and going at the ring edges.

I'm using a 10" Newtonian (an Orion Optics VX10, mounted on a dobsonian base). I have had very good views of the planets, but only on 1 or two nights a year where the seeing has been exceptional. Mostly the visuals are annoyingly muddy.

I find myself ticking off the list of things that might be contributing to dull views, and what I might do to help make them better. I once suspected that my collimation might be a problem, and I'm now absolutely sure it is not. Laser collimation is bang on and confirmed in star testing. The other culprits may be the generally limited time at the eyepiece, poor seeing and the mirrors themselves.

By limited time at the eyepiece I mean limited both in session length (usually not more than 90 minutes) and limited quantity of sessions (maximum four per month). I try to spend a lot of time physically at the eyepiece, but inevitably pushing the dob means that I occasionally lose the target at high magnification. 

Seeing-wise, there are not many stars in the sky to the naked eye and they generally show atmospheric wobbliness. Bristol isn't very high above sea level so the atmosphere is pretty thick. The planets shimmer about a bit in the eyepiece once I get to around 13mm.

The primary mirror looks in good condition. I cleaned it using the suggested methods (slightly soapy water, gentle wiping with wet cotton wool and rinsing with distilled water) last time I took the scope apart to fit a new focuser. There are a few very light scratches (I fear I could have caused some when I cleaned it) but nothing that leads me to think they may be causing the problem. The mirror seems bright. A label on the mirror side dates it to 11/07/08 - so a good 15 years. I haven't given as thorough attention to the secondary, but it seemed bright when I dismantled the scope.

Is there a way I can improve the visuals I'm getting? As I see it the options are:

1. Invest in some sort of EQ tracking system. For a 10" newtonian an EQ6 will be necessary, which feels like overkill for visual. However, there are some EQ bases for dobs - could this help me to stay at the eyepiece more and fiddle around finding the target less?

2. Recoat the mirrors: While I don't see an indication that I need to coat the mirrors, the coatings are 15 years old and show a bit of use (i.e. the light scratches) - could recoating potentially give a minor boost to the contrast and improve the planetary views?

3. Use an atmospheric dispersion corrector: I only became aware of these in the last 12 months but I'm aware they are used for visual as well as well as AP - could this help given that Jupiter and Saturn are generally low or low-ish in the sky?

I do realise I should take in the usual caveats - that I am expecting too much from the actual conditions. That if I want amazing results I should do AP... etc. so these are not the answers I am looking for - I do enjoy visual and love the time I get observing. But like most amateurs, I just want to push the envelope a little bit further. Do let me know your thoughts.

 

[bosun21]

9 minutes ago, Giles_B said:

Another year, another planetary observing season - I've managed to get out a couple of times this month in the small hours for an hour or so. I've worked out I can manage about an hour between 1am-3am and still work okay the next day if I get an early night and don't have too early a start.

As with the previous two years, I'm off to an enthusiastic start, but I'm plagued by the muddiness and lack of detail in the views I'm getting. DSO views are fine, although they lack contrast when I observe in my Bortle 5 garden. They are better when I get to the countryside. But planets... Last night I felt I could make out Jupiter's banding but not much else. After 45 minutes at the eyepiece I could barely make out the Cassini gap on Saturn, just a faint hint of the gap coming and going at the ring edges.

I'm using a 10" Newtonian (an Orion Optics VX10, mounted on a dobsonian base). I have had very good views of the planets, but only on 1 or two nights a year where the seeing has been exceptional. Mostly the visuals are annoyingly muddy.

I find myself ticking off the list of things that might be contributing to dull views, and what I might do to help make them better. I once suspected that my collimation might be a problem, and I'm now absolutely sure it is not. Laser collimation is bang on and confirmed in star testing. The other culprits may be the generally limited time at the eyepiece, poor seeing and the mirrors themselves.

By limited time at the eyepiece I mean limited both in session length (usually not more than 90 minutes) and limited quantity of sessions (maximum four per month). I try to spend a lot of time physically at the eyepiece, but inevitably pushing the dob means that I occasionally lose the target at high magnification. 

Seeing-wise, there are not many stars in the sky to the naked eye and they generally show atmospheric wobbliness. Bristol isn't very high above sea level so the atmosphere is pretty thick. The planets shimmer about a bit in the eyepiece once I get to around 13mm.

