Archive for August, 2010

Pelican Nebula in HAlpha wavelength

Date: 7/31/2010

San Bruno, CA

Takahashi FS-60C with Qhy9 CCD, HAlpha 12nm Astromik Filter and Astro-tech field flattener.

Auto-guided with 50mm Finder turned into a guidescope and Orion startshoot autoguider

9 exposures of 420sec taken under Moonlight (77% illumination). Thanks  to Narrowband imaging, it is possible to do long exposures under light pollution and moonlight illumination…

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Jupiter is back: LRGB and Infrared Imaging

Jupiter is back in the late summer sky…

I tried out some new planetary imaging techniques compared to RGB imaging I did so far:  LRGB and Infrared imaging of Jupiter. The pictures were taken at approximatively f/d 25 and I found this is a f/d ratio working pretty well in average condition, optimizing the balance between exposure time, and magnification (1 pixel = 0.25 second of arc) for a planet like Jupiter.

I worked also hard on my Photoshop skills – and try to do less aggressive sharpening in order to produce planetary images with a more “natural” aspect and a better color balance. I found the overall aesthetic aspect of the picture is improved when doing so.

I’ll post other pictures but here are two  I took this week,

First –  LRGB imaging:

Since Jupiter’s rotation is very fast, the LRGB pictures have to be taken in a short period of time, definitely less than 4 minutes. I took more frames at high speed for the luminance with an IR/UV filter. The fast exposure time (about 1/30 sec) allows to take many pictures but also to find the right window where the turbulence is minimal. Visually, seeing was at 4/10 which is far from ideal – but by taking many frames, the signal/noise ratio is improved. Then I took less exposures for RGB frames – to make sure all the frames are taken in a window of 3/4 minutes. But since RGB frames are used for color – the slight time delay of 2/3 minutes they have with the Luminance layer is not that critical and not really visible on the final picture.

As a summary – I think the LRGB technique I have been using provides superior results compared to my previous RGB technique.  I don’t know if it is a valid /  general statement – but at least in my case it seems to work better.  I think it is because of the advantage of taking many frames at high speed for the luminance layer.

– Luminance exposure: 635 frames at 1/30 sec

– Red exposure: 203  frames at 1/5 sec
– Green exposure: 177 frames at 1/11 sec
– Blue exposure: 202 frames at 1/11 sec

The result is pretty good for a mass-produced telescope at a price tag of less than $1300 in average seeing condition … What do you think?

 

Second,  Infrared imaging:

The camera I use has no IR filter. It allows infrared imaging with the right blocking filter…

For infrared imaging I used the Astronomik IR 742 pro filter – it lets the light pass above 742nm and blocks the light below 742nm.

I was able to verify, as explained on the astronomik site, that the seeing is much better in the infrared – even if the turbulence is quite high – and Jupiter is low on the horizon.

However two factors are reducing the overall resolution

a) At + 742nm the resolution of the instrument is lower than at 500nm …by +50% – given the longer wavelength…  The aperture I used is pretty small so it really affects the resolution I can get in imaging.

b) Worse, the integration time has to be raised at +1/4 sec. so the longer exposure time does not allow to take shots so easily in a “low turbulence” window.

The IR Pro planet is not a miracle filter. Still, I found this technique quite interesting – as it certainly provides better quality pictures in a bad seeing environment, and highlights different features compared to the visible wavelength.

 

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M17 with Canon EF 200mm f/2.8 L II and Astrotrac

Date: 8/9/2010 – San Bruno, California – near the SFO airport

Transparency: 3/6 – Wind between 5mph and 10mph

Mount: Astrotrac travel system

Camera: Modified Canon XTi (Standard IR filter replaced with an astrodon IR filter by Hap Griffin) with Canon 200mm  f/d 2.8 teleobjective (prime lens) opened at f/d 3.5 – at Iso 400.

22 exposures of 90 seconds with Astronomik CLS CCD clip filter, 10 flats frames, 10 darks, 10 biais.

Full Frame

M17 Area – Skytools v3

Cropped Frame centered on M17

Processing:

  • MaximDL5:  darks and flats subtraction, alignment and averaging
  • Photoshop CS4: stretching, selective sharpening
  • Noise Ninja:  noise removal

I used a Bahtinov mask as a focus help on a bright star (Antares).

This is important to use a bright star when using the Bahtinov mask, to have the right in-focus diffraction pattern.

Note that at f/d 3.5 the focusing tolerance is +/- 7 Microns!  See Thierry Legault’s site on this topic.

Interestingly, having the canon not perfectly in focus elongates slightly the stars on the frame edges.

I initially thought the lens was the culprit and using it at f/d 3.5 was “too fast”.

In fact, by having an optimized focus the stars are round on the whole frame! See below – pictures are at 100% size.


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Lagoon and Trifid Nebulae: second try

Date: 8/1/2010 – San Bruno, California

Mount: Astrotrac travel system

Camera: Modified Canon XTi with Canon 200mm  f/d 2.8 teleobjective (prime lens) opened at f/d 3.5 – at Iso 400. Installed on a tripod collar ring and  a Manfrotto ball head.

32 exposures of 90 seconds with Astronomik CLS CCD clip filter, 12 flats frames, 10 darks, 10 biais.

For Flat fields I use an electroluminescent panel from Glowhut.  This is by far the best way I found to take flats that “work” in a consistent way.

This is my second try at this with the same set up. I got much better results this time…. Even though it is taken from my backyard where usually magnitude 3 stars are barely seen. Transparency was a little bit better than usual  – I would say between 3/6 and 4/6 (magnitude 3.5 stars seen at best) and M8 was quite low (below 30 deg.) so imaging was still a challenge!

The big difference with my previous posting is that my Canon XTi has been modified (Standard IR filter replaced with an astrodon IR filter by Hap Griffin) – and the response of the camera in the Red and especially HAlpha wavelength is much better…

Also this time I made sure the astrotrac polar scope had a centered reticule for better polar alignment.

I used the sane  CLS CCD anti pollution filter, same digital processing, and same exposure time as in my previous try. The Astronomik CLS CCD works wonderfully with the modified Canon.

But this time I also  used an “X-Tend a Sight mount” from Photosolve along with an Orion EZ Finder.  It really helps to find and approximatively center the objects in the canon 200mm field of view since seeing stars through the Canon XTi is almost impossible. Then I take a short shot and re-center the object.

In addition I used a Bahtinov mask as a focus help. Focusing the Canon 200mm open at f/d 3.5 is really hard: in a fraction of a turn stars get out of focus. I found the mask to be of some help in getting more consistent results (even though you do not obtain the usual diffraction patterns you observe when focusing a telescope with the mask).

Processing:

  • MaximDL5:  darks and flats subtraction, alignment and averaging
  • Photoshop CS4: stretching, selective sharpening
  • Noise Ninja:  noise removal

M8 and M20 – Canon 200mm Teleobjective and Modified XTi

Details of M8/M20- Cropped image

The following objects can be seen in the field of view: M8, M20, M21, diffuse nebula  Ngc6559, globular clusters Ngc 6544, Ngc 6553, open clusters Ngc 6530,  Ngc 6546

Skytools 3 Atlas

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