NGC 2070 in Ha (6 Hours)

[Atmos]

I was mucking around with exposure times with narrowband over the weekend, was ~75% moon phase and imaging over Melbourne CBD, absolutely blown away by the Astrodome 3nm filters! What I have learned more than anything is that I do need to work on my PEC training when doing hour long exposures. As good as 1 second multistar guiding with MaxIM DL is, without PEC training I don't think I'll ever truly get round stars at that exposure time with an EQ6 Pro.... Although I could be wrong if someone wants to correct me :P
Thinking about buying PEMPro as I have heard nothing but praise about it :D

This is a combination of single 1200, 2400 & 3600 exposures in Ha. I was slowly increasing exposure time to not only see whether I would be able to do hour long exposures without warping too much AND to see when I would become read noise limited. In Ha it seems to be near 2 hours! I was surprised from my location to say the least.

They have been bias and flat calibrated but no darks yet, my dark library does not yet extend out that high. Work in progress.
As for processing, did a light MLT, stretched and then a slight unsharp mask followed by a 5 layer HDR. About 5m work in total :)

Astrobin for larger image (http://astrob.in/222193/0/).

[Placidus]

Congratulations on an excellent image. Good composition. Capture seems faultless. Congrats on the 1-hour subs.

You've wrung out the very last drop of nebulosity in the processing - perhaps a tiny bit less pushed might be even better.

Makes us want to rush out and photograph that "Aladin's Cave" star-filled bubble at 8 o'clock and about half way out from the centre.

Very best,
M & T

[Bassnut]

wow, thats an impressive image. Whats far more impressive, is 1hr subs on an EQ6, very well done!. Yes, 3nm sure works in urban enviroments, as you have proved, smart purchase. And no darks! sheesh, awesome.

[alpal]

Hi Atmos,
that's great to get a 1 hour subframe.
I can't however make out the Tarantula Nebula when
I compare it to this other Ha shot:
https://hughsblog.files.wordpress.com/2013/04/ngc2070_ha_st_01.jpg

It looks so different.

cheers
Allan

[Slawomir]

A great result Colin - and congratulations on your 1-hr sub! :eyepop:

As for darks, I would stick to just master bias (ideally super bias), flats and a bad pixel map. Darks will only inject more noise and are IMO unnecessary, unless you have some strong camera-induced signal with longer exposures.

[gregbradley]

A beautiful and deep Ha image Colin. Very impressive.

Greg.

[rustigsmed]

excellent stuff colin, am pretty interested to see the raw frames to ascertain the difference in quality and vs the risk factor (guiding / satellites / clouds etc). 30 mins is as far as i've gone ... but that was with a dslr - haha.

looking forward to seeing more!

cheers

Russ

[DJScotty]

Awesome image Colin. So much intricate detail to lose yourself in. 1 hour subs is "Impressive, most impressive!" :fight:

[Atmos]

Thanks M & T, I do agree that it has been pushed a tad too far. I tried going as far as I could without black clipping it. I was just curious to see what could be dragged out of 2 hours of exposure. Planning on getting a fair bit more Ha, thinking I might TRY to stick with the 1 hour subs though. Takes about half an hour with T-Point to get close enough to SCP.



I was a bit skeptical of what I would be able to achieve with the EQ6. As nice as it is performing I do think I am going to have to be upgrading the mount in the future, still trying to decide what to go with. Thinking the MYT but I am still unsure and it is still a little way off I think.



My RAW subs do look a bit like that before I start any processing or when I look at them through the Low/Medium stretching through MaxIM DL.



I made a master bias from 200 frames, still worth running the Super Bias routine in PI?
I'll have to have a look at creating a bad pixel map, will save near 300 hour in taking darks :eyepop:



Thanks :) Now I just need to get another 80 hours to catch up to a proper deep field!



I did a Linear Fit and then the same processing on all three images. They're full res and ~3mb each.

1200 seconds (https://dl.dropboxusercontent.com/u/27436364/NGC%202070%20Test/1200s.jpg)
2400 seconds (https://dl.dropboxusercontent.com/u/27436364/NGC%202070%20Test/2400s.jpg)
3600 seconds (https://dl.dropboxusercontent.com/u/27436364/NGC%202070%20Test/3600s.jpg)



Thanks Scott, I am surprised at how they came out.

[Paul Haese]

Well done Colin on doing long subs.

