Helping me to shot andromeda

Started 2 months ago | Questions thread
OP jjuncal Junior Member • Posts: 44
Re: Helping me to shot andromeda

Hi Frenchy! Merci beaucoup for providing so much information!

frenchy01 wrote:

Good advice already from Dan Paris and 82Virago.

Here is my experience with a D850 (which is the DSLR version of your mirrorless Z7):

0.a. If you did not yet install Stellarium (free, see: http://stellarium.org/ ) or another astronomy software on your computer, please do as it is very useful to locate targets in the sky.

0.b. Check the weather conditions before going out, I like to use the «astronomy seeing» widget of Meteoblue (https://www.meteoblue.com/es/tiempo/outdoorsports/seeing/ourense_espa%c3%b1a_3114965) which gives some interesting info :

I'm using stellarium and some other apps but didn't know those ones. Thanks a lot

1. Although 200mm already shows a nice Andromeda, I would still use a somewhat larger focal length (at least 280mm f/4 if not 400mm f/5.6), losing 1 or 2 stops of light to the teleconverter is not too problematic for Andromeda as it is a quite « bright » DSO.

Following framing shows how Andromeda would frame at 200mm (outer frame) or 400mm (inner frame) on a D850 (so same sensor size and pixel as your Z7 – Stellarium allows one to define own sensors and telescope/lenses so that you can see the framing of different objects depending on the lens used):

Thanks y lot. I knew that I could include the framing on the sky on stellarium but never managed to make it happen. it will help me seeing how I'll make my composition.

2. I do not know which tracker you have, but you should be able to reach 30s to 60s at 200 to 400mm on a good tracker (f.i. iOptron Sky Guider) without guiding if the tracker has been properly aligned. This is good enough for a target like Andromeda. Of course longer exposure would reveal better the outer details, but you can compensate with more frames. Do not push the tracker too far of its limits or you will end up in throwing away more frames...

I do have the ioptron sky guider pro.

3. As already said by Dan Paris, the Z7 has a dual gain sensor with the second gain quicking in around 800iso, and furthermore it is isoless in this range (which means iso values higher than 800iso do not apply an analog gain on the data but simply a supplementary digital gain ; shooting at 800iso and increasing exposure by 1EV during raw development gives nearly exactly the same result as shooting 1600iso ; same for 800iso+2EV=3200iso...). It is actually even better to shoot at 800iso than 1600 or 3200iso, because you keep a better dynamic range (brighter objects would saturate faster at higher iso’s).

Ok. thanks a lot in bringing more infos. I'm even more aware how it's working. shooting mostly at native 64 iso. I never looked for much more infos.

4. On a night with excellent viewing conditions, 1h of data should already give very nice results (but if you can shoot 1.5h or more do it, with small trackers you will always have to drop some frames due to non round stars – wind, vibration caused by walking in the vicinity of the tripod, etc...).

Following image was made in a Bortle 4 site (in the Alps at 2050m), during a night with very good seeing, and comes from a stack of 74 pictures at 500mm f/5.6 30s 800iso (Fornax Lightrack ii tracker), so only 37min of data. In those conditions nearly no star ended up overblown, at 3200iso many would have been at RGB 255,255,255 so overblown without color.

Perfect! this is more or less the kind of final result that I want to achieve!

5. Use an astronomy software to get acquainted with the sky. Andromeda is not too difficult to find when you are at a dark site (with averted vision you can see the soft blob from the brighter galaxy center). It is not too far from the Cassiopea (W) constellation, so this is a good starting point to locate Andromeda, see image from Stellarium :

It is more difficult to frame with a long focal length, so if you have a zoom the best is to start at the lowest focal length (140mm if you use 70-200mm+TC2 or 98mm if TC1.4), and aim approximately where the galaxy should be (see image above, largest frame is 135mm). Make sure your lens is focused (manually, unless you have the moon somewhere around in which case you can autofocus on the moon and then switch to manual). Take a picture at 12800iso 15s f/5.6 (or 12800iso 8s f/4), if Andromeda is in your frame you should see it as a hazy ellipse as in following image at 200mm (obviously a bit smaller if you are at 140mm) :

Center the galaxy in the frame at 140mm (some trial and error necessary, be careful not to move too much when trying to center, not always easy with ball heads). Once this is done you can zoom in to 400mm (or the chosen focal length), check the focus and correct if necessary, set the ISO back to 800iso, set the exposure time (up to 30s with internal timer or use external timer if you want to go higher) and take a test image to check if the stars stay round (at 800iso Andromeda will appear quite faint on the image if exposure is 30s, but no worry the stacking and image processing should bring it to good levels ; if you want to better see Andromeda for this first check, use 3200iso or 6400iso but do not forget to set it to 800iso for the definitive frame grabs). Oh and of course perfectly polar align the tracker before trying to locate and put Andromeda in your frame center, you do not want to realign and restart your localizing work...

Perfect! it gives me some interesting technics like focusing on the moon although I'm planing in shooting in a night without moon. I guess I can get better results with less light in the sky.

And now go out and get used to fine tune the aligning of your tracker, finding out its limits, see if you can easily frame other targets which you'll have to locate in the sky, play with the softwares (DeepSkyStacker), search for online tutorials on how to post-process astronomy pictures, etc, etc...

P.S. : I started with 70-200mm where framing was easier (possibility to zoom out and then in), but later used 300mm f/4 and 500mm f/5.6 which were much harder to frame. I bought the Nikon DF-M1 dot-sight which really helps (once the dot alignment is calibrated/checked on a star) to bring the framing close to the right position (of course one still needs to know about where he wants to aim, so Stellarium is your good friend to find easy to identify brighter stars near to your target)

P.S.2: it will also take a great deal of post-processing, you'll need to stack the images (use f.i. DeepSkyStacker) to increase the S/N ratio, neutralize color casts, stretch the image to reveal the arms of the galaxy, increase saturation to reveal faint color, etc... For my images I did not use dark frames nor flats, I developed the raws in Photoshop with lens corrections as 16-bit TIFFs and stacked the TIFFs, this is probably not the best way but it did the trick for this target...

Yeah thanks. I'm considering adding some complementary software as it speeds up the process quite a bit. until know I've been using mostly just photoshop as you did for stacking.

I saw some videos online how people doing all the process I'll try to replicate it.

P.S.3: and be patient! Astronomy is a game of joy (getting out and admiring the night sky, seeing images appear on your computer as you are progressing in the post-processing, ...) and frustration (too windy to get good images, bad focusing, non optimal seeing...). Although I did image Andromeda 6 times (with theoretically good seeing), only 2 out of the 6 outings ended up with an image showing the faint blueish color on the external arms, all the others were brownish...

Yeah, I totally understand you. I already did some tries and I already can see how to improve. Here an example of my first try stacking last September in Greece made with the Z 14-30 f4 and stacked something like 20 frames.

There's room for improvement with the ioptron and all the knowledge you offered me.

Merci beaucoup pour ton aide et toutes ces précieuses informations.

Jose

 jjuncal's gear list:jjuncal's gear list
Fujifilm X100V Fujifilm X-T2 Nikon Z7 Nikon Z7 II Sigma 35mm F1.4 DG HSM Art +8 more
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