I was tooling around the other night with my AP set up and wanted to see if I could find some other cool things of which to take photos...
I found my goto was a bit wonky so I manually locked onto Aldebaran and then synched the scope using Stellariumscope and then clicked on the Rosette Nebula and whoosh, off she went there. I double checked using Betelgeuse and the camera exposure confirmed I was on target, so I went back to Rosette and took this quick snap....(25 secs at ISO 1600).
I wanted to make sure I was on target and couldnt seem to find any nebulosity.....
Questions:
1) IS this the central cluster? It sure looks like it when I compare the stars to Stellarium and Mike Sidonio's core on IOTW.
2) Assuming I am in the right spot....How come I cannot see ANY nebulosity even on a relatively short 25 sec @ ISO 1600...? Is it a VERY diffuse nebula or is it predominately Hydrogen Alpha which my camera cant do....?
I took a similar shot of M1 - Crab Nebula just next door to this and I could see the blob, so I am assuming that my theory in 2) above applies and its simply too dim to capture anything with such a short exposure?
Hope I am not going loopy and shooting the wrong thing, but Flame, M42 and as I said, M1 all stopped on the money using Stellarium scope, central to the sensor and something the camera confirmed each time...
PS. I have attached a 15 sec ISO 1600 snap of M1 as a comparison in the second photo....
Am I right?
EDIT: Hmmmm, Astrometry.net result of the plate solve shows it IS the core of the Rosette, sounds like I might be right regarding the nebula? So the original question changes to can anyone confirm just how dim this nebula is with a standard DSLR?
I have added the astrometry.net result image also...
Cheers
Chris
Last edited by Screwdriverone; 16-01-2012 at 11:07 AM.
Reason: Added M1 test snap
Well, that's only a 15 sec shot....you wouldn't expect too much to show of the neb. Maybe push it to 60 secs and some neb will begin to come out. Remember, it's a large neb, about 130ly across and is more than twice the size of the full moon as seen from Earth (it's nearly three times the size). It's integrated apparent brightness is mag 9, so although it looks bright in piccies, in real terms for a very short sub it won't be so bright.
Chris the Rosette is mainly Ha and even with my RC8 i can't see any nebulosity in a 15Sec Luminosity shot.
I have to do 15Minute shots to get nebulosity.
So with your setup try what Carl suggests or even longer.
Sort of a self fulfilling post...I answered it before I posted it, sorry about that....with the plate solve...lol
Thanks for confirming my suspicions about the diffuseness (is that a word?) of this neb, I will give it a red hot go one of these days and see what happens.
I am still banging my head against processing at the moment, trying and trying and TRYING to get some results from about half an hours worth of M42 and flame data, and screwing it up over and over......
I even captured this lot at ISO 100 for 4 minutes per sub (M42) and it LOOKS beautiful in RAW, I am struggling with getting anything of note from the stack after DSS.....its driving me NUTS even with Star Tools!!!!
Maybe I need to RTFM....? Too impatient to do it properly I spose, one of these days I may have some processed pics to show....
Sorry for the waffle, I have been doing that a lot lately....
I think you're right about the faintness of the Rosette nebula combined with the insensitivity of regular DSLRs to H-alpha.
Are you imaging with a bright moon in the sky, or from a place with lots of light pollution? The example you posted has a very bright skyglow despite the short exposure.
Am I correct in assuming that you have an f/4 imaging scope and an unmodded SLR? If so, I think you're going to struggle to see much/any of Rosette unless you're in dark skies and use very long exposures.
Here are a few pics showing what I mean (links to full quality versions of the attached pics):
1. 50% crop of a single exposure straight-out-of-camera taken under very, very dark skies: Rigel on the left, Orion's Belt left of centre, and Rosette on the right. The settings were 300 sec, f/2.8, ISO 1600 on a stock unmodded 5DmkII.
At f/4 (i.e. on your scope), this would be the equivalent of a 600 sec (10 min) sub at ISO 1600. Notice that despite exposing for 24 times longer than in your example, the Rosette is still quite dim here.
2. Same as in #1, but histogram stretched and +30 saturation applied only to the red channel. Just going by eye, it looks like both the Horsehead Nebula and Barnard's Loop are more brightly exposed than Rosette with the same settings.
3. Closer view of the Horsehead with a 59 sec, f/4, ISO 6400 exposure straight-out-of-camera - it's the equivalent exposure value to 236 sec (~ 4 mins), f/4, ISO 1600. Note from the above shots that the Horsehead was picked up by the unmodded sensor more brightly than Rosette...
The Rosette is quite a faint diffuse neb. I used to struggle with signal to noise shooting for 3-5mins with my unmodded 450D. Much harder than brighter galaxies or most bright nebs. Actually, I ended up buying my CCD after taking stacks of DSLR subs to "fix" the problem and failing miserably. That's what started me down the slippery slope of cooled CCDs.
It really is a wonderful object though, and just goes to show just how easy Mike and other top class AP'ers (appear to) make it look when we gaze at their masterpiece renditions.
Are you imaging with a bright moon in the sky, or from a place with lots of light pollution? The example you posted has a very bright skyglow despite the short exposure.
Am I correct in assuming that you have an f/4 imaging scope and an unmodded SLR? If so, I think you're going to struggle to see much/any of Rosette unless you're in dark skies and use very long exposures.
At f/4 (i.e. on your scope), this would be the equivalent of a 600 sec (10 min) sub at ISO 1600. Notice that despite exposing for 24 times longer than in your example, the Rosette is still quite dim here.