I think star shapes would be more influenced by PA, focus and tilt rather than PE, so long as your PE is corrected out via guiding. It's always good to experiment and find out what works best with your equipment.

[Atmos]

That's a good point, could end up being PA. A 23 point model was saying I was within 18" of SCP so I figured that would be close enough for hour long subs... Maybe not!!
I think I'll have to do some shorter subs to be able to check for tilt, hour long is a bit long!

[alpal]

Hi Colin,
I can see why your picture looks different to mine.
The long subframes even at 1200s have over exposed the center of the Tarantula.
The center is very bright.
I had 1 hour of subframes in Ha binned 1x1 stacked from my 8" f6 Newt. last December.
I made a cropped comparison with mine by rotating & re-sizing mine etc.
I reckon if you did some shorter subframes for the center & blended it in it would be better.
My finished picture was here when I added all the LRGB Ha together
https://www.flickr.com/photos/24719437@N03/15429377843/in/photostream



cheers
Allan

[Slawomir]

Master Bias will inject some minor noise, while Super Bias will not induce any noise. It only takes one or two mouse clicks to generate a super bias from master bias...

[Placidus]

Hi, Colin,

Rigid camera tilt will show up in a tenth of a second. It is independent of frame length. Camera tilt will show up as stars that are in focus along a line (which may be vertical, horizontal, or diagonal) across the image, but elongated 90 degrees to the line by progressively greater amounts away from that line.

Camera flop is different - the camera can be on square in some parts of the sky but move under gravity in other parts of the sky if the attachment is rickety. You can again test that with very short exposures on a bright star - just check different parts of the sky. The direction of flop can change across the sky.

Incorrect polar alignment can be spotted because the pattern of star enlargement is that of field rotation. Little star trails rotating a tiny bit about your guide star (which might be at the edge of the frame, not necessarily in the middle). I see no evidence of this in your shot.

18 seconds of the SCP is superb. Ten times that is fine, so long as the HOUR ANGLE of the error is roughly 90 degrees (say within 45 degrees) of the hour angle where you are photographing. We (and many others) routinely put our polar axis about 150 sec arc WEST of the pole, and we then photograph between hour angles of about +/- 3 hours of the meridian. It helps with guiding but causes negligible field rotation.

Out of focus stars will of course be blurry but symmetrical on-axis. On very large chips, out-of-focus stars in the corner can look like flying saucers.

Other things that can cause funny stars: wind buffet, momentary loss of guiding due to tiny clouds, and grit causing static friction (grabbing) in bearings and gears. Personally I don't think you have a problem that needs fixing.

Trish and I routinely do 1-hour subs. They give relatively clean noise-free backgrounds at our dark site. Our stars aren't exactly round, but they're not exactly round in a 1 minute shot. It's nothing to do with the long exposure, it's more about hanging 80 Kg of scope and cameras off a not-so-expensive mount. We don't ever get burn-out in nebulosity, but that's because the camera chip (16803) has sufficiently deep quantum wells. 3nM filters have no effect on nebulosity because they let 90% through, but they help prevent star burn-out.

Very best,
Mike

[RickS]

Nice capture, Colin! Loads of nebulosity there.

Wrt darks, most of the folks here that don't take darks are using low noise Sony sensors. With a KAF-8300 you may find it is worthwhile.

Cheers,
Rick.

[SimmoW]

Very impressive shot and exposure length Colin!

I'm swearing at you, because with the detail in your sub I'm gonna have to opt for 3nm filters when I go NB!

Slawomir, how many bias shots must I take to make a super bias worthwhile?

[Andy01]

Thats very cool -loads of depth there. Should be a great shot when finished :)

I purchased my belt modded and hypertuned EQ6 from Al Sam, because he regularly did 1 hr exposures with it. I'm personally not brave enough to go that long with so many clouds and satellites about, and the likelihood of losing subs, but after seeing yours maybe I can go to 30mins on the next project.

[strongmanmike]

Looking good Colin, quite amazing what can be stretched out of the Tarantula region, huh? :thumbsup:

1hr subs hey?...Oooooh scary :scared2: I haven't ventured past 15min, not because my rig can't handle it but rather I'm just time greedy and perhaps a little paranoid :nerd: about losing bigger chunks of data.

Mike

[Slawomir]

I had a master bias made from about 200 bias frames to be on the safe side, but pretty sure it will work fine with perhaps 50 or so. You need to make a master bias first, as SuperBias tool creates super bias from a master bias, not individual bias frames.

