Archive for the ‘Planetary astronomy’ Category

Planetary imaging: Which resolution for what seeing

Dear all,

I am in the process of trying to benchmark planetary imaging resolution with a given turbulence level.

The idea stems from the fact that when I first got into planetary imaging (as a sideline for clear nights when the moon is too present to do any deepsky astrophotography), I was getting dreadful images. At the same time, I was coming across those fantastic images from C14 owners. At first, I questioned the optics of the scope especially since my old C11 really was below par in that respect. As faithful readers will remember, I even got the scope checked  through a shack-hertman optical bench, whose verdict was that everything was fine… But, my images were still as bad as ever, and I just could not understand why. The main problem for me is that, being on my own behind the eyepiece, I did not have the opportunity to get advice from an expert planetary imaging guru, or look into another scope to see the difference on a given night.

I imagine that I am far from beeing the only one in that position, and the point of this article is to make the learning curve of would be planetary imagers, alot smoother than mine… So, what is needed is a “tool” or method to get an idea of the level of turbulence on a given night and be able to relate that level to a given image quality. It is not possible to gauge turbulence accurately  just by looking at a raw AVI recording of a planet as turbulence can take many different forms and affect a planet’s image in lots of different ways, which are not obvious just looking at the raw data. I came across an idea last night as focusing on Jupiter was tricky, due to the fact that focus was moving in and out and I was not sure where the best focus was. So, I slew the scope to a nearby star in Aries to achieve focus by looking at the FWHM on the screen and I also wanted to check that collimation was spot on by recording Airy patterns. I had previously done a startest with my 13 mm eyepiece behind the televue barlow which gives a magnification of about 612. The star was STARAlpheraz (alpha Andromeda) much higher in the sky than Jupiter. The image was quite good, I could just about see the star as a wobbly point with the airy disks breaking up around it; but, visually, you could still tell that collimation was pretty good. As for the turbulence, it was about as low as I’ve seen at my location. Now, the star I had on the screen looked nothing like Alpheraz I had looked at througn the eyepiece (picture on the left). I could get a fairly accurate focus on it, but, it shows what turbulence was doing at the time, which is move about by at lest 1 arc second as well as slightly moving in and out of focus. This animated GIF photo is an actual crop of the AVI I acquired, so the scale with the picture of jupiter below is 1:1 on the 640×480 picture. It does help to understand why I cannot get anymore finer details than what I have got already. So, the use of this is that I can now measure this seeing by looking at how many pixel the star is moving around by, and relate that to the image quality below. All I need to do is record a short clip of a nearby star before moving on to Jupiter. If it’s better, then I know I’m in for a treat! below are the picture of Jupiter taken last night just after the star clip.

jup271111-04

What I am hoping on doing is to build a small seeing data base and corresponding planetary images. Then, unlike myself, anybody struggling with planets can relate to the turbulence, and check whether, for a given turbulence, his results are below par or not. We’ll see how it goes.

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Crayford focuser

A crayford focuser? on a SCT? why for?

Well, Crayford focuser can be very useful for SCT but, I strongly believe, it won’t replace the built it focusing system which works by moving the primary mirror. Why? because, if you’re like me, you can do loads of different things with your telescope, such as:

  • use it for visual observing with eyepieces, barlow lenses etc…
  • use it for planetary astronomy with a webcam or a webcam style camera on top of a barlow lens.
  • use it for deepsky astrophotography, in which case you have an Off axis guider, a CCD for imaging and a CCD for guiding

And the focuser cannot do all that.

crayford

The main benefits from a Crayford focuser is that it allows you to adjust focus extremely accurately. So, really, in an ideal world, it’s useful in all cases mentioned above. But, as usual, there is a catch. First of all, even though you do get focusers with different travel courses, the overall length of your focuser is important, especially for deepsky imaging where it is critical, especially for an Edge HD, where the field is flat 127 mm back and nowhere else. Also, the deepsky imaging setup weight to about 4 pounds, which is slighly too much for most Crayfords focusers. As a final nail to the deepsky imaging coffin, the only way to position the main imager to the correct back distance, is without crayford, as it’s too far back otherwise. Which is why, in my case at least, I can only use the crayford for visual observations and planetary imaging. The crayford is great in those cases, because, it allows me to adjust the native focus knob  to a point which is mid-range to both applications, and then, lock the primary miror in place so that nothing moves, especially collimation. Also, if your telescope is subject to shifting, as in, the image moves while adjusting the focus knob, then using a crayford will cure that.

As you can see, a Crayford focuser can not do it all on a Celestron telescope, so, thinking is required before buying one. Also, it has to be said that if you buy a cheap one of poor quality, you might endup with a focuser that is worst than the original arrangement.

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