It's tough with an unmodded SLR!
Hi Dave,
Yes to unmodded DSLR, max ISO is 1600, (although I may try my son's new 550D which has 6400.....one day) and the sky at home has horrendous light pollution (plus the moon was rising at the time), so I wasnt expecting much anyway....something you can see in the 15 sec M1 snap....
The scope is a normal bog standard Skywatcher 200mm F5.
Thanks for all the info, and the time taken to post it with examples, certainly helps me understand the time needed on something so faint... I appreciate it.
The Rosette is quite a faint diffuse neb. I used to struggle with signal to noise shooting for 3-5mins with my unmodded 450D. Much harder than brighter galaxies or most bright nebs. Actually, I ended up buying my CCD after taking stacks of DSLR subs to "fix" the problem and failing miserably. That's what started me down the slippery slope of cooled CCDs.
It really is a wonderful object though, and just goes to show just how easy Mike and other top class AP'ers (appear to) make it look when we gaze at their masterpiece renditions.
Hi Rob,
Yes, the limits of the 1000D has me currently eyeing off a modest DBK21 for imaging, to eliminate the need for coma corrector, mirror mods, low profile focuser and the extension tube for visual, as well as the dedicated astro cam aspect compared to the unmodded DSLR....
I dont want to go crazy with high end stuff until I get something more permanent in the back yard, but with the LP and the skyglow....I may have to look at finetuning everything to the point where lower ISO and LOOOOOOOONG subs are the way to go with some skillful processing to remove the unwanted stuff as travelling to dark skies is not only expensive, but a pain to lug everything around.
Plus, I often forget something small, like the last time I left my T ring at home and couldnt take ANY photos at all.....DOH.
Yes to unmodded DSLR, max ISO is 1600, (although I may try my son's new 550D which has 6400.....one day) and the sky at home has horrendous light pollution (plus the moon was rising at the time), so I wasnt expecting much anyway....something you can see in the 15 sec M1 snap....
The scope is a normal bog standard Skywatcher 200mm F5.
Aah... so at f/5 (and ISO 1600) to get the Rosette equivalently exposed as in my #1 and #2 pics, you'd need an exposure of 952 seconds (~ 16 mins)!
Obviously this isn't possible with light pollution, so you have to resort to taking shorter subs and stacking them. Some mathematically gifted people have done the signal-to-noise ratio calculations for comparing stacking shorter subs in light pollution versus shooting under dark skies... check out these links, for example:
The basic gist of it is this: shoot each sub such that the sky fog peak is detached from the left edge of the histogram (looks like you've done this on your example). If your light-pollution-limited subs are say 10x shorter than the dark skies subs with equivalent settings, your total integration time has to be 10x longer - taking into account the shorter subs.
For example, your 25 second sub has a similar sky fog histogram peak to my 952 second-equivalent example in #1... i.e. from the dark skies I could expose each sub for 38 times longer. Therefore, to get an equivalent signal-to-noise ratio of the Rosette and Barnard's Loop as in that picture you'd have to shoot a total integration time of 952 x 38 = 36176 seconds (10 hours).
Since each sub is limited to 25 seconds, though, that means you'd have to shoot 1447x 25 sec subs (or 724x 50 sec @ ISO 800, etc)... just to match the one dark sky exposure! If 5 hours of dark sky exposures were shot and stacked, you'd never be able to catch up from a light polluted site since 5 x 38 = 190 hours...
Note that this is all with a stock, unmodified digital SLR: it all changes quite drastically with H-alpha modding, filters and narrowband imaging, CCD imaging, etc.
Thanks for all the info, and the time taken to post it with examples, certainly helps me understand the time needed on something so faint... I appreciate it.
Cheers
Chris
No worries, I have a research background in engineering and interest in mathematical statistics so as an astro newbie I find all these calculations quite fascinating!
Hmmm, dont think 10 hours is going to happen any time soon, LOL
Looks like I will either need to grab a BB gun and go on a street light smashing rampage, OR shoot some narrowband.....(not gunna happen with what I have) OR buy a cheapish mono DMK or similar with some RGB and narrowband filters and have a play......
OR, pick up the scope and drive for an hour or more to dark skies and stuff around setting up etc etc etc and then find another limit either in the scope alignment or PEC or something.....
I think I might just play it cool for now and practice framing, guiding, drift alignment, exposure lengths and processing with what I can see from the disgustingly light polluted back yard before getting too frustrated with what the DSLR can and cant do....and buy a dedicated astro cam.....
Although, I am looking at a cheapish DBK21 for about $380 to play with, that might keep me happy for a while....
Thanks for the info again Dave....good to know I am up against the proverbial wall and not expect so much from such short exposures.....
Chris, just be aware that the imaging chip size of the DBK21 is very small, therefore you will be limited to what it can capture, you might be dissappointed.
I suggest you download the CCDCalc tool by Ron Wodaski, then enter all your scope/camera details and it will show the field of view that it can capture for different objects.
I tamed my LP problem (I'm 12kms from the Brisbane CBD in the heart of suburbia) using a Baader UHC-S filter. I've attached the spectral specifications of this, and other, filters so that you can see what wavelengths are getting attenuated. As you can see it's all major forms of artificial light.
Is this the core of the Rosette Nebula?
If 5 hours of dark sky exposures were shot and stacked, you'd never be able to catch up from a light polluted site since 5 x 38 = 190 hours... 
