Archive for September, 2010
Another post on my 2 week escape to clear skies – Abiquiu, NM … I probably produced as many pictures in two weeks as in 6 months in the San Francisco Bay Area! Another aspect of doing Astrophotography in a Urban environment is that you need to go out very far to access clear skies.. And the Astrotrac travel system helped there – I was able to easily carry the equipment on my flight to New Mexico!
I captured these two great galaxies with two different cameras: A modified Canon XTi for m31, and a Qhy8 CCD for m33, both cameras using my Canon 200mm L lens and the astrotrac travel system. Note the Qhy8 has the same sensor as the Orion Starshoot Pro V2 ( Sony ICX413AQ).
M31 with Canon XTi and Canon 200mm L
This is a cropped picture. It is a combination of 13 frames of 3 minutes at f/d 3.5.
Stars on the image are round till the edge (on the uncropped images) which is a sign of perfect focus (with my Canon 200mm L lens).
M33 with Qhy8 and Canon 200mm Lens
This is a cropped picture.
This is a combination of 19 frames of 3 minutes at f/d 2.8, taken with the Qhy8 CCD Camera.
Comparison between the Canon XTi and Qhy8 for Wide Field imaging: my personal opinion
Having taken a couple of pictures with the Canon XTi and Qhy8 CCD using the same lens (Canon 200mm) I’d like at this stage to do a comparison between the two approaches.
This comparison assumes the CCD or Canon are used with Teleobjectives (not a telescope), and the CCD is a color one, not a black and white …
Mobility: Advantage to the Canon XTi … Non need to plug to a power cord, no power converter… Which is really in the philosophy of the Astrotrac
Filter use: Advantage to the Canon … You can use clip filters in front of the sensor (HAlpha, Anti-pollution – Astronomik) – I cannot see how to use Filters with the CCD given the back focus. The Filter has to be in front of the objective, which is (more) expensive and cumbersome.
Sensitivity and Signal to noise ratio: Advantage to the Qhy8 on a couple of points
- a) On Red/HAlpha better sensitivity (even though the Modified Canon should be very close).
- b) Larger chip pixel size on the Qhy8 which leads to better overall sensitivity (7.8 microns for the Qhy8, and about 6 for the Canon XTi)
- c) Better Signal to noise ratio with CCD Cooling (for the Qhy8 chip which is already a very low noise one even without cooling)
For faint emission nebula there is an advantage with the CCD – for Galaxy / Reflection nebula I think the difference is minimal (as the pictures above show it).
Integration with Lens: Advantage to the Canon – I did not find a way to control the f/d ratio when the Canon Teleobjective is not connected to the Canon Body. With the CCD I had to use the Lens at F/D 2.8. It is not a big deal if the lens is excellent – but it is a problem if you need to stop the aperture. You have to use a mask in front of the lens in this case.
Focus: great live focus support with the Qhy8 and software shipped with the CCD. I don’t have the latest Canon body with the “Live view” function – but the CCD software is really designed for a very precise live focus and display in real time of the Full Width Half Maximum (FWHM).
Versatility: Advantage to the Canon XTi – I can (and did) use my modified canon for daylight pictures. which is another way to travel light: one camera for everything!
Ease of Use: Advantage to the Canon XTi. However the Qhy8 CCD – as many Color CCD cameras – is quite simple to operate (vs. Back and White ones with filter wheels).
As a summary, for Wide Field Astrophotography with Teleobjectives and the Astrotrac: I don’t think the kind of pictures you do with a color CCD camera such the Qhy8 are drastically better compared with a modified Canon XTi (not modified is another story). Given the versatility of the Canon DSLR body I think it is still a good choice for Wide field Astrophotography – if a choice has to be made between a CCD and a DSLR camera.
