54 replies to this topic

[Henry from NZ]

This is my latest attempt of NGC3628. The image is composed of 8x5min subs each of R, G and B, with just DBE and a histo stretch to show what can be gleaned. This is the practical limit of how long I can expose in one night, as where I am the target is low and even at zenith it is only 40 degree high. I tried to get more subs and the weather has not been cooperating and it looks like it will be a long time before I can get more subs.

 

I am now actually fairly happy with the collimation of my 6"RC on which these subs are obtained. The stars are round enough to my eye and the field quite flat. However even though the RC has been reduced by 0.7x, it is obviously not quite fast enough from my light polluted sky to pull in the fainter detail with the limited sky time I have. (n.b. I cannot reduce it further otherwise the stars will be out of round).

 

Look at the two ends of the galaxy for example, the faint bit / arm (whatever that is called) has a pixel readout of 0.029 - 0.030 and the adjacent background is 0.027-0.028. If you squint your eye you can just perceive it but I really don't know whether Pixinsight processing can bring it out more...

 

I wonder if I should really look into getting something faster. A lot of the target I wish to image fall into similar category as this one (i.e. low in sky and short duration above horizon at best of time). However if I want to keep that focal length (1000mm) I will be looking at a 250mm (10") f/4 or a 200mm (8") f/5 imaging newt - both probably too heavy for my NEQ6/Atlas... Or shall I look at a hyper star setup?

 

Which path shall I look at?

 

This hobby is not easy is it?

 

Edited by Henry from NZ, 27 April 2016 - 02:30 PM.

[Jon Rista]

Have you considered just getting more total integration time? Last time I really imaged in my light polluted back yard, I was just sticking with my targets, and stacking a couple hundred subs or so. The results can actually be pretty good if you get enough data. This image demonstrates how deep you can go with about 10 hours from a red zone:

http://i.imgur.com/d0S4jJm.gif

Anyway, may be an option if you aren't really up for another larger scope yet.

[baron555]

Just do more imaging and combine everything together.  Why is it that you are limited to 5 min subs max?  Lack of guiding?  Lack of good mount tracking?

 

Faster optics will help but you may still have your underlying issue(s) that may limit what you can change.

[rigel123]

Stars look great, you just need more integration time, there is no way around that in an area with LP.  With more integration you will be able to pull more out of your image.

[Henry from NZ]

Yes, more subs would be the cheapest solution. Unfortunately it is difficult and not something of my control. I have about 30 galaxy and dso targets I would like to image e.g. M1, 65, 66, 95, 100, etc etc but due to my location they rise above the sky haze at 19:30 and set below the haze at 00:00. Including framing time focusing time etc 4 hour exposure is the practical limit for a night. Being galaxy targets I cannot do narrowband so I cannot image either side of full moon. That leaves about 2.5 weeks in a month or so but with the weather in Auckland I would be lucky to image 2-3 nights in that time frame. For example next week for the whole week is forecast to be cloudy. So I will wait till next month before I can acquire another set of similar exposure if I can lucky.

I guess that's why people do narrowband as they give them more sky time in cities.

[baron555]

You can combine this year's subs with last year's and so forth into the future.

[rigel123]

Yes, more subs would be the cheapest solution. Unfortunately it is difficult and not something of my control. 

I hear you!  But if you can get 4 hours on one object in a night that is a good start!

[syscore]

The pixel scale you are using right now fits these targets very well and if you go faster without also going bigger then the targets will be pretty small. Maybe you can lower the number of targets and integrate each one over multiple nights. Your results are looking very nice though. I think I am going to tape wire across my corrector plate to get diffraction spikes like yours.

Edited by syscore, 27 April 2016 - 03:25 PM.

[kf4mot]

I'm dealing with the same things. Light pollution and tree's that severely limit my time on any given target. Weather that limits me to imaging a few nights a year. I've got gigabytes of half finished images.

I've decided the solution is to go portable. I want to make those few good nights count.

[Madratter]

By shooting just RGB, you are really limiting your effective integration time.

[Rick J]

John and Warren hit the key.  Time time time time etc.  Faster optics only reduce resolution (assuming same aperture) making low contrast features even harder to see.  The ratio of background to object stays the same.  The key is more time.  Sky photons are noise and are random.  Galaxy is signal and not random.  Thus while signal adds arithmetically.  4 times the time 4 times the signal.  Background builds by approximately the square root of time so is only 2 times stronger with 4 times the time.

 

This DOESN'T necessarily mean more time per exposure.  In a light polluted environment sky noise soon drowns out read noise of the camera.  Once this point is reached there's no gain in going with longer subs but you do lose dynamic range in that brighter features and in some cases galaxy cores saturate costing detail and bloating stars needlessly.  There are many calculators on the net for this.  But I suspect your 5 minutes is plenty sufficient to keep read noise at bay.  You just need, as John and Warren say, a lot more of them.  There really isn't a better solution and what ones there are are expensive.

