Observing the Sun

Sun filters and thermal balance
Full-aperture or off-axis?
Making a wrinkle-free filter
A narrowband filter for the Sun
Sun links

Observing our nearest star, the Sun, can be very rewarding. To get a maximum level of detail, the telescope should be in thermal balance, collimated, shielded from stray light, and fitted with a quality solar filter. One of the best solar filters for white-light observing is the Baader Astrosolar filter. If you make your own filtercell for this filter, take great care to avoid wrinkles, which will adversely affect the image quality.

The only stray light of significance in my Teleport came through the shroud opposite the focuser. This is easily avoided with some cardboard:

Sun filters and thermal balance
Keeping the Teleport in thermal balance during solar observation has proven very difficult. Inspired by Mike Spooner, I made a full-aperture sun filter with spacers to provide ventilation (see photo below). I bought a digital thermometer with two sensors to investigate the thermal behaviour of my scope using either the full-aperture ventilated filter; the unventilated filter I had made earlier, or the off-axis filter that came with the scope. The results surprised me. Read on!

1. From left to right: The original unventilated filter; the ventilated filter, and the off-axis filter.

2. The spacers provide a gap that should ventilate the telescope.

Method: 
I attached one sensor inside the upper cage assembly, and the other was placed on the outside of the mirror box (in shadow). I noted the temperature every five minutes, with three repeated measurements every time. The digital thermometer isn't particularly accurate; sometimes the readout will inexlicably jump 1 degree or more. The various filters were compared over a long time period with unavoidable differences in weather conditions. I found that a little wind could affect the results a lot, so to avoid wind and get at least some repeatibility, I placed the telescope facing out the open door out to my south-facing balcony, were it was shielded from wind.

3. In windy conditions there is little heat-buildup inside the telescope. Each point is the average of three measurements.

4. With no wind, heat builds up inside the telescope. "Blocked" is the off-axis filter and a piece of cardboard on top of it, blocking virtually all radiation from entering the front of the telescope. "Extra shielding" is the non-ventilated filter and the scope wrapped up in isolating material to block any infrared radiation from entering through the shroud.

Results
In windy conditions, the temperature difference was small with both filters, but the ventilated filter kept the temperature inside the upper cage closer to the outside temperature than the non-ventilated filter. With no wind, heat built up regardless of filtering, even when I completely blocked the front of the telescope. The best explanation I can offer is that the source of the heating is not direct sunlight entering the front of the telescope, but hot air rising from the floor of my concrete flat and infrared radiation entering through the thin shroud of the telescope. If I had done the testing on e.g. grass, I suspect there would have been less heating. I draw the following conclusions from the results: 

1. Most of the heating does not enter through the filter, but come from indirect sources.
2. The tube current problem is nearly the same for a full-aperture filter and an off-axis filter.
3. The tube current problem is nearly the same for the ventilated and non-ventilated filter.
4. A little wind makes a lot of difference.
5. Watch out for changing collimation with heating!

Please note that my results are specific for the 10" Teleport - different telescope designs may behave differently.

Full-aperture or off-axis?
Based on my results, I decided to go with the full-aperture ventilated filter.

Update: I am having seconds thoughts about ventilating or not. I have seen at night that if the air inside the telescope is hot, closing the tube with the Turbofilm reduces the tube currents (this of course also slows down the cooling). If there is a thermal imbalance during solar observing, is it better to keep the tube closed? I don't know.

I have found that a full-aperture filter offers several advantages, and as the above tests show, only causes a slightly larger heating of the inside of the telescope than an off-axis filter. Using the full aperture obviously increases the theoretical resolving power relative to an off-axis filter. Brightness increases (but not nearly as much as expected, and still at a comfortable level for me), contrast is better, and in good seeing the resolution is better. And last, but not least, the exit pupil is larger, which makes observing much more comfortable. In bad seeing the off-axis filter gives a steadier - not better in my opinion - image than the full aperture filter.

Making a wrinkle-free filter
I made a new filter, since the one I used in the test was cumbersome to attach and a little on the fragile side. As shown in the Baader Turbofilm test wrinkles in the film adversely affect the image quality. When observing the sun I can see this as a lowered contrast. When I made the new (and hopefully last) ventilated filter, I made every effort to avoid wrinkles. I made my filter cell from a single piece of cardboard. I have found that the boxes that computer stuff comes in are good. Try to find an undamaged box with smooth and flat surfaces. Any irregularities or warp in the surface may cause wrinkles when the filter is attached to the cell. I use two-sided carpet tape - this attaches very well to the filter material. Note that the filter material and cardboard have different thermal expansion coefficients - in hot weather my filter is slack, while in cold weather it contracts and may become as tight as a drumhead.

5. Start with good ingredients.

6. The pattern.

7. After cutting and spraypainting with flat black. I kept a piece of cardboard above the focuser to shield my head from the sun. I cut out the center to save weight.

8. The styrofoam blocks act as spacers and support the tabs. I put wood glue in the corners of the tabs to keep the 90 degree angle.

9. The ventilation gap (right). Before viewing the Sun, I firmly attach the filter to the scope with tape.

10. The worst wrinkles will always be along the edge, so I made the filter oversized relative to the mirror (grey circle). The blue line shows the width of tape I used.

11. From the remains of the carboard box I made a storage box for the filter. I put styrofoam blocks on the front of the filter so I can place it upside down without damaging the filter.

12. Initial tests showed that some stray light entered the focuser from the opposite side of the filter, so I put more styrofoam here. I also covered the styrofoam with flocking paper. Now the contrast is better than the old non-ventilated filter, because the new filter has less wrinkles. This filter is rigid and lightweight.

13. My attempt to shield the scope from indirect heating.


Despite my efforts, I still often have problems with the thermal balance. In the morning the outside temperature often rises quickly, and the temperature inside the telescope will often be lower than outside. Later in the day the temperature is ususally higher inside the telescope than outside, but in most conditions the tube current problem is sufficiently small to be overshadowed by atmospheric turbulence. Sometimes the seeing steadies for a brief moment, and then the amount of detail is fantastic. One morning the seeing was perfect for a few seconds, and I could clearly see the individual granular cells. I will never forget it.



A narrowband filter for the Sun
After seeing the amazing images by Arthur Whipple, I ordered the same narrowband filter he uses. This filter has a 10 nm wide peak centered at 520 nm, and gives a green image of the Sun. This filter must only be used in concert with a front-aperture filter like the Baader filter. The filter is 0.95" in diameter, so I had to make a filterholder. Tom Noe then made me a special 1.25" adapter with a constriction that keeps the filter from falling out the bottom end of the adapter. The views through the filter are not quite as good as I had hoped. After getting used to the monochrome green image, the granular contrast is slightly better, and atmospheric turbulence is less visible.

The special adapter with the narrowband filter.


Sun links
Solar FAQ by Jeff Medkeff
Sunspot images by Arthur Whipple
Solar-observers Yahoo-group
Swedish Vacuum Solar Telescope