This said if you already have a CCD camera similar to the Qhy8 (as I do) – then using it can be a tool of choice for wide fields that include faint emission nebulae (e.g. in Cygnus). The overall sensitivity is I think better compared with the Canon XTi sensor (without any doubt for a non modified DSLR), and as a bonus it allows a very precise control of the focus. As an illustration, I took this picture of Ngc 7000 with the Qhy8 CCD …
Another post on the pictures I took during my vacation in Abiquiu, NM – to escape the light dome of the San Francisco Bay area.
Pictures taken with the Astrotrac Travel system, without autoguiding. For perfecting the North Pole alignment, I used the DSLR Logger Shareware.
The Lens I used in this post were the Canon 50mm f/d 2 – a cheap lens – but doing a nice job when in focus… And the Canon 200mm L – a great lens for Astrophotography.
The Camera was my modified Canon XTi. I took other pictures (see previous post) with my Qhy8 CCD camera.
Barnard’s Loop with Canon 50mm and Canon XTi
Orion was low on the horizon, even in the morning, but it was hard for me to resist taking shots of the great nebula in such a beautiful sky.
I was not able to see Barnard’s loop with my naked eye but shooting it was fairly easy.
Pictures were taken with my modified Canon XTi and Astrotrac travel system. The modified camera certainly does an easier job in capturing the light of this faint emission nebula.
This is a combination of 6 exposures of 5 minutes.
The Great Nebulae,horse head and Flame nebulae with the Canon 200mm and Canon XTi
This is an overall picture of the horsehead and m42 region with the Canon 200m L lens at f/d 3.5.
Even if the pictures are not perfect on the corner I found one more time the performance of the teleobjective very impressive.
Focus might have been slightly better since I have some level of elongated stars on the corner – which is I found a symptom seen when the focus is very slightly off. But having a focus with the DSLR at this high speed focal ratio is challenging…
This is a combination of 18 exposures of 3 minutes with Dark frame, Biais, and Flat frame reduction.
Note the nebulosity below the horsehead and m42 are not light pollution but rather extensions reaching the barnard’s loop….
Disturbing at first for somebody like me who is used to take shot from light polluted sites -and who is usually dealing with gradient from the light pollution!
This is a cropped picture of M42 – even cropped the image reveals lots of details.
Notice the fine details of the Nebulosity extensions on the left between the great nebula and the running man nebula…
Last, the Horsehead and Flame Nebulae … still crop of the same picture.
I was able to escape from the Bay area light dome and spent two weeks in Abiquiu, NM, where I used extensively the Astrotrac Travel System with a set of good Canon teleobjectives and my Takahashi fs60-c.
I’ll post most of the pictures I took – but I was very satisfied with the performance of the Astrotrac for unguided wide field photography.
Photographies were taken either with a modified Canon XTi, or with a Qhy8 CCD coupled to the Teleobjectives.
Here is NGC 7000 and the Pelican Nebulae with the Canon 200mm f/d 2.8 L Teleobjective and the Qhy8 CCD camera.
It is a combination of 27 exposures of 2 minutes, processed with Dark, biais, and Flat frames.
When coupled with the CCD it is not possible to control the F/D ratio of the Teleobjective, but at f/d 2.8 the lens performance is still honorable and shows sharp stars.
I was able to observe the Qhy8 does a slightly better job in capturing faint details of the nebulae. I suspect it is due to the pixel size of the Qhy8 sensor (7.8 microns). This is about 50% larger in area compared to the Canon XTi sensor, and captures in theory 50% more photons by pixel in the same exposure time for extended objects like nebulae.
Below is the cropped image showing details of the Anerica and Pelican Nebulae. As you can see the image is still full of details when magnified.
Combining 27 frames contributes a lot in increasing the signal/noise ratio. Focus has been done using the real time view of the CCD, and here again advantage for the Qhy8 compared with the Canon XTi. Focus is done by looking in real time at the Full Width Half Maximum measurement on a bright star – and allows a very precise focusing – which is not a small feat at f/d 2.8.