 

I have a friend who images from "central Berlin".  I can only imagine how bad that must be.  He often uses two or even three long nights to collect his data.  With the type of pollution he's dealing with a IDS LP filter is used for the luminance as well. One shot color as you are using is another problem as its color filters don't have gaps where LP tends to reside as do imaging RGB filters for mono cameras and thus the light loss of LP filters is more harmful to you than it is for him.  Try both and see which works best.  One shot is much slower to get photons as well due to the filters meaning you need a lot more time just to overcome that.

 

While faster isn't better sometimes slower is.  The increase in resolution helps low contrast features be pulled from the noise.  I work at f/10 for this very reason and don't use a compressor.  I also use more aperture which is pretty much the same as more time in that at 14" I collect 5.4 times more photons per second than you do at 6" which is the same as 5.4 minutes for every one you take even with the longer focal length.  For pulling signal from noise its total photons that count.  You might try removing your reducer (but it may be necessary for a flat field) and then just reduce the final image back to this image scale after processing.  I find I take at 0.5" per pixel but usually reduce to 1" as that's about what my skies support.  Taking at 0.5 makes processing for faint detail easier with better final results.

 

I have many targets I can only image for an hour a night.  I don't let that stop me.  I just get that hour then go to something else that gives that may also only give me an hour.  After a few months sometimes years (given my lousy weather) I now have many targets ready for processing.  Using this approach and full automation I've now over 1500 objects taken in 10 years time but often not in one night.  Other than moving to darker skies, buying a mono camera with filters and filter wheel (expensive) and going to larger aperture (also expensive) time is the only real solution I've ever found in doing this for over 60 years.

 

When I was on the north island a few years ago I found plenty of dark sites not all that far from cities.  It's a pain but for 50 years I drove to similar sites at similar distances for my imaging.  Even with the drive time the gain in dark skies allowed me to do far more than I could have with time from where I lived.  After retiring and finding arthritis making it too hard to travel any more I moved to a dark site and built an observatory but I did it the hard way for 50 years.

 

Also you have a ton of things to image down there we can't see up here at all.  Rather than do what has been taken a zillion times you have a ton of treasures rarely seen.  Those you can put all night into.  Many are well suited to narrow band but OSC camera isn't.  It can work but again needs far more time than a mono camera for similar results.

 

Rick

[happylimpet]

Aye - why not image those amazing southern galaxies? Then you can get 8 hrs on each in a night.

 

I think yr mount will be fine with an 8" f4 or f5 if you do go that way.  

[pfile]

i'd kill to get a solid 4 hours on a target each night... but i am boxed in at my home.

 

i'm routinely at 25-35h on galaxy targets to get decent SNR from here (red zone according to the maps, perhaps maybe not that bad when pointed north.)

 

because of this i'm taking much longer subs than need be, since i don't want to stack 200 5 minute subs to get there.

 

as for RGB vs LRGB i find that because of the light pollution the L exposures are really tough. the gap between the G and R filters in the astrodon G2E set is pretty decent at eliminating sodium-vapor style light pollution.

 

as an aside, i identified some g=19.1 - 19.5 mag stars in a 30h integration around M81. i don't know how to translate that to a bortle zone as the magnitudes there seem to correspond with visual observing rather than what can be seen with long integrations.

 

rob

[FirstC8]

Well for my current object it may take the whole season to do just one.😆

[pedxing]

Take the long view, it may take multiple seasons to complete some targets.

 

Plate-solving is your friend for getting reframed off a previous image.

[Henry from NZ]

It sounds like every one voted for more integration time, which is fair enough. I am little surprised there is no love for larger imaging newts. I guess a 10" F/4 (for 1000 mm focal length) is quite unwieldy and I am really opening another can of worm, a silly thing to contemplate having only a short moment ago tamed the RC collimation woes.

[happylimpet]

More integration time is key, but of course bigger aperture will reduce that. Going from 6" to 10" at the same focal length will speed things up by a factor of three (10^2/6^2). Depends how much of a rush you're in!

 

Also gives you an opportunity to really make the most of the southern hemisphere planet-fest over the next few years.

[FirstC8]

It sounds like every one voted for more integration time, which is fair enough. I am little surprised there is no love for larger imaging newts. I guess a 10" F/4 (for 1000 mm focal length) is quite unwieldy and I am really opening another can of worm, a silly thing to contemplate having only a short moment ago tamed the RC collimation woes.

If speed is very desirable then Hyperstar would be the natural option.

[Jon Rista]

It sounds like every one voted for more integration time, which is fair enough. I am little surprised there is no love for larger imaging newts. I guess a 10" F/4 (for 1000 mm focal length) is quite unwieldy and I am really opening another can of worm, a silly thing to contemplate having only a short moment ago tamed the RC collimation woes.

