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Daytime observations of Venus, Mercury and Sirius

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#1 project nightflight

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Posted 02 August 2020 - 08:45 AM

We recently acquired a Suunto Tandem. We wanted a simple handheld tool that allows reliable measurements of altitude and azimuth. For the Suunto Tandem, the manufacturer claims a precision of 1/4 degree in altitude and 1/3 degree in azimuth. We tested the device on the night sky and can confirm that these claims are correct.

 

Suunto Tandem (Precision Inclinometer/Compass)

Combined inclinometer/compass
(click to enlarge)

 

There are a lot of astronomical applications for a device like this. We find it especially useful for locating objects in the daytime sky. To do so, we look up the current horizontal coordinates of e.g. Venus and use the Suunto Tandem to look for the correct spot in the sky. The altitude value from Skysafari can be used without any correction. For the (magnetic) compass bearing it is necessary to subtract the magnetic declination from the (true) azimuth that Skysafari gives. The procedure is straightforward and makes it very easy to find Venus in the daytime sky or e.g. a comet like NEOWISE in the still bright evening twilight.

 

In the morning of August 1, 2020, we used the Suunto Tandem to point a small 2.6“ Apo on an alt-az mount at several celestial objects. The best way to do this is by holding the Suunto Tandem against the sides of the zenith prism, using the two contact edges of the device - first to adjust the azimuth, then the altitude. This makes it pretty easy to point the refractor at the desired horizontal coordinates. All objects we visited with this method were nicely centered in a low-power field of view.

 

During our observations the sun was already 25 degrees above the horizon. To be safe from accidentally pointing the scope at the sun, we had set up in the shadow of a small hill on our property. For locating the objects we used a 24mm Panoptic, for detailed observations we switched to a Hyperion zoom, used without and with a 3x Barlow. Here is what we saw:

 

Venus was a bright silvery crescent against a deep blue sky. With all magnifications from 17x to 50x it was very easy to observe, a true showpiece.

 

Mercury was a bit harder to locate at -0.8mag. But once we had detected the planet in the eyepiece, we could boost the magnification up to 150x. With a diameter of only 6“ it showed not only its gibbous phase but surprisingly also a dark marking on the southern hemisphere. After checking with albedo maps we suspect that we saw „Solitudo Iovis“, a prominent dark feature. Amazing.

 

Sirius was the real heartstopper. It was at an altitude of about 23 degrees hovering above a treeline on the horizon. With the 24mm Panoptic at 17x the bright star was standing there, sparkling in a light blue sky above the intensely green treetops. This was one of the most eerie astronomical sights we ever had.

 

 


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#2 Simcal

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Posted 02 August 2020 - 09:24 AM

I remember as a kid going to a science day at UofToronto, and there being a beautiful Questar set up on the science building rooftop.. getting a turn to see Jupiter in the daylight.  It was so long ago now that it might have been a mis-remembrance, but my memory of it, was that it was COOL!


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#3 clearwaterdave

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Posted 02 August 2020 - 10:02 AM

I found having your scope already focused on infinity was key to finding things in the daylight.,It doesn't take much "out of focus" to make things invisible in the blue skies.,

   I was able to locate things using the dec setting on an eq mount.,Very cool beans,.observing Venus in the day time.,Cheers.,


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#4 starblue

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Posted 02 August 2020 - 07:48 PM

I've spotted, naked-eye, Sirius a few minutes before sunset and Arcturus at sunset but it requires very clear air; both were near the meridian at the time. Daytime Venus is easy as long as you can localize your scanning in some way--say, a nearby Moon. I even located Venus mid-afternoon with binoculars only 4 days before the last Venus transit and watched it set that evening naked-eye.

