A Night of Achievements

Posted by XTSee on 21st February , 2010

Good nights for astronomy are often few and far between in the UK. We’ve had the usual run of miserable, rainy, cloudy nights. Sometimes when the early evening sky looks promising, inevitably the clouds roll in by the time we’ve had our supper!

So when a good, clear, cold night comes along you grab the opportunity, and make the most of it. Last night was one of those times, so I planned to get several things done which had been waiting far too long;

  • Become more familiar with Polar Alignment
  • Check the collimation of my C6-SGT (Schmidt-Cassegrain)
  • Try viewing Mars again
  • Try out my SPC900NC webcam now that I’ve done the Steve Chambers long-exposure mod (quite excited about this)
  • More testing of my home-made Dew Controller

Polar Alignment

In a previous post I explained how I had installed my new polar scope in my EQ5 mount, but that it absolutely did my head in. Well I’ve done a fair bit of reading up on it, and now have it set up correctly.

  1. I re-centred the reticule inside the scope body (I’d made the classic mistake of undoing the adjustment screws too much and it had dropped down inside).
  2. I have done a daytime sighting up on a distant abject and collimated the polar scope reticule for rotational centred alignment with the RA axis.
  3. I’ve learnt how to set the polar alignment RA index scale and date circles to give correct indication of Polaris transits for any day/time (always wondered what those scales were for!).
  4. And using PolarFinder.exe I can double-check I’ve got it right (although you must remember to set your longitude every time you run the program, pity it doesn’t remember this setting).

So this time polar aligning went fairly well. I used the date circles to ensure I knew how to use them - practice makes perfect, and compared the RA rotated position of the reticule for the Polaris circlet, with the screen display of PolarFinder. Finally careful alt/az adjustments of the mount to bring Polaris into the circlet.

Tip: The reticule markings in the Polar Scope are difficult to see in the dark. Shining a red torch at an oblique angle into the top polar scope mount opening allows you to see the markings, although holding the torch there while also adjusting altitude and azimuth, and crouching below the mount to peer through the polar scope eyepiece, is quite tricky!

Polar Alignment Resources

Some of the web links I provide below mention finding the “transit of Polaris” for a given date. At first I thought this was some special date which occurs only once in a while! In fact any star “transits” the celestial meridian twice every day, at its highest and lowest points;

Transit: An astronomical transit occurs when a celestial body crosses your local Meridian due to the Earth’s rotation, about halfway between its rising and its setting (east/west). At this time it reaches its maximum altitude (highest point in the sky). For instance, the Sun transits the meridian at “solar noon” which may deviate from our clock-driven “local noon”. Observation of meridian transits were once very important for timekeeping purposes before the advent of standard time zones and accurate clocks.

Upper Culmination: An astronomical object’s Upper Culmination occurs at the time and place in the sky where it reaches its highest altitude on your Meridian. For polar alignment, it is this point we are interested in determining for Polaris, so we can set the date circles just once, and it will be reasonably accurate for all dates/times thereafter. Of course this must be set correctly for your longitude on earth (local meridian), for the date and precise time Polaris would be highest (say today), and once you know that time (using Polarfinder, an ephemeris or astronomy software like Stellarium), and you subsequently set the date circles, they will allow you to rotate the mount to the position Polaris has rotated to at the time you are setting up your mount.

Lower Culmination: An astronomical object’s Lower Culmination occurs at the time and place in the sky where it reaches its lowest altitude on your Meridian. If it is below the horizon at that time it is not visible so this term is usually applied to circumpolar objects.

The absolute best pages I found for describing the Polar Alignment procedure are as follows:-

Checking the SCT Collimation

Frequently while viewing through my SCT I’ve noticed stars had a slight flaring to one side of them, which made focusing to a fine pin-point difficult. Ever since buying the scope I’ve trusted that the factory had adjusted the collimation, but really I should have done something about this well before now.

So tonight I performed a “star-test” and sighted up on Sirius dead centre in a 10mm eyepiece to give a good degree of magnification, and then de-focused pre and post focus point to show the rings and pay attention to them.