[Paul Haese]

I agree with MnT, that 18" is very good. 1-2 minutes is usually ok.

[SimmoW]

Thankyou Slawomir!

[Atmos]

Good point, I guess I'll have to take some 400-600s exposures to even out the core.



It is in the same direction as PA (I had my guider fail during the night, USB issues to which I will be trying another one tonight). I had an aspect of 17% on the 1200s, 16% on the 2400s but 28% on the 3600s so I am guessing it was a guiding issue more than anything.



I was thinking that it may have been the opposite with a 200 frame bias, opening it up and running the script is dead easy to do though.



I have done a fair bit of tightening and fiddling with the focuser and I think I have managed to remove near all of the tilt, I probably should do some testing tonight to see how it has gone, or whether it is gone!
It is a nice and rigid focuser, it almost feels like I get more flop in the mount (mounting plate stuff) than the focuser :D

My report said that MA was spot on and ME was 18" off. Now, it was just a 23 point model so maybe it could be off by further in both directions as a 23 point model could very well be too small. I'll have to keep that in mind, keeping a bit west of SCP if imaging near the meridian. Food for future thought.

The KAF-8300 with its 25,500 shallow wells doesn't take TOO much before they're full. In the future I would definitely consider getting a 11002 sensor, more real estate and same size wells as the 16803.



More nebulosity than I was expecting! It was your image from a month or so ago that inspired me to give this a shot :thumbsup:
I am definitely thinking that I'll need to take darks for the hour subs. The vast majority of the "stars" within the image are just hot pixels.



I started off purely as a test to not only see how the 3nm filters would perform but also to see what the mount was capable of. I bought it second hand (well 4th hand really) off of IIS, it had been belt modded by the previous owner but I think he only used it once or twice after that.



I am pretty sure that anything over ~12 minutes for me doesn't bring in any more nebulosity, hitting my sky limit for that night. I am still really in two minds as to how worthwhile it is pushing well past the sky limit and heading for read noise limit. Realistically I hit read noise limit at 90-120m (so happy to stick at 60m) but trying to figure out whether it is better in the long run to go longer just to grab the extra signal. Looking at the raw subs, mathematically it would appear that I would be getting a better SNR with 3x20m subs than a 1x60m.

That is calculated by sqrt(2)*3*SNR and then comparing the two. I did blend the 1200 & 2400 together and then compare that against the 3600. The single 3600 was still less noisy and better to process. Decisions decisions!

As I mentioned earlier my guider did drop out during the night while I was fast asleep waiting for predawn to shut down :( The first image shows what I got (full frame) while the second is just a crop on a star, it stopped working about half way through the previous image so this is an hour long sub without any guiding.
I assume that the oscillating is the PE on ~7.5m cycles but would I be correct to assume that the streak length is longer than what I would get with an 18" offset from SCP? Working at 1.5"/pix, 741mm focal length.

[Octane]

Differential flexure?

H

[Atmos]

I do have that in mind, also trying to decide whether to get an OAG or not.

[alistairsam]

Nice one Colin
did you get the O3 as well? I got the 5nm Ha.
Did you dither the images?
it'll really help removing thermal noise that shows up in the full res.

Cheers
Alistair

[Atmos]

I did try getting O3 but I think there may have been some passing cloud over night so the first one was a bit blurred but not too bad. The second one, the guider failed half way through and then my SII had no guider at all :-)

[Atmos]

What I noticed last night is the tracking got worse as the night went along. Although I should be able to do 15 minute tracking without issues all night, 60 min subs can only be achieved for the first 1-2 hours tops. I am assuming that this is differential flexure between the guide scope and main.

I guess now the decision of whether to try to decrease flexure as much as possible or taking the plunge with an OAG. I was talking to Cris earlier this morning and he suggests that the guide scope is the better overall guiding at 741mm FL.

[Atmos]

Collected some more data over the full moon, really want to compare it against a new moon set in a few weeks is possible. I know there is some tilt in this set, I think I need to retighten everything from night to night to stave off movement, tilt on this night but not on the previous. I have a feeling that my PA may have been a bit off, I think I am getting some field rotation towards the corners/edges of the image.

23x900 with 3nm Astrodon Ha.

Full Res version (http://astrob.in/223163/0/).

[E_ri_k]

Wow, thats very cool Colin, impressed by the exposure time! Your stars look nice and round. There certainly is a lot going on in the image, like a spider in its web!

Erik