Viewing stars and planets at daytime
Normally we can only see stars at night. The reason is obvious: with daylight the sky background is too bright to see the stars, the relatively faint points are washed out by the scattered sunlight. For us, the Sun appears 550000 times as bright as the next bright object: the full moon and even 1e10 (ten billion) times as bright as Sirius the brightest star in the sky. Even stories such as 'from deep pits or mine shafts it is easier to find stars on the smaller portion of the sky' are incorrect: one is 'blinded' even by the small portion of sky one sees just like one sees normally a large part of the sky.
But there is yet a possibility to see the stars at daylight. It has no scientific value but it is just a challenge until how faint one with a certain telescope (or even with the naked eye!) one can see astronomical objects in daylight. It can be fun when seeing Sirius or Betelgeuse on a hot North American or European summer day with the telescope ! It is even possible to see double stars at daylight (e.g. Mizar, Cor Caroli, Alpha Centauri). For viewing first magnitude stars a small telescope and a power op 20x is required. It is also fun when showing it to friends, etc. My experience is that people are very amazed seeing a star or planet in bright daylight.
Alignment of the telescope should be done with objects currently visible to the naked eye. In nighttime this is no problem but in daytime there are only a very few naked eye objects available (in many cases only the Sun). In the latter case the telescope should be pointed at the Sun for alignment. This can be done safely when the requirements in the yellow box below are met.
EYE SAFETY WHEN POINTING AT THE SUN
Note: Watching astronomical events in which the Sun is involved or objects close to the Sun requires special safety !
NEVER look through an optical device like a telescope or binoculars at the Sun or your eyes will be damaged resulting in BLINDNESS or at least you get an arc eye!!
When pointing the telescope at the Sun, e.g. for alignment reasons, use a safe objective filter, e.g. Mylar sheet or, better a glass filter e.g. Thousand Oaks or otherwise look from a distance for a bright spot of a few mm across centered in the eyepiece which tells the Sun is near the center of the field. Only close to sunset and sunrise and a very hazy sky the Sun can safely be viewed unfiltered.
When no filter available you can align by watching that the bright spot is centered in the eyepiece without looking through the eyepiece !!!
See the picture above left. In this case, to prevent overheating eyepieces or star diagonals when pointing to an unprotected Sun during alignment, mask off the objective lens with a cardboard shield with a hole of at most 5cm in it. The picture above center shows a cardboard blind of 4cm on the Televue Genesis. The black is a piece of duct tape for removing the filter easily. A full aperture of a 10cm pointed at the Sun will dissipate 8 Watts of heat in the eyepiece and a 20cm 30 watts ! It is the same as pointing a burning glass of the same diameter to the eyepiece which you would never do !
With Newtonians or Dobsonians
, make the hole eccentrical.otherwise the hole is in front of the secondary mirror and no light at all passes into the telescope, see image above right. When putting the cardboard shield on front of a Newtonian tube, be careful no dust or small pieces fall into the tube and do not exert force to the spider legs of the secondary mirror. Take care that the shield does not fall off when you point the scope at the Sun.
When you put the telescope in the shade of a well-foliated tree, the sunlight is even more attenuated (but looking to the eyepiece is still unsafe!).
It is important that these conditions are met:
- Sky shoud be sufficiently clear, about the background color of this page. Hazy or milky skies with lots of dust attracting moisture or much cirrus cloud covered skies are useless unless one wants to look up Venus or the Moon. Dry and high altitude locations are the best areas like Utah, Atacama desert, Namibia, and the very best is the for the most of us inaccessible Dome C in Antarctica. But your home location can be suitable at times when the sky looks like the background color of this page. It seems contrary, but even in humid tropical environments like Indonesia, particularly during the rainy season between showers or on a clear day the skies are usually deep blue, better than in the dry season. Clear nights also show many stars in dark areas in such locations.
- Telescope should be properly aligned with an error no more than half a degree.
- Eyepiece should be prefocused properly in focus for infinity for a relaxed eye. Do this by pointing the telescope at a remote object (more than several hunderds of meters away, or better, a bright object such as the Moon or Venus, or possibly an overflying airliner). Otherwise you'll be peering into the blue void without seeing anything ! You can put a marker on the focusing barrel.
- Unless you have a goto mount you must know the coordinates of the object(s) to be found. Currently most smartphones or laptops can run programs which show these coordinates. Smartphone applications such as Skysafari for iOS and Android, Starmap for the iPhone or iPad, Palm Planetarium for the older PalmOS (for which is emulation on Android, iOS and Symbian) are star charting programs which display the altitude of the selected object and refresh automatically.
For example, below examples of Skysafari (Mac OSX, iOS and Android, $3) which shows the altazimuth coordinates of the object centered on and are refreshed constantly. Stellarium is a freeware app for Mac OSX, Linux and Windows. Although designed for 'normal' night use these apps are an excellent helper for finding objects in the daytime.
