1 hour Ha subs

[Rodd]

I decided to try a 1 hour sub.  The attached image shows significant elongation (mostly when you zoom on smaller stars).  My guiding was good--rms errors less than .1 pixel (.2 arc sec at a scale of 2.06 arcsec/pix).  Do you think the elongation is caused by field rotation due to the target being circumpolar?  My polar alignment was as good as can be achieved using the right angle polar alignment scope and computer APP.  But polar alignment  accuracy of less than 1 arcmin is difficult to get in this fashion.  Does anyone have experience with hour long subs and if so, do you think it is the field rotation or something else?  Any suggestions regarding fixing this elongation would be greatly appreciated as I do like the  signal strength of the hour long exposure.

Thanks---Rodd

[ChrisLX200]

1hr subs might be useful for some very faint objects - but not one as bright as the PacMan neb. You will still need a good set of these subs to average out the noise and produce a smooth result. Anyway, yes - if the guiding is working as expected then field rotation becomes the prime suspect. The 'fix' is to get better polar alignment.

ChrisH

[Rodd]

9 minutes ago, ChrisLX200 said:

1hr subs might be useful for some very faint objects - but not one as bright as the PacMan neb. You will still need a good set of these subs to average out the noise and produce a smooth result. Anyway, yes - if the guiding is working as expected then field rotation becomes the prime suspect. The 'fix' is to get better polar alignment.

ChrisH

Thanks Chris--As expected.  Surely, hour subs can't hurt provided you have the time and work out the kinks (rotation).  10 1 hour subs would produce a nice stack.  I know, you will probably say that 10 30min, or even 30 20min subs would be better.  BTW   Is there an add on in Maxim for getting spot on polar alignment?  

[ChrisLX200]

2 minutes ago, Rodd said:

Thanks Chris--As expected.  Surely, hour subs can't hurt provided you have the time and work out the kinks (rotation).  10 1 hour subs would produce a nice stack.  I know, you will probably say that 10 30min, or even 30 20min subs would be better.  BTW   Is there an add on in Maxim for getting spot on polar alignment?  

No it won't hurt - just not making optimal use of your time perhaps. More eggs in the basket - higher chance of plane/satellite trails and very expensive if you happen to lose a sub for any reason etc.. My own mount is PA'd to within a few arcsec but then, I don't need to set it up each time. I don't use Maxim so I'm not aware of its features, but the QHY Polar alignment camera is supposed to get you within 30arcsec easily enough and that's usually sufficient for most people. You seem to have halos around the bright stars - not something I expect to see with an Ha filter, more common with OIII though. That's another point - if you are intending on creating bi-colour or HST palette images you will need to capture the OIII and SII channels as well and these are often much weaker than the Ha. I often use 20min Ha and a longer (30min) OIII to get something that's easier to balance out, the Ha often over-powers the other channels which means I actually have to _reduce_ the brightenss of the Ha when compositing.

ChrisH

[ollypenrice]

That looks very like field rotation to me. The centre is pretty good. The top two corners look like my field rotation when I get it (due to slight loss of mount position over a long time) but the bottom two corners are not so clearly rotating in the same way. This is affected by the position of your guide star, though. I'll still back field rotation and, as Chris says, you need better PA.

I also agree with Chris that an hour is a long time on a target which is not predominantly faint, though some 'Pacmen' do tend to leap out of a black sky all at once while good ones emerge more gradually because the faint outer parts have been caught. I settle for 30 minute Ha subs as a good compromise. I know that Tim Jardine has tried multi hour subs in seach of faint planetaries so I wouldn't rule out the ultra long sub per se. You're right to give it a try though. Experiment beats theory (or rather refines it.)

Olly

[Astroboy239]

I never knew if a target was circumpolar it would be effected by field rotation!. New for me☺

Varad 

[graemlourens]

12 hours ago, Astroboy239 said:

I never knew if a target was circumpolar it would be effected by field rotation!. New for me☺

Varad 

Imaging close to the poles will be effected even more by field rotation than imaging west or east (depends obviously on your location)

Based on my knowledge, the only time you do not suffer from field rotation (assuming non-perfect polar alignment, or alt/az mount) is if you are standing on the equator and imaging perfectly west or east.

