Hello! I read the Sky and Telescope review of ZWO's new camera - ASI120MM - and it came with a 2.1mm wide angle lens that covers up to 150 degrees.
I had a strange thought. Could such a set up be used for for bright star photometry by pointing the camera straight up and taking images periodically without the need to direct the camera to different star fields?
The review article stated that the visible stars were captured on the test of the wide-angle lens. I scanned some other websites, and I wonder if the magnitude limit might be 8 or so, perhaps more with longer integration times?
When I checked the FOV with the 12D string FOV calculator, with 3.75mm pixels and the 2.1mm lens at f2.0, the coverage is 4.58'/pixel and the chip coverage 98 x 81 degrees (I'm not sure why this differes from the 150 degrees from the commpany).
Bright star photometry needs defocused stars, so defocus to FWHM of 10' would be reasonable? Additionally, long integration times are possible with the ZWO, though it is uncooled, so frequent dark frames would be needed.
With flats and darks, could this setup perform bright star photometry without the need to direct the camera? Simply point it straight up and take images when the star fields are above 30 degrees? Thank you and best regards.
Mike
It doesn't seem we could expect a high precission from this setup, but it's very simple and does not require a telescope and mount.
I expect to make some tests soon with a camera and short c mount lens. At the first time I plan to develop a wide field sky monitor, but it could be fun and useful to be able to measure bright stars too.
Hi Mike, interesting question !
The first issue is the very small photon flux that lens could get. The aperture is only 1 mm² instead, for example near 2000 mm² for my usual 200mm F4 lens on APS-C. This means a factor 2000 ! or 8.25 mag.
I think a survey of bright stars is only interesting if a good accuracy is achieved (some bright stars expert advise here ? ).
With the 200 mm lens I can achieve a good accuracy, let say ~ 0.01 mag, for mag 6~7 stars in 10 x 10 sec. without tracking and by the way some trail. We could increase the unit exposure time with a 2 mm lens, maybe 100x would be too much giving the fact the sensor is much smaller ! (blending issue). If we do 10 x 100 sec that means a gain of 2.5 mag. Then we could expect a better scintillation integration and a true gain of 3.3 mag. This leaves at mag 1~2 ... Not a lot of stars.
As you mention the dark currents would become critical but the most difficult is probably to get a good flat for such lens. We should remember the flat made with large diffuser near the lens is not a good match for the vignetting of the collimated light from the stars, and the vignetting of such lens is probably terrible ! Another aspect is such lens have usually a strong geometry distortion, this seriousely affects the flat too...
I think refining such large FOV technique would need a lot of work ! ( and fun ? )
Clear Skies !
Roger (PROC)
Pointed stright up, it would range from an airmass of 1 to 3.8! You had better be able to calculate your extinction coefficients (in each color) precisely and apply them through some weird flat field technique! Ought to be able to do that theoretically, but who would believe it?
It would certainly require lots of testing. What would happen if someone put their thumb print on the lens?
Hello! Thank you for the insights. I thought that the binocular variables might be a good list. The comps are about 10 degrees apart, so the air mass and field curvature effects, etc., might be small enough so that the effects might be able to be ignored.
Of course, that would leave a lot of field unused, but I thought that there would probably be 1 to 3 binocular variables rising into the 130 degree field each hour, so taking a series of integrations each hour for stacking and use the rest of the hour to take lots of darks? I'm not sure how many would be needed to get a the error range to the acceptable level, but it may be too many. That would give 7 or more images, each with 1 or 3 bright binoculars on it?
I had not thought of the flat problem with such field curvature. Would this be an effect that would need to be considered if only 10 degrees of the field is used? How might the flat problem be addressed? Thank you and best regards.
Mike
Perhaps it could be done this way:
First take an image a uniformly lit overcast sky, with clouds so thick that you can't tell where the sun (or gibous moon) is in the sky and use that for a flat.
Get a (min 1 inch) thick, very light gray, flat, transparent, acrylic plastic plate. Take a second exposure through it the same overcast sky.
Remove image defects using the first flat. Measure the ratio at the zenith of the images to determine how optically thick the plastic plate is. The image should get progressively dimmer towards the edges as light travels through more and more plastic (just as what happens through the atmosphere). Now, scale that second image to remove the extinction for the conditions encountered on the night you want. You'd need to have a library of many (say 50) airmass correction images to counter the extinction. If the ratio at the zenith was 0.5 (representing an extinction coefficient, you'd need to multiply the flatted "extinction correction" image by xxxx to get a correction