Swan Nebula Mosaic in Ha *toned down burnt core*

[multiweb]

One of my favourite areas north of Sagittarius. Two panels of 1h each with the C11/Hyperstar combo last night. Again same image tilt problem crept in at the bottom of the field. Very annoying. It's following a pattern and it's not random. I know it is definitely not initial collimation. I'm starting to think it is something that is temperature related with the mirror lock or corrector cell. Will figure it out eventually.

There's a lot of faint nebulosity in that shot. For once it's calibrated :P . I finally figured out last night the sweet spot for the intensity of my flats with the hyperstar. So I can calibrate the field nicely now and what a difference. Brings up all the faint stuff effortlessly.

There's a better res here (http://www.multiweb.com.au/astro/M17_HS3_ha_sf.jpg) [610KB] withouth JPEG artefacts and a big full field here (http://www.multiweb.com.au/astro/M17_HS3_ha_f.jpg). [1.18MB] so it's download friendly. :)

Enjoy the shot, got plenty more to process from the same night. :thumbsup:

[David Fitz-Henr]

That's a nice shot Marc! Lots of nebulosity, stars, details and dark lanes across the entire wide field!
The hyperstar attaches to the Schmidt plate, right? How heavy is your camera - could it be flexing the adapters / plate?

[gregbradley]

Super shot Marc. Great widefield. What is the corrector you are using?

Greg.

[multiweb]

Thanks David. Nah, the corrector is built like a pizza stone. It won't budge and M17 was right up so it's not flex, like M16 the night before. Same thing happened. Bugger if I know what it is :shrug:


Thanks Greg. That's the Hyperstar. It's a triplet that replaces the secondary in an SCT bringing the whole system down to F/1.8 or 504mm FL.

[DavidU]

Very tidy Marc.That's a lot of photons in a short amount of time.

[multiweb]

Thanks David. :thumbsup: With flat fielding working now I'm cookin' :lol:
Just need to sort out this field tilt. At the moment it's rolled like a chicken wrap and I'm more of a pancake man. :)

[Bassnut]

Cookin alright, awesome detail for short exposures, the hyperstar's workin well Marc.

[mswhin63]

Hi Marc,

Great shot and can definately see loads. I noticed something that looks like a comet, but I am not experienced in decifering photos.

It at co-ordinates 297 x 463 in the large image. Maybe it is something else but it seems to have a head on it. Could even be a lop sided galaxy :)

[renormalised]

Marc...could the hyperstar be slightly out of collimation?? If it is, it might need just the slightest of tweaks. Could be another temp related thing.

[RB]

Beautiful detail in there Marc, glad you got calibration working for you.
Wow F/1.8, incredible, the large version is lovely.

:thumbsup:

[avandonk]

Marc that is a very impressive result.

I had major problems with focus shift with ambient temperature change with the 300mm lens. I now keep it at a constant temperature by thermostically heating it with dew straps. For example at the moment the temperature is set at 16C so as ambient falls below this the lens stays at the temperature I focussed it. Radiation into a cold clear night sky is also taken care of.

I suspect what is happening with your corrector plate and tube is that it is getting colder at the top (the bit that 'sees' the sky) and the bottom (the bit that 'sees' the ground) as the night passes. I can tell this by the orientation of the Swan Nebula and where it is at this time of year.

I use this thermostat as it fully controllable for set temperature and hysteresis interval.

http://www.oceancontrols.com.au/controllers/novus/thermostats.htm


I would start by heating the corrector plate end with two dew straps and set the temperature just above ambient when you collimate and focus.

You could always add more dew straps later say one at the mirror end to stop temperature gradients changing alignment.

I would not bet two on being correct but I am willing to bet one!

Bert

[multiweb]

Thanks Fred. :thumbsup: Nearly tamed the beast. Not long now.


Thanks. Got better eyes than me. If you highlight the area I can check the raw file and post a crop. :thumbsup:


I don't collimate the hyperstar. I have a aligned the scope optics in such a way that if screw the hyperstar dead flat on the corrector I'm good to go. When the scope is "warm" everything is fine. It's at the end of the night when it gets very cold that things shift. I need to figure out what.


Thanks Andrew. :thumbsup: Yeah flat fielding will do wonders on any pictures as far as even illumination goes, even if you don't have any vignetting. I'm stoked it works now. :) Although I got almost nill vignetting with the hyperstar the central illumination of the imaging circle can be quite overwhelming and calibration spreads and levels all this, bringing out stuff that I've never seen before. I now can now push the data and capture dark dust that is lower count than the sky background ADU in 5min subs. Pretty cool.



Thanks Bert. :thumbsup: Always thinking outside of the box... :) I think you're on to something here. The top being colder than the bottom. Never thought about it this way. I already have a heating wire along the whole corrector cell that runs all night but I'm going to cover the top of the scope with some rags see if it makes a difference. It could very well be a temperature differential within the glass. Well spotted. :thumbsup:

PS: Actually now the more I think about what you just said I think I'm going to wrap the tube and insulate it like what Monte did on his FSQ106 a while a go. What would a good insulation film be?