Kind regards, Graem

[ChrisLX200]

If you have two (or more) of these subs then when you come to stack them if there is any rotation it will be obvious from the way they overlap each other. In any event, I don't think you have a serious problem here and simple refinement of the PA would resolve it anyway.

You don't say what scope you use but I dimly recall you use an NP101is? (I might be wrong about that). The 'photographic speed' - f/ratio in other words - will make a big difference as to what the optimal sub length might be for your system. Mine is f/5.2 which, if I'm correct about your OTA, is similar to your own. I find 30min is the optimal balance between recovery of faint detail and generating enough subs within a given amount of time to reduce background noise in post-processing. Only in exceptional circumstances would I go longer - for example, I want to capture the Squid (Outers4) in OIII and that is very faint indeed so 1hr subs may well help there.

With enough 30min subs you should be able to use post-processing to reveal even the faintest parts of the PacMan, doubling the exposure time and halving the number of subs I personally doubt would lead to an improvement. However, if you want to collect 10 x 1hr subs then OK, but given my own skies (and frequent cloudy nights!) that would represent a massive investment in imaging time.

I found my old data from 2014 on this and it seems I used 10 x 30min subs (I thought it was 20mins but there you go, my memory isn't too good :-) ). At this time I hadn't learned how to create a star mask which included stars embedded in bright nebula so deconvolution didn't work too well! :-) This is with the NP127is @ f/5.2.

ChrisH

[Gina]

Nice PacMan Chris

My experience with my MN190 or Esprit 80ED both about f5 is that 30m is the optimum exposure.  As for polar alignment, I reckon to get within 15 arcsecs with my EQ8 mount and AstroTortilla using Atik 460EX mono CCD camera.  I think I should be able to do better but it's a balance between fiddling and imaging time when the odd clear night arrives.  I do have my EQ8 on a permanent concrete pier though so once set up it stays put. 

By the autumn and DSO season I'm hoping to have a ZWO ASI1600MM-Cool which I gather likes shorter exposures and more of them so optimum exposure is likely to be a fair bit less.

[Rodd]

4 hours ago, ChrisLX200 said:

If you have two (or more) of these subs then when you come to stack them if there is any rotation it will be obvious from the way they overlap each other. In any event, I don't think you have a serious problem here and simple refinement of the PA would resolve it anyway.

You don't say what scope you use but I dimly recall you use an NP101is? (I might be wrong about that). The 'photographic speed' - f/ratio in other words - will make a big difference as to what the optimal sub length might be for your system. Mine is f/5.2 which, if I'm correct about your OTA, is similar to your own. I find 30min is the optimal balance between recovery of faint detail and generating enough subs within a given amount of time to reduce background noise in post-processing. Only in exceptional circumstances would I go longer - for example, I want to capture the Squid (Outers4) in OIII and that is very faint indeed so 1hr subs may well help there.

With enough 30min subs you should be able to use post-processing to reveal even the faintest parts of the PacMan, doubling the exposure time and halving the number of subs I personally doubt would lead to an improvement. However, if you want to collect 10 x 1hr subs then OK, but given my own skies (and frequent cloudy nights!) that would represent a massive investment in imaging time.

I found my old data from 2014 on this and it seems I used 10 x 30min subs (I thought it was 20mins but there you go, my memory isn't too good :-) ). At this time I hadn't learned how to create a star mask which included stars embedded in bright nebula so deconvolution didn't work too well! :-) This is with the NP127is @ f/5.2.

ChrisH

Chris,

Nice image.  I am a bit perplexed.  I have had my scope set up for about a week now (no rain but frequent clouds).  My polar alignment is better than I have ever been able to get it using the right angle PA scope of AP--Polaris has remained precisely where it is supposed to be for 7 nights with no tweeks.  When I went to tweek PA, I did not do anything because there was nothing I could do to improve it without resorting to the drift method, or using Maxpoint, which, as Gina indicated, takes up imaging time (in my case allot of imaging time because I do not know how to use it).  The image attached is a stack of 6 30min subs.  Note there is still field rotation but only about 1/2 as much (to be expected).  The eccentricity of the 1 hour sub is something like .8 and the eccentricity of the stack of 30 min subs is around .55--still above .4 and you can see it in the tiny stars.  My guiding was working very well if rms errors are any indication (.08 to .15 pixels).  By the way you are correct, I use the np101is.  A couple of other interesting stats--the SNR of the 1 hour sub was around 24 and the stack of 6 30min subs was around 12.  Does that conform with what you would expect from theory?  Also, the FWHM of the 1 hour sub is a whopping 5.73, but it is not much better in the stack of 30 minute subs at 5.61.  Seeing was not great--but it wasn't that bad.  When I shot the Heart Nebula I did not have these issues and that target is as close to the pole (if not closer).  I love this target and really want to go all in, but I must get it right before investing the time.  No processing was done to this stack at all except aligning and stacking--no calibration was done.  A far cry from yours.  How do you get that clearness, that sharpness (lack of graininess?) Thanks for helping. 