It is easy with apps such as Skysafari and Stellarium: search for the object with the app's search function and center the object and on top left of the screen the altazimuth coordinates are shown. See the images below (click to enlarge).
How to find Aldebaran with Skysafari
How to find Aldebaran with Stellarium (ocular plugin)
- Otherwise you can obtain other sky charting or ephemeris apps them from several internet sites.
- As the eye pupil dilates to only 2-3mm in the daytime, the minimum power required to use the full aperture of the telescope is aperture / 2 till 3 mm which means a 10cm telesope requires at least 35-50x power. Using 20x which is OK at night but at daytime it 'degrades' a 10cm telescope to a small 5cm one and stars are only 25% of brightness which means 1.5 magnitude loss compared to 50 or more times power. A 20cm telesope requires at least 80x for the same reason otherwise its performance is degraded. So the rule is (also applies during 'normal' night operation) aperture is the smallest value of real aperture and power times eye pupil dilation. The latter is 5-7mm at night and 2-3mm in the daytime. This also means that 15x70 binoculars perform actually the same as 15x35 in the daytime and 10x50 as 10x25, assuming the same optcal quality. This makes binoculars unsuitable for stars at daylight unless you look for Jupiter, or, close to sunrise/set, a 1st magnitude star. When using a Newtonian or Dobson telescope, put the telescope in the shadow as when setup in sunlight, turbulence in the telescope tube will cause much turbulence degrading image quality considerably. Refractors don't have this problem or at most that it does not deteriorate image quality.
The best choice is an ultrawide eyepiece (like for deep-sky observing) such as a 15-25mm eyepiece with an angle of 82° such as the Naglers. Otherwise find first brighter objects (> +0.5 mag) with a low power one with an actual FOV of at least 2 degrees, which possibly degrades the aperture. For finding fainter objects or objects close to the Sun you can better use an eyepice with a higher power (but a smaller FOV) which uses the best of th aperture. E.g looking up Arcturus with a 25mm with an apparent FOV of 50° has an actual FOV of 2°, with a 10cm f/5 scope shows the star rather dim and then barlowing the eyepiece considerably brightens the star image, as in the latter case you make full use of the aperture and in the first case the aperture is smaller due to the too large exit pupil. Limiting magnitude is for the naked eye in full daylight about -4 and for a 10cm telescope with at least 40x power about +1 for 'standalone' objects and about +2...+3 for objects nearby another (brighter) object because it is easier to find. This is valid for high sun (> 45º altitude). For low Sun (e.g. 10º) the limiting magnitudes are about 1-2 magnitude fainter. Every 1.6x larger diameter (and magnification) increases the limiting magnitude by 1.
- Seeing issues: When the seeing is very poor, this will degrade visibility of stars in the daytime as the twinkling of the star image is blurred out which is particularly the case when using higher powers (40x or more) and on hot days. In the daytime the seeing is poorer than at night but usually this is not an issue when using powers up till 50x.
Putting the telescope in the shade makes observing of daylight objects more comfortable, particularly on a hot summer day. Moreover there will be less turbulence in the (mostly Newtonian) telescope which degrades image quality considerably even more than bad seeing.
What do you actually see ?
Depends on the object.
Venus looks very bright and when close to inferior conjunction, it has a nice thin crescent. Mercury can be seen at best close (but not too close : min 7º from the Sun !) to superior conjunction then it is at its brightest.
Jupiter appears as a faint fuzzy ball but when further from the Sun it is clearer and larger telescopes might even reveal its fout larger moons. With some effort it can be seen as close as 10º from the Sun. Saturn has a lower surface brightness so it required more effort to be seen in the daylight.
The brightest stars (mag 0 or brighter) appear rather easily when not too close (< 30º) from the Sun. Stars with an orangish color such as Betelgeuse or Arcturus are a bit in favor due to the color contrasting to the blue sky.
Second magnitude stars require some more effort. Sometimes one sees the star to be found and it turns out it is easier to be seen than expected after peering into the blue void.
It is just the challenge 'how far can I go with my telescope'. The image below gives an impression of what you can actually see. The term 'naked eye' means without optical device other than ordinary eyeglasses or contact lenses.
Calculating positions of bright objects
This can help you to get the proper coordinates of objects for aligning the mount.
Show / hide position calculation
There are more methods to view in daytime. These are:
|Object||Used optical device||What do you see|
|Moon||Naked eye||Easy to find if more than 2-3 days from New Moon|
|Venus||Naked eye||In most cases particularly when more than 30° from the Sun easily to be found|
|Jupiter||Naked eye||With very clear sky, shortly (< 2-3 months) before opposition after sunrise or after opposition before sunset when not too low in the sky one might find Jupiter after looking it up with binoculars. |
|Venus||Binoculars||Easy to be found, even as close as 7 ° from the Sun under very clear conditions and at least 50mm diameter (CAUTION DO NOT POINT AT THE SUN !!) such as close to an inferior conjunction.|
Click on one of the links below to see what a 10 cm telescope will show in a clear daylight sky about 90 degrees or more from the Sun at a power of 30x (unless stated otherwise).
