Deneb and NGC7000 with a 135mm f3.5 CZJ Sonnar

[MercianDabbler]

I picked up this lens for not much money a couple of weeks ago and last night was just about good enough to tempt me out to try it out on my EOS 70D. The camera was piggybacked on my SP102 so that I could use the SP mount and drive for tracking. Actually my ST80 was also piggybacked because that has the camera mount. Rather cumbersome (and now on the 'to do' list for improvement) but it worked.

I was out for a couple of hours but clarity aroud Cygnus was in very short supply. Only Jupiter and Saturn were clear for the first hour or so. In the end I got just 6 and a half minutes of usable light frames - selected manually by losing the cloud affected frames from a bigger set. All were 10s exposure at ISO3200 with the lens set to f5.6. Focussing was done using live view (with 10x magnification) using Jupiter as the target.

The 39 light frames were stacked in DSS along with 14 darks, 26 flats and 11 dark flats. The 32 bit TIFF was then edited in Darktable. It's the first time I've tried Darktable for AP (it is not an AP tool) but I use it a lot for more 'grounded' images and love the non destructive nature of the editing. Changes were confined to stretching the contrast, cropping (down to a little under 50% of the original frame) plus the haze removal and local contrast modules.

I initially stacked without any flats but there was a big splodge of dust which turned the 'Yucatan Peninsula' into a black patch so this evening I got the camera out again (I had not touched the lens) and took some flats. The flats seem to have fixed this but I suspect there will still be some loss in that area.

So what do I think?

• Firstly I'm impressed by the overall sharpness of the image for such a small financial outlay on the glass.
• There is pretty much no colour in the image (not quite none but almost). I think this is down to the lack of exposure time. Hazy sky glow probably didn't help either.
• There was so much 'stuff' in the frame and the camera was at an odd angle (the scope was almost vertical) so it took me over an hour to find my bearings around the image. I was even doubting that the bright star was Deneb for a while, even though I knew where the camera was pointing.
• The diffraction spikes around Deneb are pretty spectacular. For the less bright stars I dont find them too unattractive. I'm not sure if using wider or narrower apertures would help to tone these down in future?

 

 

[powerlord]

you can see North America nebula and pelican. The lens is clearly nice and sharp. What was the aperture shut down to ?

stu

[MercianDabbler]

57 minutes ago, powerlord said:

you can see North America nebula and pelican. The lens is clearly nice and sharp. What was the aperture shut down to ?

stu

Thanks. Lens aperture ring was on f5.6... Conveniently positioned at the bottom when mounted on my camera I guess that's a problem with non bayonet mount lenses.

[powerlord]

haha. yeh, My current count of 'vintage' lenses is around 7 I think - M42s and a few pentax. I need to start using them more - so much to do !

stu

[alacant]

Hi

Yes, the Zeiss 135 is great value. Nice shot.

On our example, 'infinity' for astro is as indicated. You should also observe a red halo when you've gone too far toward the lens stop. The best way to do this is live view on a decent resolution screen. Your 'phone for example.

eos70d? Lose the dark frames.

I don't use DSS but are you sure you debayered the frames before stacking? 

Cheers and HTH

[MercianDabbler]

1 hour ago, alacant said:

Yes, the Zeiss 135 is great value. Nice shot.

On our example, 'infinity' for astro is as indicated. You should also observe a red halo when you've gone too far toward the lens stop. The best way to do this is live view on a decent resolution screen. Your 'phone for example.

eos70d? Lose the dark frames.

I don't use DSS but are you sure you debayered the frames before stacking?

Thanks. My astro focus is in about the same place as yours. Better to have it slightly short of the stop than slightly beyond it I suppose.

I have some more checking to do and I really need to practice again with my tablet controlling the camera over WiFi, I have had it working indoors in daylight but so far have not had it connected out in the dark. No particular issues but it's always easier to stick to what I've done before when I'm at the scope.

I'm usually somewhat dubious about the value of the various calibration frames but that may be because I rarely end up with as much exposure time as other folks. TBH the darks are not too much of a pain to get - I usually take them while I am packing up so I usually do them 'just in case'. After this photo I have now seen the value of flat frames.

I'll have another play with DSS. It does have multiple debayering options and I have messed with it in the past but I think it is currently set to something 'sane'.

... but mainly I need to reduce the amount of hardware needed to get the camera to track. In the short term sorting out the mounting direct to the SP102 should be very little effort but I think it would be even better to attach the ST80 direct to the SP mount so that I can avoid carting the SP102 in and out if I am doing wide field AP. The ST80 is a better 'finder' than the 102.

[MercianDabbler]

14 hours ago, MercianDabbler said:

I'll have another play with DSS.

Did that...

Checking the settings I didn't find anything that was obviously losing colour info.

I exported the image with the adjustments embedded (i.e. not applied). I then applied the history stack from the previous image in Darktable. I had to alter the stretching because the TIFF was not stretched by DSS. This produced a similar result but with an obvious yellow cast.

I then tried a different debayer algorithm - superpixel. This reduced the image size because the debayering combines multiple pixels into one. The result of this is below. There is definitely colour in it but it's mostly either an overall cast or else noise. The star diffraction patterns also seem to look oddly banded at some degrees of zoom. I don't think it is an improvement.

I still think that the lack of colour in the earlier stack happens because of the short exposure time rather than being something wrong in DSS.

I think DSS is applying a colour balance on output with adjustments applied because the histogram shown by DSS has the red peak shifted to the right. With the adjustments embedded there is no colour manipulation by DSS and the differing position of the histogram peaks shows through as a colour cast.

Whether the colour adjustment done by DSS is a good thing or not... I am undecided.

I shall revisit NGC 7000 when another opportunity arises and hopefully get a good deal more data. Before that I plan to do some more tests with the Sonnar lens to see how the star diffraction patterns change with aperture - it shouldn't need a major stacking exercise to figure that out.