 

One day in late May in unusually clear air I spotted Sirius naked-eye 2 degrees above the horizon and watched it set; this was ~45 minutes after sunset when twilight skies were still rather bright. You always hear about Sirius's "heliacal rising" where it predicted the flooding of the river Nile in ancient Egypt--this got me wondering about its heliacal setting--was it possible to see it? With the sun setting later and Sirius setting earlier, the sky was getting brighter, and subsequent days were hazier at the horizon. Even so, binoculars allowed me to follow its setting every day. Unfortunately, clouds returned a couple days before the day both set together, and I missed it, but based on my success up to that point, viewing Sirius's helical setting looks doable, maybe not naked-eye but with binoculars for sure. 

 

 

 


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#5 project nightflight

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Posted 03 August 2020 - 03:21 PM

I found having your scope already focused on infinity was key to finding things in the daylight.,It doesn't take much "out of focus" to make things invisible in the blue skies.

Thanks for pointing this out! Good focus is definitely needed for telescopic daytime observations. The small Apo we used for the observations mentioned above has an engraved index scale on the focuser. That came handy to set the focus position.


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#6 project nightflight

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Posted 03 August 2020 - 03:28 PM

I've spotted, naked-eye, Sirius a few minutes before sunset and Arcturus at sunset but it requires very clear air; both were near the meridian at the time. 

Wow, great observation, real inspiration! We will try to see this.


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#7 davidmcgo

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Posted 04 August 2020 - 04:01 PM

So I just got a Suunto Tandem figuring I would like to try this myself as well as help with pre aligning my mounts when I set up around sunset and it doesn’t have any declination adjustment.  
 

Not sure if I want to find an older one that has it or just do the subtraction in my head every time.  I’m wondering if Suunto got rid of that feature because it affected calibration or just due to cost.

 

Dave


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#8 Shopaholic

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Posted 05 August 2020 - 12:25 PM

I have been using the apps in iPhone , compass and "Clinometer" with 0.1 degree accuracy to spot Polaris for my PA and also spot my targets at night with my Star adventurer for imaging.


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#9 halx

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Posted 05 August 2020 - 01:15 PM

Compass is often nearly useless near the metal of a typical telescope. So there is no apparent difference if your compass is uber-expensive as that Suunto ($200-300), or just a smartphone compass (the latter at least capable of subtracting the magnetic declination for you automatically and shows the Azimuth on the big screen not on tiny round dials, even with the Suunto loupes).
 
All you actually need is a trivial $20 digital inclinometer (eg https://www.amazon.c.../dp/B078JNS7V6/), which is better than Suunto (with its 0.1 deg accuracy after calibration, large screen, magnetic base, and 100% reliability).
 
First, put the inclinometer on the OTA using it's magnetic base so it's not rocking. Point the scope into the horizon and zero the inclinometer (yes that might be tricky if you have no horizon anywhere in the view, but there are methods to help with that, however that Suunto cannot be zeroid to the optical axis at all). There are a bit better inclinometers with the bubble level which can help if you have no way to figure your true horizon. Also see the "Side Note" below.

Then, point the scope in the approximate Azimuth direction using the same star chart app (ideally if it has the "compass" mode, showing the chart aligned with the sky when you hold it with the screen facing downwards as it takes care of the magnetic declination and shows the target object automatically for you), or an ordinary $1 compass, or just measure from the known Sun position (Azimuth setting circles are helpful with that, or you can use the compass again, keeping it in the fixed position on the telescope to avoid magnetic field changes, or determine compass error from the known Sun Azimuth, there are many other ways for that as well, and the result will be much more reliable than relying solely on the high price of that Suunto delivering you something grin.gif).

Finally, set the absolute altitude of an object from the star chart on the inclinometer.

Now you can be sure that slowly scanning the field of view by rotating the scope only left and right you will catch your daytime (or night) target within a minute.

My point is: you don't need that expensive Suunto to treat yourself with daytime celestial views sometimes.

