Rosette Nebula, bicolour

[Tommohawk]

Hi all. Finally got around to trying my hand at the Rosette - its tricky from home because of limited views but I was fortunate enough to enjoy a week break in Spain and the weather was pretty good, much better than the forecast. Some windy nights, but most of the gremlins had to do with the mount binding in RA. Utter pain, because I'd tested it many times before - something to do with being close to the meridian I think. I tried some adjustments and a re-grease which gave slight a improvement, but sadly kept losing the guide star. Well... I think it was a re-grease - the tube was all in Spanish, so it was Super-Something - maybe super-glue?

Anyhow, my first go at narrowband so lots to learn. The Ha images looked great, the OIII less so with larger stars. I also tried some B images because I had a vague plan to map Ha to R, OIII to G, and then use some broadband blue. There was so little data on the blue subs though that I gave up on this. I think this approach might work well on a target with some reflection, but it's definitely in the experimental category. Lots of LP at this site so not ideal for broadband.

Ideally I would also have tried some SII but there just wasn't enough time - I was staying with friends so apart from imaging I was supposed to be socialising too!

To add further fun, my CTRL keys packed up on the laptop, so I couldn't get Stellarium to "go to" and sync, as I use CTRL+1 and CTRL+3 for this. Figured out how to remap the keys, but turns out Stellarium has user defined shortcuts - thanks to Alex Wolfe for swift response on that. I must say Stellarium is an amazing piece of software.

Net result was a very limited number of subs, but better than no subs. The flats were problematic. DSS creates a smearing effect sometimes which no amount of fiddling will eliminate, and also I found the vignetting quite different on the OIII compared to the Ha. Maybe thats no surprise and when I have time I'll redo the flats for each filter. For now I mostly fixed it by cropping.

For processing I used DSS, using the best Ha image as the reference for the OIII stack. Tweaked the mono images in PS, the created a synth G by blending the Ha and OIII 50/50. The OIII also had 3 rounds of "reduce star size". The colours were stretched selectively to boost blue in the nebula but reduce blue in the stars, as the OIII has slight haloes. I'm not really sure about the colour rendition at all - it looks sort of pleasing to my eye, but nothing like other bicolour Rosettes I've seen. Too much colour probably. I'll have a go with Steve Cannistra's method when I have time but spent way too long at the PC for the last couple of days!

Very happy for any input/criticism.

So:

EQ3 pro with ASI1600mm at -10 with Tamron 300SP at F4, ZWO 31mm filters with ZWO EFW, ASI 290mm for guiding.

13x Ha and 43x OIII subs all at 300s

[thomasv]

Very nice, that's a lot of oiii subs, but it works well. I'm on the lookout for a 300mm lens, which version do you have exactly, google comes up with a zoom 70-300mm one only, which is f5.6 at 300mm. Is yours a prime lens? Thanks.

 

[Tommohawk]

On 2/16/2018 at 15:07, thomasv said:

Very nice, that's a lot of oiii subs, but it works well. I'm on the lookout for a 300mm lens, which version do you have exactly, google comes up with a zoom 70-300mm one only, which is f5.6 at 300mm. Is yours a prime lens? Thanks.

 

Thanks. The lens is quite old - its a Tamron SP 300mm F2.8 360B model (prime). I bought it because I liked the idea of a very fast lens to getting images quickly, especially given the limited clear sky time we get.

It's proved to be a very good lens - but a couple of things to bear in mind. The star shapes at F2.8 are a bit odd in the corners - thats why I shot the Rosette at F4 -  though typically I'd crop a bit which gets rid of the worst of this. I now have a much better Bahtinov mask for this lens so I may try again at F2.8 sometime. Also its heavy - nearly 2.5kg - which makes it pretty unwieldy on the DSLR especially given that the adaptall mount isnt 100% rock solid. It good, but I certainly wouldn't dangle the lens on the camera.

TBH if I was doing it again, I would probably get a short focal length dedicated scope like the TS optics 60mm F5.5 or similar. Half the weight, easier to transport, and way easier to focus. I paid about £300 for the lens 2nd hand, and I guess you could probably get a used apo for a similar price.

Anyone out there got any opinions on the colour pallette? Still hoping to get some input on that please!

[thomasv]

Thanks for the reply, interesting, not a cheap lens then, and like you say probably too heavy, as I'd like one to use with the star adventurer. I guess those small apos still need a flattener which pushes the price up somewhat. I'll keep looking.

 

[Tommohawk]

3 hours ago, thomasv said:

Thanks for the reply, interesting, not a cheap lens then, and like you say probably too heavy, as I'd like one to use with the star adventurer. I guess those small apos still need a flattener which pushes the price up somewhat. I'll keep looking.

 

Yes, flattener required and as you say all adds to the cost. 300mm is also pushing it a bit for focal length I would think with the star adventurer - maybe an F4-5 200mm lens would work well both for weight and budget.

The colour palette.. on the rosette nebula... anybody?? 

[JemC]

29 minutes ago, Tommohawk said:

The colour palette.. on the rosette nebula... anybody?? 

I'm no expert, and i have tried this target without much success, 

I must say though that it is a nice image, a really nice image, But! not to sure on the pink to be honest   then again never seen it in pink before, i find red more striking (but that's just me)

[Tommohawk]

13 minutes ago, JemC said:

I'm no expert, and i have tried this target without much success, 

I must say though that it is a nice image, a really nice image, But! not to sure on the pink to be honest   then again never seen it in pink before, i find red more striking (but that's just me)

Thanks - yes its the pink that seems weird. I prefer the colours of Rodd's image here - plus his image has phenomenal clarity. I need to have another go at the processing. Rodd did straight bicolour with OIII on both G and B - I tried that briefly and it looked funny. Back to the drawing board!

[ampleamp]

I am no expert......., so now that's out the way; with your dataset, could you try R = H, G = O and B = O as a classic HOO option (and i might even try splitting the O into two stacks), or another combination i quite like is R = H, G - (H x O) x 1.5, and B = O (this is easy in PI, no idea how it could be done in PS). Both will give magenta stars but they are relatively easily dealt with. I use a histogram to rebalance the rgb components and then tweak saturation as necessary.

[Tommohawk]

Thanks for that. I've just re-done the HOO version, and its OK but I can only make it work with a false Lum layer created from a 50% opacity Ha. This seems closer to Rodd's rendition.

The G as (H X O) x1.5 suggestion is probably similar to the synth G version I did originally as above. But maybe my blending of Ha and O doesnt work well. I don't have PI but will have another fiddle and see what I can come up with .

Having all sorts of grief uploading PNG files at the moment. Not sure if its timing out or what. I'll try smaller file size. Hmmm - still struggling to upload the HOO and its only 5 MB. Something funny here. OK got it now after clearing the editor. Heres HOO with Lum from Ha