Viewing stars and planets at daytime

Vdeoclip of observing Capella and Betelgeuse at daytime with a 25cm Dobson 2010 March 27.

Introduction

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.
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, Aplha 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!).

Requirements

• 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. It seems contrary, but in humid tropical environments like Indonesia, particularly during the rainy season the skies are there between showers or on a clear day better than in the dry season. Of course , the best areas are arid environments 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.
• Telescope should be properly aligned with an error no more than half a degree.
• Eyepiece should be prefocused properly in focus 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 Starmap for the iPhone or iPad, Palm Planetarium are star charting programs which display the altitude of the selected object and refresh automatically. Otherwise you can obtain them from several internet sites.
  I made myself a standalone HTML / javascript application which displays these coordinates in tabular form. It is on this site you can fill in your latitude and longitude and click on 'now' to get coordinates (azimuthal and equatorial) of several stars, planets, sun and moon. It refreshes every 20 seconds. As azimuthal coordinates change rapidly. On modern browsers (Firefox 3.5+, Opera 10+, Chrome, Safari 4+, iPhone/iPad, Android) the page will be stored automatcally on the device so when the site is accessed again, no internet connection is required. So you can use this site with 'live' coordinates near the telescope, even offline.
• 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 meand a 10cm telesope requires at least 35-50x power. Using 20x which is OK at night degrades the 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.
  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.
• 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 telescope which degrades image quality considerably even more than bad seeing.

What do you actually see ?

Setting up

• GOTO mount, the easiest way. When you have a telescope with a Goto mount which usually has a tracking clock drive as well (if aligned properly), you are lucky. Some portable mounts allow two-star alignment which you can align on the Sun (watch out, do not look into the eyepiece !) and the Moon (or Venus, one of them should be available in the sky). Just type in the object name (or coordinates if there is no object database in it) and go.
• Orion Intelliscope: (models XT-6/8/10/12 and XX-12/14/16) with the PushTo handcontroller: setup can be done, using the altitude and azimuth diagnostics mode, as follows:
  Vertical stop adjustment (which is also required for 'normal' night use) should be ok. Refer to the user manual how to do this.
  Have azimuthal coordinates ready, see above how to get them.
  Put a 15-25mm wide field eyepiece in the telescope (a Nagler or an UWA/SWA is best but the supplied Orion Sirius 25mm is OK) and focus properly on a distant (>few hundred meters away) object or better the Moon if available. You can also set a marker by a CD writing pen or a sticker on the focusing barrel to remember it for later use.
  The base of the telescope should be perfectly level as well, as the azimuth and altitude relative to the base should match with true local azimuth and altitude. One or two tapered wooden strips to put under the feet ca do this.
  Get the current azimuthal coordinates of a bright object in the sky (e.g. Moon, Venus or when no object is available even the Sun). Point the scope to that object without using the intelliscope. When it is the Sun, see the yellow box above. .
  Now grab the handset, press and hold enter and press 'on' and release enter. You'll see 'ALT AZ TEST'. Press Enter. Now you will see lots of number in the tightmost column +000.0 and +000.0 above each other. The top number is the altitude and the bottom number the azimuth. Note that azimuth is, unlike what is standard, counterclockwise. Now lower the telescope until it is level. The top right number should be within 0.5 degrees from the given altitude of the Sun with a minus sign, e.g. -023.5. If not, repeat the level alignment or it is too long ago (> 2 minutes) you calculated the coordinates of the Sun. Recalculate. Rotate the scope until the bottom right numbers read the given azimuth of the Sun without minus sign. Now the telescope points to the north point of the horizon. Turn off and on (the latter with enter held down) the handset and you'll see +000.0 and +000.0 again. When you used the Sun for alignment you can remove the shield.
  Now you are ready. Decide which bright planet / star you want to find. Recalculate the horizontal coordinates (with the javascript site mentioneed above) and you can rotate the scope until you see the proper altitude + negated azimuth (e.g. 45° should be -045.0 on the handset's display. Look into the eyepiece and you must find the star.
• DIY setting circles on Dobson. When you make setting circles on a Dobson you can use the abovementioned XT10i method with any Dobson (or other altazimuth mounted scope) without handcontroller. Just a smartphone or other handheld device showing 'live' altazimuth coordinates suffices. I made them by making the circles with a vector graphics drawing program like Inkscape and then printing the drawings. You can also make them with a pen and ruler, as long as the accuracy is at least 0.5 degrees.

   
  This is how I made my setting circles on the XT-10i.

  I made an adjustable aluminum ring with a degree scale on it which can be preset on the azimuth of an alignment object (Sun or Moon). The telescope should be exactly level. Then alignment can be done of presetting the azimuth ring to the correct azimuth of the alignment object.
  Another manual to make setting circles can be found here. See also this Cloudynights discussion.
• Setting circles equatorial mount. Such a device requires that you know the equatorial coordinates of an object. See above for more. The telescope should be pole-aligned properly. This can be done the night before or by aligning them on easily visible objects as the Moon, Venus or when these are not avaiable , the Sun (safety !). When you have a carpenter's level indicator (available for a few dollars in the local hardware shop), it is even possible to use the zenith as an alignment point. For more info see [url=eqmount.php] this page[/url].
  
• Nearby bright object. This can be the Moon or Venus, e.g. during a conjunction at the daytime. When you know the location of the object relative to the Moon, it can be found that way.
• Steady telescope method. This method is useful for binoculars as well and requires no goto mount or setting circles. Find on a clear night an object with the same (+/- ½ °) declination and point the telescope at it. Calculate the difference in right ascensions and add this to the current time. E.g. to find Sirius (RA 6:45:09 and declination 16°43') take the star ι Cap as 'guide star'. The coordinates are RA:21:22:15, Dec: -16° 50'. Then the coordinates of the latter must be subtracted from that of Sirius: 6:45:09 - 21:22:15 (+24:00:00 as the latter is larger) = 9:23 hours. This difference should be multiplied by 23:56 / 24:00 (sidereal day) : each 24 hours the sidereal time is four minutes more ahead of the clock time. So this makes 9/24 * 4 =~ 1.5 minute less which results in about 9:22 hours to add. Note that time (add 9:22 hours to current time), let the telescope stand at that location and then Sirius should be in the field. It is even easier to use a terrestal object (e.g. tree top, roof corner) and find a spot to deploy the telescope where the 'guide star' is just positioned close to that object. Note the location and (sidereal) time where/when Sirius is to be found there.

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).