On a side note:
There is an Android app which has a dedicated support for that exact inclinometer pointing method (it will work with that Suunto too). It is resolving the issue with the optical axis alignment with the true horizon and meridian, and also helps with the settings circles. All you need is a single real object in the sky (the Sun in our case, or the Moon, or the single star/planet you can recognize in the dusk). Just point the telescope to it and enter its numbers from your uncalibrated inclinometer, unreliable but fixed ucompass, or your fixed settings circles, and it will show you what numbers you need to dial on them in order to point to any other target (it's simply calculating all the offsets involved and applying them to real objects coordinates which it shows as Altitude and Azimuth on the chart to match what you actually see on your gauges).


Edited by halx, 05 August 2020 - 07:37 PM.


#10 halx

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Posted 05 August 2020 - 01:22 PM

Just by the way, I would recommend this particular inclinometer: https://www.amazon.c...r/dp/B082LTG163
It's advantages include the inverse LCD screen, which is more darkness adaptation friendly (surely, after applying the dark red film or painting the screen light LED).

And the bubble level helping with the alignment and its verification. Also, it has a grove in the magnetic base for good stability on the round OTA. My tests shows that it's good enough to judge an angle even for 0.05 degrees (but it must be thermal stabilized to reproduce that reliably).
 


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#11 project nightflight

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Posted 05 August 2020 - 02:37 PM

I have been using the apps in iPhone , compass and "Clinometer" with 0.1 degree accuracy to spot Polaris for my PA and also spot my targets at night with my Star adventurer for imaging.

Nice!

For sure the digital version works well, too.



#12 JMacd62

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Posted 07 August 2020 - 11:26 AM

Not something I have done for many years. actually decades.

I can’t recall seeing Sirius before sunset, certainly shortly after, Canopus, Vega and a few other bright ones as well,
I used to find planet’s before sunset or after sunrise.  After sunrise was easier having been able to keep them in view.

 

on occasions I used to find a plannet early or mid morning, Venus several times Jupiter and Mars can’t recall looking for Saturn.

it was. Something I did for amusement. Because I could. 
Plannet only reflects light so to find in daylight they have to be on the opposite side of the sky to the sun.

I used to do it by pre calculating tha azz and altitude of the planets or stars I was interested in.

I would occasionally take an observation of a plannet in the morning to cross with an observation of the sun and get a simultaneous observation,

lit was mor for an academic exercise, curiosity or fun  rather than practical, the cross generally poor due to being close to 180 apart 

Sirius could be found in daylight I don’t recall trying it. More than once or twice.

 

i routinely used stars and occasionally planets  at Twighlight, the moon had to many annoying correctness to apply so I only ever used it for academic purpose.

 

I would pre calculate the altitude and aszzimuth of the one I chose to use

usually cheating with the air tables.

 

i would draw a 360 rough diagram like a pie chart show the azzimuth and altitude for the local time of the beginning or end of civil twighlight depending on morning or evening observation. 

Id start looking for the brighter Stars soon after sunset starting in the east where sun would loose the horizon first  then taking the west last

preset the altitude on sextant and use compass for azzimuth even with the releativly low power 4 x scope. I would find them  

Most night when I could see something  I would take azzimuths to check the compass.Using an azimuth ring. With a simple prisim. Occasionally I would do this in daylight with a Planet 

 

I haven’t tried this in decades. Should not to hard with a telescope. Something interesting to try.



#13 halx

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Posted 07 August 2020 - 11:47 AM

Curiosity only it is!
When the Sun is transitioning to/from the night, there is plenty of unwanted turbulence in the air cooling or warming up as it moves across the land. In the broad light, the bright skyglow as well will be ruining the contrast of planets, not to mention the telescope itself cooling or warming up with the ambient air temp changing.

For curiosity, I've been once trying to see stars from the ~60 feet deep dried-out water well in Kazakhstan's "prairies". But even though Deneb supposed to be near the Zenith at the time, the field of view was probably too narrow to fit it (like just 2 fingers wide).




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