 

It's not that there is no love. I would love to have a nice, fast imaging newt myself (just not sure if my mount can handle it as well with the long moment arm.) You just sounded a bit hesitant to get a new scope...so I wanted to offer an alternative solution that might save you money and hassle getting a larger scope working. It is certainly not the only solution, however...gathering more light in a given unit of time is just as good an option! Total light integrated is ultimately what counts...and a bigger faster scope gets you the same in less time. 

[syscore]

 

It sounds like every one voted for more integration time, which is fair enough. I am little surprised there is no love for larger imaging newts. I guess a 10" F/4 (for 1000 mm focal length) is quite unwieldy and I am really opening another can of worm, a silly thing to contemplate having only a short moment ago tamed the RC collimation woes.

If speed is very desirable then Hyperstar would be the natural option.

 

 

Hyperstar is not a good option for these types of targets because everything comes out tiny.

 

Been there, done that.

 

To keep the same pixel scale and be faster you have to increase aperture.

[FirstC8]

It sounds like every one voted for more integration time, which is fair enough. I am little surprised there is no love for larger imaging newts. I guess a 10" F/4 (for 1000 mm focal length) is quite unwieldy and I am really opening another can of worm, a silly thing to contemplate having only a short moment ago tamed the RC collimation woes.

If speed is very desirable then Hyperstar would be the natural option.

Hyperstar is not a good option for these types of targets because everything comes out tiny.

Been there, done that.

To keep the same pixel scale and be faster you have to increase aperture.

According to several advanced users, Hyperstar can resolve the same level of detail as the same scope with a reducer, or even at native focal ratio.

So if the right imaging chip is used, and the sky allows it, one can still get comparable detail by the use of drizzle to enlarge the objects during post processing.

In theory that is.

[syscore]

"According to several advanced users, Hyperstar can resolve the same level of detail as the same scope with a reducer, or even at native focal ratio.

So if the right imaging chip is used, and the sky allows it, one can still get comparable detail by the use of drizzle to enlarge the objects during post processing.

In theory that is."

 

But that then defeats the purpose of going to the Hyperstar. In other words, if you use pixels that small (and they don't make cameras with pixels that small) then your speed will be the same as it is now, without the Hyperstar. You can't gain speed and maintain the same resolution without bigger aperture.

Edited by syscore, 28 April 2016 - 10:09 AM.

[rgsalinger]

An 11" F2.2 scope has about 620mm of focal length. Using 3.8 micron pixels this will give an image scale of around 1.25 arc seconds per pixel. At that level you're probably not getting the best possible detail out of a DSO. Whether drizzle techniques can take such an image and get that detail I don't know, but you're not likely to find cooled cameras with much smaller pixels. Small pixels mean that the well depth is not very high and so the SNR you get won't be too great either - you'll need more integration time to get to whatever you think is the magic number. I think that's the way to look at it. Now if they build a 14" RASA, then that might get interesting!

 

Rgrds-Ross

[Jon Rista]

"According to several advanced users, Hyperstar can resolve the same level of detail as the same scope with a reducer, or even at native focal ratio.
So if the right imaging chip is used, and the sky allows it, one can still get comparable detail by the use of drizzle to enlarge the objects during post processing.
In theory that is."
 
But that then defeats the purpose of going to the Hyperstar. In other words, if you use pixels that small (and they don't make cameras with pixels that small) then your speed will be the same as it is now, without the Hyperstar. You can't gain speed and maintain the same resolution without bigger aperture.

Hmm? Maybe we are interpreting that differently. It sounded to me like he was saying Hyperstar resolves the same as an Edge with a reducer, or an Edge at native FL. That would indeed be true, since it is the aperture that determines the resolving power. The speed with hyperstar is definitely faster than an Edge w/ Reducer or Edge Native, because the focal ratio dropped. The size of the pixels doesn't really affect imaging speed per-se. Smaller pixels just divide the focused light more finely. The same light would still be incident on the sensor regardless of the size of the pixels, though. The actual speed for the pixels to become saturated would then depend on the gain used.

[syscore]

"Hmm? Maybe we are interpreting that differently. It sounded to me like he was saying Hyperstar resolves the same as an Edge with a reducer, or an Edge at native FL. That would indeed be true, since it is the aperture that determines the resolving power. The speed with hyperstar is definitely faster than an Edge w/ Reducer or Edge Native, because the focal ratio dropped. The size of the pixels doesn't really affect imaging speed per-se. Smaller pixels just divide the focused light more finely. The same light would still be incident on the sensor regardless of the size of the pixels, though. The actual speed for the pixels to become saturated would then depend on the gain used."

 

The end of all that is pixels. If you try to realize the same resolution of the scope by using an appropriate pixel size, then you are right back where you started. In other words, if the OP wishes to maintain the same pixel scale as he has now, and do it faster, he will have to invest in a larger aperture. Making the scope faster through optical reduction (Hyperstar) and then making the pixels smaller to keep the same (pixel) resolution (called pixel scale) will cancel each other out, and the speed will be the same.