Indeed as suspected, the rings were not concentric and shifted off to one side, so I carefully adjusted the three collimation screws on the front corrector plate. They were very tight, and it was necessary to loosen off one or other screw, to allow another to be tightened.

Only the tiniest adjustments must be made because each adjustment moves the star away from the centre of the view. If you make too large an adjustment the star disappears out of the field of view altogether. Re-centre, then make another small adjustment.

It didn’t take long to get the hang of it, and soon I had the correct concentric/parallel rings showing both pre/post focus, and when perfectly focused, the flaring had gone. So that’s two things ticked off my list.

Viewing Mars again

Mars is magnificent currently, very prominent in the night sky, bright and very obviosuly red. When I last viewed it through the scope a couple of weeks ago, it was very washed out by the blue Moon which was very close.

Tonight the half-moon is low down and in a completely different location, and now that I’ve got the scopes collimation sorted, I inserted my highest mag eyepiece and barlow combination. Wow! It’s a perfect disc, and I can see quite clearly the white polar ice-cap shining back at me, and even make out some of the darker markings on it. My scope is only a 6 inch, and so I’m quite pleased with this, although I must admit to getting “aperture fever”, and would love to get a bigger OTA now. Problem - money! Oh well, one day.

Try out Long Exposure mod

Now for the thing I’m most excited about trying out tonight.

Indoor testing of my webcam modified for astrophotography to allow long duration imaging, has been very encouraging. Putting the camera into dark cupboards, or experimenting in the bedroom with the lights turned out(!), has amazed me just how much the SPC900NC is able to make use of what little light there is.

I’ve been imagining wonderful vistas recorded of M42 Orion Nebula, or the blue wispyness of the Pleiades.

So I connected up the webcam to the laptop, and …… Boo, hiss, humbug. Nothing! Nada, zip! The laptop would not recognise it when I plugged it into the USB port. No amount of wiggling wires, unplugging and re-plugging would make it work. Sod it. I guess there’s a dodgy wire come loose. Which means I will have to dismantle and check it all over again.

So annoying.

Dew Controller performance

It’s very cold (in fact later after I had packed up we got snowfall), and tonight dew has been a big problem on the cold surfaces of the scope, the eyepieces, and also on the front corrector plate glass!

Fantastic! No, really this is great as it will be a good test of the dew controller!

So earlier I had turned on my newly built dew controller to see how well it works. It seems to be working ok-ish to keep the glass clear, but there is still some light coverage of very fine moisture on the glass, and as the night is getting colder it seems the dew controller is struggling to keep the glass clear.

There is a metal band around the front of the scope which holds the corrector plate, and I usually have the dew controller heater band positioned just below this at the “neck”, with an AC900 dark-cap observing shield wrapped around the metal band (to shade the scope from stray light).

I removed the dark-cap, and moved the heater band to wrap directly round the metal band of the corrector plate, then re-fastened the dark-cap shield over the top of the heater band. Within a few minutes the corrector plate completely cleared of moisture. Excellent!

Back to some real astronomy

The trouble with all of these gizmos and adjustments (some of them quite necessary), is that you spend an age getting ready, and get side-tracked from the simple enjoyment of just looking at the stars.

Being disheartened with the webcam not working, I decided instead to just get on and use the Tour feature of my Goto mount, and was soon looking deep into space marvelling at the faint fuzzies; Eskimo, Crab, Ring nebulas, Sombrero galaxy, M66, M67, M82 galaxies, M36/37/38 clusters, and a couple of others the tour pointed at, but for the life of me I couldn’t make out! Last of all I spent some time looking at several Double-Stars which I’ve done very little of.

By this time, the dew controller has paid its toll on my 12v PowerPack, the glass is starting to dew up again, and finally when I try to slew the mount round to another double-star, the motors just make a pathetic buzzing sound which means “Time for bed”.

Overall a good satisfying night of astronomy, with plenty of things achieved despite the failure of the webcam, which I can fix tomorrow, err I mean today!

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