The primary mirror looks in good condition. I cleaned it using the suggested methods (slightly soapy water, gentle wiping with wet cotton wool and rinsing with distilled water) last time I took the scope apart to fit a new focuser. There are a few very light scratches (I fear I could have caused some when I cleaned it) but nothing that leads me to think they may be causing the problem. The mirror seems bright. A label on the mirror side dates it to 11/07/08 - so a good 15 years. I haven't given as thorough attention to the secondary, but it seemed bright when I dismantled the scope.

Is there a way I can improve the visuals I'm getting? As I see it the options are:

1. Invest in some sort of EQ tracking system. For a 10" newtonian an EQ6 will be necessary, which feels like overkill for visual. However, there are some EQ bases for dobs - could this help me to stay at the eyepiece more and fiddle around finding the target less?

2. Recoat the mirrors: While I don't see an indication that I need to coat the mirrors, the coatings are 15 years old and show a bit of use (i.e. the light scratches) - could recoating potentially give a minor boost to the contrast and improve the planetary views?

3. Use an atmospheric dispersion corrector: I only became aware of these in the last 12 months but I'm aware they are used for visual as well as well as AP - could this help given that Jupiter and Saturn are generally low or low-ish in the sky?

I do realise I should take in the usual caveats - that I am expecting too much from the actual conditions. That if I want amazing results I should do AP... etc. so these are not the answers I am looking for - I do enjoy visual and love the time I get observing. But like most amateurs, I just want to push the envelope a little bit further. Do let me know your thoughts.

 

Place the scope somewhere dark and shine a handheld torch at the rear of the primary mirror. If you can see the light cone coming through the mirror then the aluminum coat on the mirror is failing and will require to be recoated . 

[John]

I find that I get the best contrast of planetary features when observing when there is still plenty of light in the sky. As the background sky gets darker, the more subtle contrasts and colour tints get noticeably harder to see.

If you can combine the above with the target planet being as high above the horizon as possible, and stable seeing, the results can be spectacular.

 

[dweller25]

I found my 10” Newt was slightly too bright for best planetary contrast so I bought this 50% transmission Neutral Density filter and it worked a treat…..

 

Another option would be to buy a pair of adjustable polarising filters to fine tune the brightness……

https://www.firstlightoptics.com/moon-neutral-density-filters/astro-essentials-variable-polarising-moon-filter.html
 

 

 

Edited August 7, 2023 by dweller25

[John]

I hate to mention the "C" word but good collimation is important to getting good sharpness and contrast. 

 

[Louis D]

Try a #56 green filter to block unfocused red and blue wavelengths on bright planets to see if it improves overall sharpness.  It sharpens up Venus tremendously for me.

[Giles_B]

2 hours ago, Louis D said:

Try a #56 green filter to block unfocused red and blue wavelengths on bright planets to see if it improves overall sharpness.  It sharpens up Venus tremendously for me.

Yes, blue worked well for me on Venus too (didn't try green) - not sure why I didn't play with filters the last couple of times. I'll make sure I try it next time. I didn't find brightness was a big problem once I'd got to high magnifications.

Has anyone had good experiences using an atmospheric dispersion corrector? The way my Newtonian is now set up I use a 3.5" extension, so I have a *lot* of potential in-focus (which seems to be the major barrier to using an ADC on a newt).

[globular]

I find my ADC very beneficial on the planets - essential even, especially when they are low.

I get much sharper focus and improved contrast.

The bigger your aperture, the higher the magnification and the lower the planets are, the bigger the impact the ADC makes. 

They can be a faff to use when you first try them - but I wouldn't be without mine when I'm doing a long planetary session / sketching study.

 

[John]

One trick that I tried this year with Venus, suggested by @Stu I think, was to observe it using my Lunt solar wedge with the ND and polarising filters removed. The wedge on it's own passes through just 5% of the light but it does so across the whole visual spectrum and the optical quality of the prism within the wedge is excellent. I found that this method delivered glare-free, sharp and contrasty views of the 2nd rock from the Sun with my 100mm and 120mm refractors.

Of course I was VERY careful to replace the filters within the wedge immediately after using it for Venus so that it was ready and fully safe for solar observing again.

 

 

[dweller25]

Also good cooldown is important to getting good sharpness, contrast and stability.

