From: Ray Byrne <ray@in4media.co.uk>

Date: February 24, 2010 10:51:03 AM MST

To: Astro_IIDC@yahoogroups.com

Subject: Re: [Astro_IIDC] REPORT: Experiment with a pretty big focal length (C11 @ f.92 = 26 meters)

Hi Alberto,

I read it all and found it fascinating!

On 24 Feb 2010, at 15:44, albe albo wrote:

Hello to all,

some days before the 74" telescope image published by Jim,    i did my BIG experiment pushing  my new C11 at f. 92  .

http://tech.groups.yahoo.com/group/Astro_IIDC/files/Planetary/100207-BigMars-Mayer.jpg 

I know that i'm out of the theoretical resolution power for that OTA but i noticed that many amateur astronomers went beyond their OTA limit perhaps because of the multi image stacking that in the latest years changed the methods of creating lunar and planetary images freezing and then averaging the seeing deformations.

Perhaps we are "exploiting" the seeing which could behave like an additional lens modifying the characteristic of the OTA.

Even reading the book written by  Mobberley i noticed that he had the suspect about the different limits in webcam astronomy.

Considering the defects of my image (noise and blurriness) i was pleasantly surprised by it because there are lot of obstacles with such focal length.

1) The camera worked at 15 fps but the gain was almost at the upper limit for each channel with very high levels of  noise. I didn't want to lower the fps at 7.5. In addition my DMK31 shows a lot of diagonal banding when it's used at 15 fps with a very high gain.

2) even if the camera gain was at max the level of the CCD  illumination is still low.

3) With such magnifications i guess that even the eyepiece that i used for the the projection becomes a very critical component of the optical chain  because of its extreme usage  (9mm orthoscopic by University Optics).

4) Another serious problem is that it is almost impossible to perform a decent automatic selection between the good and the bad frames even if some good frames are present!  Often i find very good frames alternated with useless blobs both classified  at the top of the "sharpest frames". Since I took 3400 image x channel it's hard to select them manually!

5) an unexpected problem was represented by the big size of the planet that clearly shows the rotation more than smaller size not allowing very long sessions of IR-R-G-B sequences. In this case from the beginning to the end the sequence took 20 minutes!

Note: for this sequence I did an additional mistake that usually doesn't happens. I took this sequence as R-G-B-IR while usually i do IR-R-G-B. For Mars it is mandatory that IR & R must be very similar while in this case i noticed a big rotation between R (the first) and IR (the last)

All those factors together afflict such kind of experiments not allowing a "fair" comparison with shorter focals.

I was wondering and dreaming about a camera that could grab at 60 FPS even with such low condition of light but keeping a low noise but i guess that such camera don't exist or it is veeery expensive.

Example: being able to capture this huge disk with the same speed and sensitivity of a f. 20 (60 fps&low noise) i guess that the sharpness would increase and the noise would decrease a lot allowing better processing and a better evaluation.

Perhaps even using a more sophisticated "magnifier" (barlow, ocular, powermate, etc) could help for a better result.

Unfortunately i don't know a (cheap) camera that could perform such ideal sensitivity and low noise so... for the moment i must postpone this verification.

Nobody else experimented the TIS diagonal banding?

I'm in touch with TIS engineers and i sent them some stuff.

I noticed the banding only at 15 fps. with high gain (not at 7.5 or 3,75 or 30)

Such banding shift across the screen so that if you stack the frames without aligning them the banding seems to cancel itself while the banding itself seems to be used like an "hook" to align the images otherwise after the alignment process it should statistically disappear but, on the contrary, often it becomes more visible.

That's all for the moment (did someone resist to read all that?     )

Cheers

Alberto

http://tech.groups.yahoo.com/group/Astro_IIDC/files/Planetary/100207-BigMars-Mayer.jpg