[Bolts_Tweed]

Smooth as ice cream mate - thats a good thing trust me.

The clarity of Ha makes it look just damn good - some shots are just better in Grayscale.

Well done mate

Mark Bolton

[multiweb]

Thanks Mark. I like ice cream too. :thumbsup:

[peter_4059]

Top image Marc. The swan takes on a completely different form in the wider field image.

Peter

[multiweb]

Thanks Peter. :thumbsup: It does look good in Ha and even better in narrowband. There are stacks of clouds in the area. But I think it's being a bit "eclipsed" by its famous neighbourg M16 although I reckon the Swan in HST palette is way way nicer than M16.

[avandonk]

Marc on thinking further the camera and Hyperstar are shielding the bottom bit of the corrector from the cold night sky. This would not happen at the zenith but by then the temperature differentials are already established. You only have to see the extreme lengths planetary imagers go to too make their optics perform at anywhere near the so called diffraction limit by getting rid of temperature gradients.

Why this effect has not been seen with SCT's before is that a small field of about two degrees maximum would not be noticeably affected.

I hope this is correct as it will stop you slowly going insane while chasing phantoms of collimation or alignment.

I would try bubble wrap covered with aluminium foil just as a test. This covers convection and radiation as a cause of differential cooling. A well insulated dew shield or hood would also really help.

I just looked at all the available evidence and this hypothesis is the only one that fitted all the known facts.

Bert

[jjjnettie]

Oooh Ahhhh
Very nice Marc.

[multiweb]

Thanks Nettie - glad you liked it :thumbsup:



Hey Bert, thanks heaps for your analysis. :thumbsup: Here's a pic of the rig. I should have started with that, showing you a picture of it.

The dew shield is made out of flute panel and it is flocked inside. It's approx. 400mm in length and bolted onto the corrector cell in 4 points. So I'd assume it is a good isolation as the material itself is hollow.

The hyperstar + camera are inside the dewshield. I have one thick wire bonded to the outside of the corrector cell casting and I run 12V through it off a 75AH Gel Cell - no regulation. It gets fairly hot to the touch (you won't keep your fingers on it for very long). The heat transmits to the whole cast onto the glass and keeps the corrector relatively dew free if I start it early at the beginning of the evening. If the corrector is too cold then the wire won't get it warm enough again to clear it completely that's why I start early and keep it to temp.

The hyperstar itself gets pretty hot too because the front of the camera gets hot (nosepiece and spacers) when the CCD is cooling and the heat transfers from the camera to the body of the hyperstar as well overtime. I can feel it's warmish when I take it off afterwards.

[avandonk]

Marc the whole point of dew heaters is to radiate enough heat to the optic surfaces to counteract the heat loss by radiation into a cold clear night sky. Heating by conduction is far to brutal and leads to temperature gradients.

Get a kendrick dew heater strap instead of the wire and place it on the dew shield close to the corrector end. Not to close!

Your problem is temperature gradients. The heating you are applying is most probably leading to the temperature gradients.

Thermostats are the only way.

Lucky I only bet one!

Bert

[strongmanmike]

That's quite a dramatic field Marc...I think I like square fields really....

Mike

[jase]

Copious amounts of nebulosity. Impressive Marc. Well done.

[multiweb]

Thanks Bert - I'll look into getting a thermostat.


Thanks Mike. I wonder why... ;) See you at the SPSP?


Thanks for the comments Jase. :thumbsup:

[multiweb]

Toned down the core on the Swan itself after a few comments. Probably got a bit too excited pushing the surrounding and burn the main object. :P Looks a bit more balanced now with more details. Updated download links.

[LGT150]

je ne la connaissais pas sous cet angle elle est très détaillée

[multiweb]

Merci Lucien. Oui, il y a beaucoup the nuages tres dense dans le champ de vue mais ils sont aussi tres sombre et pas facile a capturer.

[Hagar]

Nice image Marc, The core is quite visible with the barring showing up very well. Detail overall is very nice. Reading the other posts I have a question about the hyperstar as well. Do you find the air movement caused by the fan shifting a lot of air inside the dew shield causes any distortion to the image? Always wondered about shifting so much air in a small contained area.

[multiweb]

Thanks Doug. TBH I don't think the air flow caused by the camera causes any distorsion in the image. It does actually help in keeping the dew off the corrector as well so I think it's a plus. Also at an image scale of almost 3asp you would need some serious distortion to show in your images. We're almost talking about widefield like image scale.

I've enclosed everything inside a semi-rigid dew shield and run the wiring through holes inside the dewshield because I'm extremely paranoid about knocking the hyperstar off the corrector plate and break everything. I'm always on the move and I have slightly bumped the dewshield many times when moving the scope around. You can't be too careful. One knock and that would be the end of the gig. :sadeyes:

Also I have enough slack on the camera connection inside the dewshield so in the event a wire gets caught or someone step on a cable it won't pull on the hyperstar. It'll pull on the dewshield and the whole scope will move which is safer too.