Monday, April 30, 2012

Observing Stars and Planets during the daytime

You may be surprised at how many planets and stars you can see during the daytime! You don't need a big or specialized telescope, Although it's particularly easy if your mount has go-to capability. But there are a few things to keep in mind if you want to do this safely.
  • NEVER look at or near the sun unless you are using an appropriate and correctly-fitted filter on the telescope objective. If you're not 100% sure about what you're doing, stick to observing at night.
  • CAP the telescope objective whenever slewing near the Sun; if the scope slews across the sun, heat may damage your telescope. Make sure your cover is secure and won't fall or blow off.
  • ALWAYS keep your finder telescope's objective covered, and cover its eyepiece, too, just to be extra safe. You won't be using your finder anyway. Even safer is to remove it from your telescope.
  • DON'T do slow search slews near the sun while looking through the eyepiece. If even a small portion of the Sun's disk slides through the field of view for a tiny moment eye damage would result!
If you get the feeling from all this that it's not particularly safe to look near the sun, GOOD.

Now, assuming you're not completely scared out of trying this, here's how to spot daytime stars.

If possible, the night before do a good polar alignment followed by a multi-star alignment so that its go-to is ready. Some mounts let you retain the go-to information with a Hibernate mode. If your mount has that feature, use it.  The next day, awaken your mount from Hibernation.

Use the go-to to find the brightest object other than the Sun. This may be Venus or the Moon; both are easy to see during the daytime when far enough in angle from the Sun. "Sync" your mount to the object if possible. Syncing is useful for doing go-tos to targets near the synced object. For example if you have synced to Venus in Taurus, you might select Rigel, Capella, or Betelgeuse to look at.

Once you've gone to the target star, you may have to hunt a bit. Use a low-power eyepiece and scan outward from the center of the field of view. I use a slow spiral when searching. Don't bother using averted vision, as it's ineffective with the daytime brightness. Remember that you're looking for a tiny sparkle of light, not a glaring bright star.

Your luck will be best when you look farthest from the Sun, when the Sun is low in the sky, and when your target star is well above the horizon. The larger your telescope's objective, the dimmer the stars you'll be able to see. Even with a small telescope there's a lot you can see. If the sky is hazy or there's a thin layer of clouds, you will probably be limited to only the brightest sky objects.

Here's one days observations I made using a 72 mm ED refractor on a CGEM mount. Most of the time I was using a 12.5 mm or 9 mm eyepiece, giving magnifications of  about 35X and 48X. I tried a variety of color filters, but only a yellow #15 seemed to help.

Planets I observed:
  1. Venus was my starting point and on this day was a brilliant crescent.
  2. Jupiter (-2.00) was a ghostly disk
  3. Mars (-0.3): Much easier than Jupiter at -2.00, a distinctly red disk.
I tried Mercury, but the the sky was too bright.

Stars I observed (and their magnitudes):
  1. Capella (0.08) was easily seen
  2. Betelgeuse (0.45) was easy when it had risen high enough, and showed distinct color
  3. Aldebaran (0.87), fairly easy and had a distinct reddish tint
  4. Procyon (0.4) not too bad
  5. Pollux (1.16) easy
  6. Castor (2.0 and 2.9) This double was split cleanly with a 6mm (72X) eyepiece and #15 yellow filter. The reason I could see the faint companion was that I knew exactly where to look, which makes a tremendous difference.
  7. Alhena (1.93) no problem
  8. Sirius (-1.44) Quite easy despite being low in the sky.
  9. Rigel (0.18) fairly easy.
  10. The three belt stars of Orion, from east to west (Alnitak, 1.74; Alnilam, 1.69; Mintaka, 2.25) only Mintaka was moderately difficult
  11. Saiph (2.07), the SE foot of Orion. Easy
  12. Bellatrix (1.64), the NW shoulder of Orion. Also easy. With Betelgeuse, Saiph, and the belt stars, it's easy to see all the the "signature stars" of Orion during the daytime!
  13. Regulus (1.36) easy
  14. Algieba (2.01) fairly easy
  15. Dubhe (1.81) Very clear.
  16. Merak (2.34): Yes.
Not bad for a late morning and mid-afternoon's work. Unfortunately, clouds rolled in and I didn't get to do more during the late afternoon when more stars would have certainly been visible.
This shows how you can see stars down to almost 3rd magnitude during the daytime with a small telescope. If your telescope is larger, you can probably do better than I did.