 

[Rodd]

21 hours ago, ollypenrice said:

That looks very like field rotation to me. The centre is pretty good. The top two corners look like my field rotation when I get it (due to slight loss of mount position over a long time) but the bottom two corners are not so clearly rotating in the same way. This is affected by the position of your guide star, though. I'll still back field rotation and, as Chris says, you need better PA.

I also agree with Chris that an hour is a long time on a target which is not predominantly faint, though some 'Pacmen' do tend to leap out of a black sky all at once while good ones emerge more gradually because the faint outer parts have been caught. I settle for 30 minute Ha subs as a good compromise. I know that Tim Jardine has tried multi hour subs in seach of faint planetaries so I wouldn't rule out the ultra long sub per se. You're right to give it a try though. Experiment beats theory (or rather refines it.)

Olly

Olly-My polar alignment is better than its ever been--I went to tweek it after the 6th night and found I could not improve it without resorting to the drift or Maxpoint--both of which are beyond me and require too much time.  I attached a stack of 30 minute subs (6 of them) further down this thread and rotation is still evident (though not bas much--eccentricity of .8 vs .55).  The odd thing is I did not have a rotation issue when shooting the Heart Nebula (even with the Celestron at 1,960mm FL), and that target is in about the same area no?  Anyway--would a focal reducer help with rotation by reducing exposure time?

[ollypenrice]

I would have thought that a reducer would make it worse because it would widen the FOV. Rotation is around the guide star and the further you are from it the worse it gets, I'd have thought. If you are aiming to crop the reduced image to the size of the unreduced one then the reducer will not reduce exposure time. (The F ratio myth.)

The best test of your PA is to stack just your first and last sub together. Then look at the edges of the frames. How roatated are they relative to each other?

Olly

[Rodd]

9 minutes ago, ollypenrice said:

I would have thought that a reducer would make it worse because it would widen the FOV. Rotation is around the guide star and the further you are from it the worse it gets, I'd have thought. If you are aiming to crop the reduced image to the size of the unreduced one then the reducer will not reduce exposure time. (The F ratio myth.)

The best test of your PA is to stack just your first and last sub together. Then look at the edges of the frames. How roatated are they relative to each other?

Olly

Just a thought on the reducer--fishing as it were.  regarding stacking the 1st and last--I took 6 30min subs of the Pacman and the same elongation is present--but much less (eccentricity of .8 goes down to .55).  But I think all 6 subs are the same.  When I stack them they line up with each other perfectly.  The time difference between the 1st and last is about 2.5 hours.  If its not field rotation, what else could it be?  (I am becoming suspicious that I have another species of gremlin infesting the setup).

[Rodd]

14 minutes ago, ollypenrice said:

I would have thought that a reducer would make it worse because it would widen the FOV. Rotation is around the guide star and the further you are from it the worse it gets, I'd have thought. If you are aiming to crop the reduced image to the size of the unreduced one then the reducer will not reduce exposure time. (The F ratio myth.)

The best test of your PA is to stack just your first and last sub together. Then look at the edges of the frames. How roatated are they relative to each other?

Olly

Olly--sorry, I forgot something--you say the rotation is around the guide star.  But my elongation is in the same direction in every star (left to right) in my image.  If rotation is around the guide star, then the rotation should be in different directions in all areas of the image, no?  So, maybe its NOT rotation?