Edited August 8, 2023 by dweller25

[Giles_B]

Hopefully cooldown is not a big problem as the scope is kept in the shed.

I think probably the Herschel wedge will dim Saturn too much. May get away with it on Jupiter, although I find the ND 0.9 a bit dim, so maybe not.

Good to know you get good results from the ADC @globular - and this might be a good route for me to try - although I will need to double check that I do have the focal range needed. I suspect the proof will be in the trying. I presume you use a 2x Barlow in the train with your f10.5 SCT?

[globular]

2 hours ago, Giles_B said:

I presume you use a 2x Barlow in the train with your f10.5 SCT?

My SCT could focus with just about anything in the light train as the mirror can move a long way - but I find the sharpness gained by the ADC is lost again if you move the mirror too far from it's optimal back focus position.

With my ADC I therefore use a T2 prism diagonal (good quality but short light path) with my Pierro Astro ADC (which has a shorter light path than some other ADCs) and this gives exactly the same back focus as my 2" BBHS prism without ADC.

I can therefore switch between with and without ADC by simply changing the diagonal - and everything is kept bang on the optimal back focus for my scope - keeping the views as crisp as conditions allow.

Your newt is different, of course, so you will probably need a barlow (maybe only a very weak one?) to reach focus.

edit: I see you have a 3.5" spacer currently.  Removing / reducing that should be enough to make room for an ADC without needing a barlow.

Edited August 8, 2023 by globular

[Louis D]

5 hours ago, Giles_B said:

Hopefully cooldown is not a big problem as the scope is kept in the shed.

Sheds here in Texas can heat up in the direct sun to 20 or 30 degrees F above outdoor ambient temperature.  That would be extra thermal load to dissipate at night.

[doublevodka]

On 07/08/2023 at 15:06, Giles_B said:

 

Seeing-wise, there are not many stars in the sky to the naked eye and they generally show atmospheric wobbliness. Bristol isn't very high above sea level so the atmosphere is pretty thick. The planets shimmer about a bit in the eyepiece once I get to around 13mm.

I do realise I should take in the usual caveats - that I am expecting too much from the actual conditions. 

 

Personally, I think these couple of sentences tell the story for me.

Seeing was particularly rubbish for me for planetary season last year, just looking up you could see how unstable the atmosphere was and this massively affects things, especially at higher mags.

To give an example about 3 years ago I had a Nextstar 4SE, I saw more with that including Cassini Division than I did with my current Skymax 127 last year so don't be too hard on your kit.

However, if you want to boost contrast in the newt I'd recommend flocking it, it's not particularly difficult or expensive but is worthwhile doing for all types of observing 👍

[PeterStudz]

For what it’s worth here’s my experience with my 8” Dob and planets.

I flocked my whole OTA. The difference is minimal but I’m convinced it makes a slight improvement on contrast. And with planets you are looking for slight contrast differences, especially in colour contrast.

I’m in an urban environment and use a dew shield to try and keep out stray light.

Let your OTA cool down. I find that I need at least an hour before things are at their best.

Get a jet-stream forecast. It might not always tell the whole truth but some of the best views I’ve had are when the jet is well out of the way.

Do NOT get you eyes use to the dark. You see colours and colour contrast better if you aren’t use to the dark. I’ll look at a light (eg my phone screen) or pop back inside for a bit - eg make a cup of coffee.

I have a EQ platform - DIY - made from instructions on this forum. It’s great having a planet at high magnification stay within the FOV for minutes on end. It means you can relax more. When relaxed you can see more.

For some reason I’ve had my best observations when it’s not completely dark. Very often this has been in the early hours as the sun is coming up. I’ve also had good views during the summer months. I’m in an urban environment and one thing that might be going on is that there’s less rising heat as the night goes on. Also houses aren’t heated in the summer. Last year on Jupiter and Saturn was typical. By the autumn I struggled to get good seeing when I was observing in the evening. Often it was obviously at least partly due to rising heat.

[Giles_B]

This is great - I feel like I'm getting a good list of ways to incrementally improve things. Flocking, experiment with an ADM, and I'll look for the EQ platform instructions on the forum. A good clear night tonight so I'll get out later and try Saturn and Jupiter again, with the filters at the ready.

For the first time I've mounted the scope alt-az and this might help to keep the target in view in a bit more of a relaxed way - just have to see if the mount will take it - it should do.

