Hello to all,
I am an amateur astronomer (Ischia - Italy) and I am very happy with the birth of this new section dedicated to the research of novae.
I'm a fan of this field of research and for some years I dedicate myself to photographic research of novae.
I use a DSLR camera Canon 600D with lens Canon EF-S 18mm-55mm f/3.5 - 5.6 IS Zoom, mounted in parallel to the telescope on Eq6 Pro mount.
For my photographic research I use the maximum focal length 55mm f/5.6 with 60 seconds of exposure and ISO 800.
With this configuration I take large areas of the Milky Way 24x16 degrees down to magnitude 10.5-11.
Recently, however, I was thinking of buying a fixed focal length lens because everyone says that these lens are better than zoom. However, I am very undecided whether to take it and especially what to take.
First, I am undecided whether to take a 50mm lens or one with greater focal lenght, for example 85mm or 100mm. If I increase the focal length then I have to take many more images to cover the areas of research but, probably, I have a higher resolution and could increase magnitude limit.
However, if I decide to stay on the focal length of 50mm, I wonder if it's worth actually get one fixed focal length or continue with my zoom lens.
You who are definitely more experienced than me, can you give me some advice?
Best regards and clear skies.
Luigi
Hi Luigi,
I use old Nikkor lenses with longer focal, mostly a 200mm F4 and also a 135mm F2.8 and a 85 mm F1.8. They are available at surplus for a low cost ( 100 Euros) and they are usable with a Canon body thanks to a small adapter ring. What is important is the fact those lenses offer a much larger aperture than most zooms and short focal, they have a very accurate helicoidal focus ring with which it's easy to recover a marked (de)focus point, the geometry distortion is very low, low aberations, much more uniform images and defocus...
The aperture surface of the 55mm at 5.6 is 76 mm², the 200mm at F4 is 1963 mm² !! This is 26 times more photons and electrons. With it you could work at ISO 100 and have a very large dynamic that is 8 times what you have at ISO 800, and more, a very high SNR. At ISO 200 you get a full dynamic and the full sensitivity ( but leser SNR) as single electron/photons are recorded ( one photon/electron per ADU at ISO 200). The 135mm F2.8 has the same aperture surface, it's ok. The 85mm at F1.8 has the same aperture too. But it's difficult to use it at F1.8 due to the high vignetting and more aberrations at the image periphery, the defocus is not uniform enough. I am obliged to stop it at F2.8.
There are number of problems at short focal length in addition to the small aperture, the sky background is much higher, it's very difficult to make flats with a good accuracy, they need a software correction of the Lambertian law, the geometry distortion is significant...
On accuracy point it's much better to use long focal like a 200 mm on APS-C . But ok if you are interested in just detecting novae another compromise is maybe to use an 85 ~100 mm. But remember you will access a much higher dynamic and mag range with a long focal, large aperture and lower ISO, in a much darker sky.
Clear Skies !
Roger
thanks for your
Hi Roger,
thanks for your help.
I would buy the lens Canon EF 85mm f1.8 USM.
What do you think about this lens?
Here I find a little recension for his use in astrofotography.
http://www.astrosurf.com/comolli/strum29.htm
Hi Luigi,
Certainly a good choice for your nova search. This lens is known being a very good performance/price compromise. The vignetting at full aperture on a 600D reduces the transmission to about 66% from the center one. The geometric distortion is about 0.1%, no issue. Probably possible to use it at full aperture, but the flat is difficult to make at such vignetting level. Stopped at 2.8 it will be much easier, depends the accuracy you are looking for.
Have fun !
Clear Skies !
Roger
Hi Roger,
thank you very much. I'm planning seriously to take the 85mm lens!
But I have another question.
In my photographic nova research I never took any flat or dark. I only take two images of each sky area, a shifted slightly relative to each other (10 arcsec). I do this to check for hot pixels or cosmic rays.
I have never felt the need to take flat or dark.
Do you think that they are important in the search for novae?
Regards and clear skies.
Luigi
Just to clarify: So
Hi
Just to clarify: So your focus is mainly on detection/discovery, not light curves of known objects, right?
Cheers
HBE
Yes, Bikeman, only detection/discovery of novae.
In that case, I don't see a strong argument in favor of using flat fields or darks, also because the total exposure time is so short.
Cheers
HBE
Hi Luigi, Heinz-Bernd,
If you use jpg files the 600D applies it's own synthetic flat when the lens and settings are known (Canon lenses).
If you use the RAW image output files and the 85mm at full aperture it's probably easier to visualize and compare the images without the vignetting. The 85mm at F1.8 has a strong vignetting (even if well manageable) and it could be difficult to compare images in this case. But ok, you don't need an accurate flat, make one and use it for months of observation.
Normally there is no need for dark with a 600D CMOS sensor upto about 60 s of exposure under 20°C ambient or below, in a single image or a stack with shifting.
Anyhow it's possible to use a "master dark" technique that avoids to take any dark during the observation. Many believe it's not possible with DSLR but it's not true. First, DSLR have temperature probes into the sensor micro-chip that are usable and, second, it's alternatively easier to directly measure the dark currents impulses into the sky images and then rescale the master-dark to it before subtraction. This second solution is far the most accurate of all (exists in IRIS).
Clear Skies !
Roger
thank you
Hi Roger, Bikeman,
thank you for your advice.
I have purchased the Canon 85mm f1.8 lens and in these days it should arrive.
As soon as possible I will try to make some tests and I will keep you informed of the results.
Regards and clear skies.
Luigi
I have
That would be nice,
I have decided to replace (or maybe supplement) my ole Olympus E420 with a Canon EOS 1100D. I can adapt my legacy Pentax K mount 50 mm lens to the EOS , but not my Four-Thirds 85l mm lens (some Korean no-frills model), so that one would need replacement as well. I'm curious how well your new 85 mm lens will perform.
CS
HBE
I just
Hi Bikeman,
I just purchased the Canon 85mm f1.8 and in these days I'm doing some tests. As soon as possible I post here the results.
Regards and clear skies.
Luigi
I have tested my Canon
Hi,
I have tested my Canon 85mm f1.8 lens in two nights, taking photos with different values of aperture, ISO and exposure time. I must say that I am very pleased with the performance of this lens. The image quality is very good. The stars at the edges of the field are perfect until about f3.5. For large apertures (f3.2-f1.8), the quality at the edges of the image degrades a little, but it always keeps on acceptable values.
Anyway, for my nova search, I decided to use f3.5 aperture, because, from the area where I make observations, the sky isn't very dark and there is always a certain light pollution. For more large apertures the sky background becomes very clear.
At f3.5, ISO 800, exposure time 60s, I reach the visual magnitude of about 13.5 - 14. Compared to my previous setup (Canon lens 18-55mm at 55mm f5.6), I increased the limiting magnitude of about 3, and also the image quality is improved significantly.
In attachment there are two images of the same star field made with both lenses at ISO 800, exposure time 60s. The brightest star is Sadr (gamma Cyg).
Regards and clear skies
Luigi
Impressive comparison, thanks for the info.
CS
HB