Yesterday, during full moon, I practised my polar aligment method. The classic drift method but using a webcam behind my 550mm refractor increases accuracy even more.

After the polar aligment which took me 30-60 min, the resulting DEC drift was really small.  I did a superlong DEC drift measurement of more than 8hours, while sleeping. The total DEC drift was 80", about 0.25"/min. Useless, as RA guiding error caused by periodic error is close to 40" in 10min. But fun nonetheless.

And the first time I combined exposures from two nights. The first night I took H-alpha exposures and the other night I took RGB exposures, both with the EOS20D and the Takumar 135mm. I used my 550mm refractor with the DSI camera as guiding scope for the first time. This worked very well.

Here is the resulting image and more info: http://www.astro.pwng.nl/item/north-america-nebula-in-hargb

This guiding scope is more heavy than my previous  guiding "scope", at 70-200mm zoom lense. So there is the risk of too high load of the mount resulting in vibrations during long exposures.

All went fine. The weight is OK, I managed to shoot 20 lights with 5min exposure time without any star drift.

Here is the result: http://www.astro.pwng.nl/item/andromeda-nebula-2

So I came up with this idea to reach excellent polar aligment within 5 minutes: I glued a laser diode to the hour axis of my Vixen GP mount. This laser beam is about horizontal when the mount is polar aligned. The position of the tripod is fixed with a little hole in my terrase that hold one leg of my tripod. <add some photo's>

Then I do the best possible polar aligment possible. I have spend two evenings using the well-known drift method to reach near perfect polar aligment. I think I have obtained a  DEC drift  less than 0.25" /min. The Polar alignment error is less than 1'. This is much better than what is possible with a polar axis finder scope. is Now I mark the laser dot on a wall 6 meters away.

In subsequent sessions all I have to do is put the tripod in its fixed position using the hole for the single leg and then adjust the Azimuth and Height adjustment knobs of the mount to aim the laser to the marked location.

I will report the accuracy I obtain using this method.

Unfortunately I had to discard half of them during stacking because 17 had unacceptable drift due to guiding errors. Adding these frames to the stack would have made the end result worse.

In fact during the night I switched from 10 min subframes to 5 min subframes because I saw guiding errors in most of the 10min frames. So I need to improve my guiding quality. This can be done by doing better polar alignment, using a longer focal length for my guide scope of by tweaking the parameters of PHD. But I expect the best improvement by using a PCMCIA-USB adapter on my Thinkpad T30. This will increase the USB speed from USB-1 to USB-2. At the moment, with USB-1 speed, PHD looses the guiding signal from the DSI guide camera for a few seconds while a frame is downloaded from the DSLR camera. When the guide signal is back, PHD overshoots and lacks accuracy during 10-sec-3min after the frame download. This sometimes leads to a guiding error that is just visible in the next subexposure.

Here is the end result of my long exposure record:  http://www.astro.pwng.nl/item/leo-triplet

BTW 1: apart from the guiding problem, when looking at the histogram values, 10-12min@ISO800 seems to be about the max for the combination of this telescope (f/6.9), camera (modded 20D), and my local skyglow conditions (SQM-L 20.5).

BTW 2: My new 12V adapter for the Canon 20D worked great. It worked from 21h-3h without an accu reload.

After viewing a few examples, these are not very appealing pictures, and in general one converts these H-alpha pictures to black-and-white which give nice shades of grey. Here is my first result with one 15 min exposure and a 135mm lense:

http://www.astro.pwng.nl/item/rosette-nebula

Another technique that uses H-alpha frames is the so-called HaRGB method. This combines H-alpha frames with one-shot colour RGB frames and this will enhance the contrast and quality of the Red channel.
I will need much more study and experimenting to master this technique.  Here is some literature to start with:
http://starizona.com/acb/ccd/software/ps_hargb.aspx
http://www.robgendlerastropics.com/HARGB.html
http://www.robgendlerastropics.com/LRGB.html
http://www.mlunsold.com/process/HaRGBCombine/HaRGB.html

More to follow..

After a switch to an EOS 350D I couldn’t use it anymore because this camera type uses a different battery type. My 12V adapter was in a drawer for a year. Now I have an EOS20D and I can use the adapter again.

But it does not work well. After 1-3min the camera switches itself off with ERR99 and resets itself. I can use it again after removing and reinserting the adapter. For several nights I blamed a voltage drop of the power supply during use. I tested the voltage today during use and it is OK.
The EOS20D works without a problem with a normal battery and I don’t see the problem with my 300D.

I did notice that the EOS 20D has a higher power consumption than the 300D (during a long exposure: 420mA for the 20D versus 220mA with the 300D). Now I think the camera switches off  because it is getting overheated by the adapter.  I noticed Astronomiser changed the AST300DDC3CPro converter by adding a separate voltage converter box. I  think this will be the solution for my problem.

[edit 26-feb]. My AST300DDC3CPro power converter for EOS 20D has arrived. The converter is in a seperate little box outside the camera. That should keep the camera cooler. I did a few long exposures of 1h or more. In the 3rd exposure I had another ERR99 again! I think I  can blame the 7Ah 12V battery. I will recharge it again and will test if 10x1h is possible.