[ChrisLX200]

3 minutes ago, Rodd said:

Just a thought on the reducer--fishing as it were.  regarding stacking the 1st and last--I took 6 30min subs of the Pacman and the same elongation is present--but much less (eccentricity of .8 goes down to .55).  But I think all 6 subs are the same.  When I stack them they line up with each other perfectly.  The time difference between the 1st and last is about 2.5 hours.  If its not field rotation, what else could it be?  (I am becoming suspicious that I have another species of gremlin infesting the setup).

Well if they're still perfectly aligned after 2-1/2 hours then it's not likely to be rotation. I must say the outer field on these TV refractors is not perfect which is most apparent when using my 16803 sensor. Televue do sell a 'large field corrector' to address this and I actually have one on order, but with smaller sensors I wouldn't expect it to be such an issue. As I said earlier, the QHY PoleMaster is very good for achieving PA to 30" quickly, and if I had to set my mount up each time I would surely invest in one.

I would hazard a guess (in the absence of seeing your full resolution data) that your focus is significantly off - I know what these refractors are capable of and that razor-sharp detail just doesn't seem to be there. Mis-focus doesn't just affect the stars it also affects the fine wispy detail in the nebula - perhaps even more so as you can potentially do something about the stars but recovering blurred nebula detail is not really possible. Deconvolution helps when applied gently, as will an unsharp mask, but it's easy to destroy the image by excess sharpening. My example above is not good - as I said, I failed to protect the embedded stars with the mask and I should re-do it. On the other hand, I'll probably collect more data this year so I'll wait until it's all together.

As far as de-noising is concerned I use StarTools which (IMHO) has the best de-noising system available. I have PixInsight too and I can't get close to the effectiveness of StarTools (but that doesn't mean someone else could not - I'm no expert using PI). The thing about StarTools is that it records stats on image changes at every processing step, it generates a log of how the noise was generated, and at the last step - which is when tracking is switched off and the de-noise algorithm applied - it already has a pretty good idea of the optimal settings needed to get rid of it, which it does in a multi-step process.

ChrisH

[ollypenrice]

Yes, if the stars and the frames align after 2.5 hours then it isn't rotation after all.

The TV refractors I've seen here (only two of them) were very sensitive to cooldown, sufficently so to make one hour subs impossible, I'd have thought. Within that hour the focus drift would be fatal.

Olly

[Rodd]

9 minutes ago, ollypenrice said:

Yes, if the stars and the frames align after 2.5 hours then it isn't rotation after all.

The TV refractors I've seen here (only two of them) were very sensitive to cooldown, sufficently so to make one hour subs impossible, I'd have thought. Within that hour the focus drift would be fatal.

Olly

Funny--I have always thought my TV very insensitive to temp changes.  Granted--my scope is well equilibrated by the  time I start imaging.

[ollypenrice]

2 minutes ago, Rodd said:

Funny--I have always thought my TV very insensitive to temp changes.  Granted--my scope is well equilibrated by the  time I start imaging.

Environment plays a part. The Tak FSQs get varying reports of focus-temperature sensitivity, too.

Olly

[Rodd]

31 minutes ago, ChrisLX200 said:

Well if they're still perfectly aligned after 2-1/2 hours then it's not likely to be rotation. I must say the outer field on these TV refractors is not perfect which is most apparent when using my 16803 sensor. Televue do sell a 'large field corrector' to address this and I actually have one on order, but with smaller sensors I wouldn't expect it to be such an issue. As I said earlier, the QHY PoleMaster is very good for achieving PA to 30" quickly, and if I had to set my mount up each time I would surely invest in one.

I would hazard a guess (in the absence of seeing your full resolution data) that your focus is significantly off - I know what these refractors are capable of and that razor-sharp detail just doesn't seem to be there. Mis-focus doesn't just affect the stars it also affects the fine wispy detail in the nebula - perhaps even more so as you can potentially do something about the stars but recovering blurred nebula detail is not really possible. Deconvolution helps when applied gently, as will an unsharp mask, but it's easy to destroy the image by excess sharpening. My example above is not good - as I said, I failed to protect the embedded stars with the mask and I should re-do it. On the other hand, I'll probably collect more data this year so I'll wait until it's all together.

As far as de-noising is concerned I use StarTools which (IMHO) has the best de-noising system available. I have PixInsight too and I can't get close to the effectiveness of StarTools (but that doesn't mean someone else could not - I'm no expert using PI). The thing about StarTools is that it records stats on image changes at every processing step, it generates a log of how the noise was generated, and at the last step - which is when tracking is switched off and the de-noise algorithm applied - it already has a pretty good idea of the optimal settings needed to get rid of it, which it does in a multi-step process.

