Last updated on
4/11/
2006

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Solar Observing

The sun is a magnificent view in a good scope, and for an overview of the features of the sun visible through telescopes see Anthony Seal's Solar Web pageBut if you donít use a solar filter you will likely ruin the scope and definitely cause yourself permanent blindness - all reputable discussions on telescopes include this warning and theyíre not kidding.

Solar Filter A new film developed by the Baader Planetarium in Germany, was introduced a couple of years ago. Reviews of the Baader film have been extremely positive - the sunís color remains a true white with this film and image sharpness appears to be better than for others. It is available in the US market from Celestron (part #94162 for an 8Ē SCT, about $115) . Celestronís filter (shown here on the right) is made to attach to C8ís like the U2K and it attaches snugly by fitting into the front cell and locking with a slight turn, just like the dust cover does. I like this feature because it assures that the filter cannot be accidentally bumped off while you are looking at the sun through an eyepiece.

If you donít have a Celestron, Kendrick Astro Instruments sells a Baader filter to fit SCTs.  But note that you have another option from Kendrick. If you delve into astrophotography you may wish to get the Kendrick Kwik Focus, and Kendrick makes a Baader filter (Kendrick #2057) that fits into one of the three cut-outs of the Kwik Focus.  Itís an option worth considering. (The Kendrick solar filters and the Kwik Focus attach firmly to an SCT with a nylon locking screw.)

There are also two types of older solar filters that have traditionally been sold - one with a flexible mylar film and one that uses a metallic film on glass. The mylar filters are less fragile than the glass ones but they make the sunís image blue in your eyepiece.  So if you can avoid being clumsy the glass filters are better than the mylar - they display the sun as orange which is closer to reality and more pleasing. Thousand Oaks makes the solar filters that have been the most highly regarded in years past, and I was happy with mine. But itís about $120 and I ultimately replaced it with the Celestron Baader filter for the true white color.

Note that on the U2K the counterweight bar on the bottom of the scope prevents the collar of some solar filters like the Thousand Oaks glass filter from fitting snugly on the front of the scope.  If youíre anal like me you can chuck a half-inch diameter sanding drum in a drill press and sand a notch in the aluminum filter collar so that it fits properly. Just be sure to clean all the dust and filings from the filter before you place it on that expensive corrector lens on the U2K. Also note that the Thousand Oaks filter (and perhaps others) comes with a strip of self-stick felt to be used inside the rim of the filter collar to assure a snug fit over the front of a scope.  Be sure to apply this strip so the filter fits tightly - the last thing you need is for the filter to be accidentally dislodged while someone is looking at the sun through the scope.

Kendrick Sun Finder As noted above, you must protect your eyes and telescope optics from the sunís intense light when viewing the sun with your scope.  This includes covering your finder scope! Because of that, it becomes somewhat difficult to accurately aim the scope at the sun - you canít look through your finder scope so the common aiming technique is to observe the shadow of the scope on the ground and maneuver the scope until its shadow is minimized.  This will get you close, but you will still need to wiggle the scope around until the sunís image appears in your eyepiece.

The Kendrick Sun Finder makes this process a whole lot easier. It fastens to your Telrad Finder (providing yet another reason to get a Telrad) with an elastic cord, and provides a pin-hole in its front that projects a small circle of light at a brass pointer on its back when the scope is aligned with the sun. 

It sells for about $30, and is marketed by Kendrick Astro Instruments.  I highly recommend this product. Iíve really enjoyed observing the sun, and the Sun Finder has greatly simplified placing the sun in my eyepieces.  Depending on how you value your time the $30 price is well worth it, even though like many Kendrick accessories it feels a little expensive for what you get (in fairness this is probably a result of high-quality production for a small customer base).

Important Note:  The Kendrick Sun Finder is made to fit on top of the Telrad with a shock cord, so that you can easily remove it to change the batteries in the Telrad. The problem with this system is that the shock cord doesnít really hold the Sun Finder in place very well when you insert and remove the scope in a case like the Orion soft case. If you have a drill press you can carefully drill a pair of holes in the side of the Sunfinder and the Telrad, and screw it in place with a pair of small machine screws.  This will keep the Sun Finder securely in place and still allow you to change the Telrad batteries when needed (which isnít very often - the Telrad batteries are good for a long time even if you forget to turn it off periodically, not that anyone would forget after a long late-night observing session...). Obviously Kendrick doesnít want to sell you an attachment to someone elseís product that requires you to go into your workshop and modify that product (hence the shock- cord), but in this case itís a good idea for you to make this change.

Hydrogen-Alpha Filters The solar filters sold in the $120 price range (that I discuss above) are essentially neutral-density filters (albeit much, much denser than photographic neutral density filters) - they dramatically reduce the amount of sunlight entering the scope, and allow you to see sunspots on the sunís surface. There is a more sophisticated type of solar filter that blocks all parts of the solar spectrum except the alpha spectral line of Hydrogen. An H-Alpha filter can allow you to see fine detail on the solar surface or prominences - some of the most dramatic views you can get with your scope.