[edit 28-feb] During 6 hours I used my camera with the 12V adapter with exposures of 10-30min. Not a single problem.  The amp glow is much less than with an in-camera battery. Up to 20min exposures I can hardly see amp-glow in the lower right corner. And even on 30min exposures the upper right amp-glow is absent.

 http://zalmstra.nl/astro/sub-exposure%20calculator.xls

The spreadsheet uses measured SQM-L (Sky Quality) values if you have them, but you can do without.

This is just a first usable version. It needs some more calibration and I want to include filter support.

To put the discussion about optimal sub-exposure in perspective, here is a quote  from an email I received from Kharusi about this subject: "Most beginners under-estimate how much impact itegration time makes. Worrying about the length of subs is trivial compared to the impact that shooting more integration time will make."

His advises:

• do not use longer sub-exposures than advised from this calculator. It might lead to worse results caused by seeing, sattelites, planes, guiding errors, saturation.
• Do not overestimate the advantage of high ISO's. In general 1600 is ok, but go back to 800 or even 400 to prevent overexposure. (Lodriguss advises to use 800 en 400 at low temperatures)
• maximize total integration time (number of frames x sub-exposure time). Consider to combine sub-exposures from 2 or more nights.

In (very short) summary: One can improve the SNR by using longer exposures and by using more exposures.
I want to find out what is the optimal sub-exposure time (=time per frame) in my situation, using a DSLR with different optics under fairly good Dutch skies. To me it seems the longer the better, up to a certain limit, and I want to find this limit. I think there are two limits on the maximum exposure time:

1. Guiding.
2. Sky fog

This blog is about the 2nd limit. Long perfect guiding is not easy, especially with long focal length. But when imaging with a short 135mm lens I have mastered this and I can autoguide for any length of time using PHD and a 200mm lens with a DSI guiding camera.

The max exposure time due to skyfog is affected by ISO value and by focal ratio. Doubling ISO from 800 to 1600 halves the max exposure time. Doubling the focal ratio from f/3.5 to f/7 quadruples (4x) the max exposure time. So when I know the max exposure time for one combination of ISO value and focal ratio I can calculate the max exposure time for any other combination.

I tried 5 and 8 min ISO 1600 exposures of M45, The Pleiades, with my 135mm f/3.5 lens on the EOS20d and hope to find out if I reached the maximum exposure.

To begin with: I need much more knowledge of the theory behind deepsky astrophotography. Apart from the above mentioned article series of Stark Labs I have another source that will help my knowledge: optimum exposures from Starizona. In it, there is a link to another in-depth and instructive discussion from Hiddenloft about optimal exposure times. Both sources mention the characteristics of popular CCD camera's. Many recommendations depend on these camera characteristics, but they are not well known for DSLR camera's. Here I found the characteristics of my EOS 20D camera.

There is soo much information on the internet about deepsky astrophotography and not all of it is equally true or instructive.  I am a bit overwhelmed by the complexity of the subject. And sometimes even experienced astrophotographers that made a big study of this, seem to concur about essential conclusions. Just Google up "optimal sub-exposure" and start reading....

One important finding up to now after reading some of the theory: apart from a maximum exposure time determined by sky-fog (which is quite high, as I found out), there is the more important minimum exposure time. This minimum is determined by the read-out noise of the camera and it is affected by the amount of sky fog reaching your sensor, so it depends on skyfog itself (more skyfog, lower minimum), the ISO value of my DSLR and the focal ratio of my optics (faster optics, lower minimum).

Several astrophotographers agree that, as long as this minimum exposure Tmin is met, there is  not a big SNR difference between N frames of Tmin and one frame of N*Tmin exposure wich will take equal amount of valuable imaging time.  Apart from that, using one or few long exposures has a few disadvantages over many short ones:

• it is harder to guide error-free for a long time. 
• Seeing-effects on the atmosphere may incur the sharpness of the image when using longer sub-exposures 
• there is more chance of a problem with an sattellite or clouds ruining your subframe.
• the minimum pixel value will be higher  so you will lose some dynamic range.
• There is the risk of clipping in bright parts of the objects. For instance you can lose usefull colour information in bright stars.
• In addition, DSLR's compared to (cooled) CCD camera's can add another source of noise (dark current) during long exposures, getting more problemous when the camera is heated during long exposures.

This all favors short exposures as long as the mimimum Tmin is met.

After reading the excellent weblog of Samir Kharusi  on this subject (he states that, given the total imaging time, the minimum sub-exposure Tmin is as good as any longer sub-exposure time  http://www.samirkharusi.net/sub-exposures.html)  and after some email discussion with him about this SSL-regime I decided to skip searching for maximum sub-exposure time and instead: 

• to do tests to determine this minimum exposure time following Mr Kharusi SSL regime, depending on local conditions and the camera&optics used. His weblog entry  http://www.pbase.com/samirkharusi/image/37608572 shows how to use an SQM-meter (wich I have) or short testframes to determine this minimum.
• investigate if my results improve  when using longer sub-exposures without changing total imaging time. If mr Kharusi is right, they will not!
• I will do tests with and without my CLS clip-in filter. This skyfog discussion has wetted my appetite to retry this sky-fog CLS filter!

I will report results from these test, which will take several weeks (considering local cloud conditions) in another post.