Note: Planetary objects may appear larger in the telescope because the apparent field diameter of the eyepiece's is 50-60°, much larger than the angular diameter of the blue circle on this computer screen from reading distance. Stellar images may appear brighter if you are using a 20cm telescope or larger and a power of more than 50x.
My own experiences
Here some exceptional observations of daytime objects which I did in the past 30 years. From Holland unless stated otherwise.
|Mercury and Achernar at noon||2013 Feb 15 Southbroom KZN South Africa||10cm Genesis||Was at noon with Sun 73º above the north. Achernar and Mercury (-0.4 elong 18º E) were easily found.|
|Saturn in the late morning sky and some stars.||2013 Feb 3 Nice location near Kruger Park, South Africa||10cm Genesis||Morning Sun about 30º above horizon. Stars : Spica, Antares near zenith, Alpha Centauri and Alpha Crucis both as double stars. Saturn appeared clearer than with 25cm Dob at home in daylight|
|Jupiter moons Io, Europe and Ganymede|
and Pleiades stars Alcyone, Maya, Atlas and Pleione
|2012 Sep 1||25cm Dobson, 93x and 230x||Morning Sun 12º above horizon very clear weather|
|ε Lyrae||2012 Aug 12||25cm Dobson||Half an hour before sunset easily separated @ 93x|
|Ganymede||2012 Aug 11||25cm Dobson||Easily to be seen next to Jupiter @ 93x|
|Procyon||2012 Jul 22||10cm Genesis||Midday 10:30 UTC only 17º from the Sun but very transparent sky|
|2012 Jun 10||10cm Genesis||Midday 4.5 day after inferior conjunction only 5° separated. Filmed with Canon Powershot S100 behind eyepiece and stacked.|
|2012 May 26||10cm Genesis||Only 9.5° from the Sun as a very dim ball ! Filmed with Canon Powershot S100 behind eyepiece and stacked.|
|Four planets||2011 May 25 morning||10cm Genesis + Nagler 13T6||Alt Sun 40°, sky very blue, Mars (+1.4) close to Venus barely visible, Mercury and Jupiter easily visible|
|Several stars||2011 May 25 afternoon||10cm Genesis + Nagler 13T6||Alt Sun 40°, sky very blue, Several 1st mag stars, and all seven bright stars of the Big Dipper, including the +3.3 Megrez, the latter with Nagler 13mm + Barlow, With the barlowed 13mm I saw even Cor Caroli B (+5.5) !|
|Cor Caroli (+2.9) with companion (+5.6)||2010 jun 25||25cm Dobson XT-10||Alt Sun +17°, sky little hazy, companion barely visible|
|Venus / Jupiter conjunction and Antares||2008 feb 2 (Red Sea Egypt)||10cm Televue Genesis||Alt Sun 15°|
|Mercury||2007 Jan 27 ||10cm Televue Genesis, close to the Sun||(Sharm el Sheikh Egypt), around midday alt Sun 30°|
|Sirius||1986 Mar 23||NAKED EYE||After looking up with a 80mm telescope I looked along the telescope and saw a very dim star. It was 15 hrs local time (Canberra, AU)|
|Alpha Centauri||1994 Feb||NAKED EYE||Very clear, just at sunrise (Fiji south coast).|
|Arcturus||1982 May||NAKED EYE||Very clear, just at sunset.|
|Jupiter||2001 Nov||Naked eye||Early morning after sunrise, near Perpignan, France|
|Venus||2007 Aug 18||10cm telescope||On the day of inferior conjunction !!! a very narrow crescent !!|
|Venus||1999 Aug 21||5cm binoculars !!||4 days after inferior conjunction, separation only 9° !!! |
|Several stars up till mag 4.0||Feb 1997||10 cm Genesis||During a stay in the Karoo desert in South Africa|
|Here an overview of objects I saw ordered by optical equipment|
|Naked eye||Venus (often), Jupiter, Sirius, Canopus, Alpha Centauri, Arcturus||The latter three only close to sunset / sunrise, but Sirius once in full daylight.|
|Binoculars||Venus, Jupiter, once Venus at inferior conjunction|
|10cm Televue Genesis||Mercury, Venus, Jupiter, Saturn, all 1st magnitude stars except Fomalhaut, Polaris (mv=2.0), Mizar (ζ UMa) Beta Cygni (including mag 5 component !)||Alpha Centauri was easily visible as a double star in 1995 but in 2012 I needed much higher power (83x) to separate it with the Genesis, due to its orbit and reaching periastron in 2015.|