H-Alpha filters are sophisticated and thus expensive.  The quality of the filter is denoted by the bandwidth of the H-Alpha part of the spectrum which the filter passes - a narrower bandpass means less visual ďnoiseĒ and more detail. The bandpass is measured in Angstroms (one ten-millionth of a millimeter) and traditionally the less expensive H-Alpha filters have been 1.5 Ň filters while the most expensive have been 0 .8 or 0.7 Ň filters. However, there is a difference in what you can best see, between these two types.  (My thanks to Gary Hand of Hands On Optics for his assistance in explaining this difference.)

Solar Prominence Filters
The wider-bandpass (and less expensive) 1.5 Ň filters can show solar prominences, and may work better for prominences for two reasons: a) they have wider acceptance apertures (3Ē) and a wider bandpass so they transmit more light and thus display the (relatively dim) prominences better, and b) their wider bandpass encompasses the doppler shift in the H-alpha wavelength caused by the sunís rapid rotational speed, so you can see prominences on both the leading and trailing edges of the sun. (Click on the thumbnail above to see a photo of the sun through a Lumicon Solar Prominence filter, taken by Sylvain Weiller with a Celestron C8.) The disadvantage of the 1.5 Ň filters is that they show much less detail on the sunís surface itself, than do filters with a bandpass <1.0 Ň .  1.5 Ň filters are for these reasons marketed as ďsolar prominence filtersĒ (truth-in-advertising actually occurs here <grin>). The Thousand Oaks 1.5 Ň H-Alpha Prominence Filters sell for $840 to $930 depending on the size of your SCT , and the Lumicon 1.5 Ň H-Alpha Solar Prominence Filter sells for about $900 plus an extra $70 to $90, again depending on the size of the SCT. 

Coronado Instruments, a manufacturer of what have been viewed as among the best H-Alpha filters (see below), has introduced the Personal Solar Telescope (PST-40), a 40mm-aperture solar telescope with a 1.0 Ň filter that sells for a very attractive price ($500).  It is a compromise between the broader-band solar prominence filters discussed above, and the true sub-Angstrom H-alpha filters discussed below. It is important to note that it comes as a self-contained unit with the standard ľĒ-20 threaded base used for cameras, so it can be easily mounted atop an SCT using a piggy-back bracket or a camera adapter on a dovetail plate, and thus does not require one of the expensive front plates needed to mount an H-alpha filter on an SCT, as discussed below.  So it is a very cost-effective compromise.

H-Alpha Filters
Narrow-bandpass H-alpha filters (0.8 Ň or less) may not display prominences as well, but they will show them and in addition they show the convection-current-driven granularity of the sunís surface, much better than a 1.5 Ň prominence filter is able to do. (Click on the thumbnail to the right to see a larger version of a photo of the sun through a Coronado Solar Max H-Alpha filter, taken by Jack Newton.)

The 0.8 Ň or lower H-alpha filters have traditionally been much more expensive than 1.5 Ň filters, but Coronado Instruments sells the Solar Max, a 0.8 Ň filter that sells for not all that much more than the 1.5 Ň filters from the manufacturers discussed above (more on this below). Here is a comparison table of 0.8 Ň or narrower-bandpass H-alpha filters:



Note that for the Solar Max and for DayStar filters, there is a hidden cost for larger scopes like an 8Ē SCT, as noted in the above table. All H-alpha filters comprise a part that fits on your scopeís front cell and another part that fits on the rear cell.
For example, in the case of the the Solar Max the front cell houses a narrow-band Fabry Perot etalon which is tuned to H-alpha and an energy rejection filter, while the rear cell holds a secondary interference filter that serves to block the peaks adjacent to H-alpha.  The front-cell components are fastened to a plate that fits on the front cell of your scope.  The price of the Thousand Oaks 1.5 Ň Prominence Filter discussed above, includes the front cell mounting plate for an 8Ē scope. But the Solar Max does not include that mounting plate for the front-cell components, and Coronadoís price for a mounting plate for an 8Ē scope is ď$350 and upĒ, which must be added to the $995 for the rest of the H-alpha filter assembly. (If you do decide to get a rich-field telescope you can mount a Solar Max on it and the price of the front cell mounting plate would drop to around $135, but you would probably need to purchase a higher-magnification eyepiece as well, since the eyepiece you would use for wide-angle views of deep-sky objects with a rich-field scope probably wouldnít provide the higher-magnification view you would want for viewing solar details.)  Front plates for DayStar filters are also an additional cost although less than for a Solar Max. I should mention that DayStar filters also have an excellent reputation.

The Solar Max has a very small acceptance aperture (40mm or 1.7 inch) so the amount of light transmitted to your eyepiece will be more limited, but it does include a tuner that allows you to overcome the doppler shift problem described above. (The DayStar T-Scanner filters have even a lower acceptance aperture - 30mm). Because of this tuner you can see prominences (as you can with a 1.5 Ň prominence filter) but you can also view the sunís surface with these more expensive narrow-bandpass filters (the Solar Max or one of the DayStar filters).

Many observers note that the view of the sunís surface through a Solar Max is stunning.  And observers who have used any of these H-Alpha filters note that once you use one you never regret the price.  However, be aware that these H-Alpha filters have in the past been somewhat difficult to get.  For example, Coronado typically has had a long delivery time (several months) although this wait time has apparently been reduced to 6 weeks - if you order one you should be certain you know what your delivery time will be.  However, customers do seem to be happy with their products once they have received them.
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If H-Alpha filters interest you, note that Coronado maintains an interesting chat room for H-Alpha filter users at Solar Chat.
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