Try this out for yourself and let me know how you did. You might be surprised at how pretty stars are during the daytime!

Friday, April 20, 2012

Stupid Imaging Trick #2

I'm impatient. When it comes to imaging, I want as much time with the shutter open that I can get. My setup, currently a cement pier, allows me to polar align my mount one night and leave it in place so long as good weather holds. Normally I polar align and also do a two-star alignment as well. When I'm done I return the mount to its "home" position and power it down.

The trick comes into play when I start up the next night. Instead of repeating the two-star alignment, I simply go to the alignment star nearest to my imaging target. For example, the last week that has meant Mizar, which a short distance from M101. Normally Mizar shows up somewhere in the finder scope's field of view. I then sync the mount to it with a little slewing. Then using go-to I move to M101, and it's right in the center of my CCD's field of view.

This doesn't produce a huge time saving, but syncing is faster than slewing to several stars. Every saved minute is a minute sooner you're able to pack up and go to bed!

Tuesday, April 17, 2012

M101 Progress

This Friday I may be able to complete the acquisition part of my M101 project. At the present time I have 13 hours of H-alpha, two hours of luminance, 40 minutes of red and 40 minutes of blue. (Both the R and B are binned 2x2 on board my ST-8300. I feel that I need to add at least 20 minutes of red, 20 minutes of blue, and an hour of green before I'm ready to process it all. I have the dark frames ready to go.

Friday night is a gathering of imagers out at Onan Observatory, and the forecast is for clear skies. There should be a good four hours of imaging time between the start of darkness and RA end-of-travel. Deduct from that the time for polar alignment, (re)focusing, and the usual snags and glitches.

The last several years my clear-sky luck at Onan has been terrible. I won't be surprised if the clear forecast busts.

Thursday, April 12, 2012

Stupid Imaging Trick #1

So lately I've been imaging with my camera cooled to -25C. Last night I shot several hours of light frames, but alas, I have no matching dark frames. Tonight I thought to do it after dark, but it's too warm, the camera can't get down that far. What to do?

I put the camera in our chest freezer that runs at about -18C so that the USB and power cable trailed out to my laptop. I set the camera's temperature to -25C, and it's now collecting the needed dark frames. All need to do is remember to seal the camera in a zip-loc bag when I remove it from the freezer, so that condensation won't build up in and on it.

Kind of dumb, but I'm sure I'll come up with others.

Focusing; imaging at a dark site

I recently posted that I was going to test the parfocality (if that's a word) of my Baader LRGB and narrowband Ha/SII/OIII filters. The test has been done, and it was less than conclusive. If you use a Bahtinov mask to do focusing, all the filters are essentially parfocal. That is after focusing for one filter, they're all in focus. They all have nicely centered spikes in the test images.

However, the centered appearance of the central spike when estimated by eye is only approximate, and the range of error is about the same as the range over which final focusing is done. I think you're still better off handling focus with the assistance of software that quantifies the sharpness of focus. Likewise, I think every time you change filters, it's time to refocus. At least that's what I'm going to do for the foreseeable future.

Last night I gathered another bunch of photons for my M101 project, this time from the dark-sky site of Cherry Grove. It's really nice to have a dark place to image from only an hour's drive away. The site has a warming house, too, which was nice. Imaging was largely automatice, so I could while away the hours reading and listening to the Twins game in comfort, as the temperature outside fell to around 0C.

Added note, 5/22/12: Further imaging with the Baader filters suggests that the LRGB filters are essentially parfocal when used with an apochromat. Likewise the Ha-SII-OIII filters.

Saturday, April 7, 2012

Parfocal test of Baader narrowband filters


There aren't many nebulae in my sky right now, and the full moon means there's no point in traveling to a dark sky site to work on my M101 project. So tonight I'm going to investigate how parfocal my Baader Ha/SII/OIII and LRGB filter sets are. I don't expect the two sets to be parfocal with each other.

It's a simple plan at this point. I'll use 20 second exposures of bright stars taken through a Bahtinov filter. The stars are chosen to span the spectral range from blue to red: Altinak (spectral type O), Sirius (A) and Betelgeuse (M). Best focus of Ha will be the starting point, and then I'll cycle through the other filters and shoot another Ha to catch any focus drift.

Once the narrowbands have been tested, I'll repeat shorter exposures with the LRGB, initially focusing the L filter.

After that I may move on to shooting more long-exposure images in Ha of M101.