[Giles_B]

Much better sky quality last night, and much better views - I spent two hours (1-3am) outside with 2/3 of this time spent viewing Saturn with a barlowed 9mm eyepiece - i.e 267x - I found that the Baader Contrast Booster gave me the most consistently good views of the Cassini gap and brought out some of Saturn's banding. The improvement for Jupiter was less profound, but still satisfying. Difficult to tell how much of the difference was down to the filter and how much to the conditions.

Setting things up in alt-az was helpful - the ability to tweak the position accurately meant interruptions were shorter and having the scope up high was more comfortable. Downsides were that there was a wobble each time I tweaked the focuser, and the size of the scope made the controls hard to reach (it turned out my little-used extenders wouldn't grip the controls). All things that would prompt me to consider some sort of equatorial mount. I think this would also make an ADM more usable.

Thanks for all the advice. Definitely feels like I've got some options beyond just grinning and bearing it!

[John]

3 hours ago, Giles_B said:

Much better sky quality last night, and much better views - I spent two hours (1-3am) outside with 2/3 of this time spent viewing Saturn with a barlowed 9mm eyepiece - i.e 267x - I found that the Baader Contrast Booster gave me the most consistently good views of the Cassini gap and brought out some of Saturn's banding. The improvement for Jupiter was less profound, but still satisfying. Difficult to tell how much of the difference was down to the filter and how much to the conditions.....

 

Jupiter does not generally respond as well to higher magnifications in the way that Saturn does. I usually find that I need to step down the magnification to get the best contrast on Jupiter.

I'm glad you have seen some improvements though 🙂

[IB20]

Yes agree with @John, Saturn and Mars seem to eat up the magnification but Jupiter not so much and I usually settle on 120-170x for best contrast. I have used a variable polariser on Jupiter which I found really helped tease out the detail in the equatorial belts and zones.

One of my favourite ever Astro observations was Uranus in the 200P, no really surface detail and I haven’t clocked any moons yet but the colouration is quite beautiful. When you think what you’re actually looking at, it’s very moving and profound.

[PeterStudz]

And I agree with @IB20 . I was recently observing Uranus at & after sunrise with my 200p. I certainly haven’t seen any surface detail or moons, but it’s obviously a disc/planet and I can see shading towards the edges. It’s a lovely shade of blue and I was surprised how long after sunrise I could see it. And looking at the blue of the planet against the blue of the sky was quite special. Helped along by the dawn chorus - great stuff!

[John]

One of my best views of Uranus was when it was very close to the Moon a few years back. I could get the limb of the Moon and Uranus in the same field of view at 200x and the colour of the Uranian disk was very striking.

I have seen 2 of Uranus's moons but needed my 12 inch dobsonian at high magnification to get those. Neptune's moon Triton is a little easier and I have seen that a few times with my 130mm refractor, again though high magnifications are required.

 

[Ratlet]

23 hours ago, IB20 said:

Yes agree with @John, Saturn and Mars seem to eat up the magnification but Jupiter not so much and I usually settle on 120-170x for best contrast. I have used a variable polariser on Jupiter which I found really helped tease out the detail in the equatorial belts and zones.

One of my favourite ever Astro observations was Uranus in the 200P, no really surface detail and I haven’t clocked any moons yet but the colouration is quite beautiful. When you think what you’re actually looking at, it’s very moving and profound.

I always call this the 'you can feel the distance in your bones' moment.  They're brilliant.  I had it the first time I saw Saturn back in 2012.

[Mutley]

Thermal problems? Does the VX10 have a mirror fan? I too used to think keeping the scope in a shed was a cure-all for thermal management - but it's not. 

I've had some good results recently by running an AC desk fan aimed at the mirror for an hour or two before use. Have the scope on the floor in the shed with the door propped open for practicality. If nothing else is a good test to find out if your problems are thermal.

[Second Time Around]

Yep, my VX10 has a mirror fan. In fact, the whole series does.

[Mr Spock]

What I use for large apertures is a variable polarising filter. Controlling the brightness of the subject reduces glare which can overwhelm faint features.

For sharpness - collimation. I adjust my 12" before each session and it's bitingly sharp. There is no 'near enough is good enough'; if collimation isn't spot on, views will be fuzzy.