ChrisH

I do use a LFC even though my chip is not that big.  My focus may be a bit off--but I don't see how it can be significantly off.  I use a B mask and zoom to get very close and spend allot of time tweeking in very small increments to get the spike exactly in the center.  I'm talking very tiny movements.  It cant be THAT far off.  I looked into the QHY Polemaster and they don't make an adapter for the Mach 1.  Figures.  Ive heard phd has a good polar alignment tool.  I can't use PHD to guide with the dual chip camera without delving to to nasty patches and what not.  But I could just use it connected to the main camera to get polar alignment--think that would help?

[ChrisLX200]

The problem with TVs are that they have a rear element which is surrounded by a lot metal, cool-down is slow and unless you have a temperature probe _inside_ the OTA (not really possible) you have no idea what is going on. An external probe is useless because it will have apparently stabilised whilst the OTA is still far from it. The only solution is to keep re-focusing every 30-60mins regardless of what the temperature probe is indicating.

It's difficult to display in full resolution the detail which the NP127is is capable of capturing, I don't know if this crop jpg is any better but I'm well satisfied that (for a 5" aperture scope) it's tough to beat when you have it focused. Yeah, it's still noisy when viewed at 100% :-) Only 10 subs so I need more...

ChrisH

[Rodd]

3 minutes ago, ChrisLX200 said:

The problem with TVs are that they have a rear element which is surrounded by a lot metal, cool-down is slow and unless you have a temperature probe _inside_ the OTA (not really possible) you have no idea what is going on. An external probe is useless because it will have apparently stabilised whilst the OTA is still far from it. The only solution is to keep re-focusing every 30-60mins regardless of what the temperature probe is indicating.

It's difficult to display in full resolution the detail which the NP127is is capable of capturing, I don't know if this crop jpg is any better but I'm well satisfied that (for a 5" aperture scope) it's tough to beat when you have it focused. Yeah, it's still noisy when viewed at 100% :-) Only 10 subs so I need more...

ChrisH

The questions is --how far off is my focus.  Is teh B mask really that bad.  TV makes a digital focuser--not motorized as they stopped making there focus mate driver.  But there digital focuser measures movements to 1 micron.  I am thinking of getting that.  The problem is I do not know what focuser will be compatible with the  scope.  What would you recommend? I use Maxim so I do have the potential for autofocus at least with the TV

[Rodd]

50 minutes ago, ollypenrice said:

Yes, if the stars and the frames align after 2.5 hours then it isn't rotation after all.

The TV refractors I've seen here (only two of them) were very sensitive to cooldown, sufficently so to make one hour subs impossible, I'd have thought. Within that hour the focus drift would be fatal.

Olly

I put several frames into the blink tool and they are not lined up after all.  I originally thought this was due to dithering, but the movement is in 1 direction only and far greater the longer the subs are apart from one another.  So I have 2 major things to fix--focus and polar alignment.  Both are proving difficult.  Televue stopped making the fosus mate driver, so the best I can do with TV stuff is a 1 micron digital manual focuser.  1 micron is pretty small and it should work.  I don't mind manual focus if it works.  I am tired of learning curves.  

[ChrisLX200]

6 minutes ago, Rodd said:

I put several frames into the blink tool and they are not lined up after all.  I originally thought this was due to dithering, but the movement is in 1 direction only and far greater the longer the subs are apart from one another.  So I have 2 major things to fix--focus and polar alignment.  Both are proving difficult.  Televue stopped making the fosus mate driver, so the best I can do with TV stuff is a 1 micron digital manual focuser.  1 micron is pretty small and it should work.  I don't mind manual focus if it works.  I am tired of learning curves.  

Nobody said it was easy :-)  However, focussing happens to be one of the easiest to fix - just use an autofocuser because manual focussing simply won't hack it, and yes - I've tried both. You just _have_ to get perfect focussing or don't bother with the next step (which is to capture an image). No amount of post-processing will correct for poor focus. The Televue series are not too bad for rigging up a DIY stepper motor, and it's a darned sight cheaper to do that than buy the Televue focuser which if I remember correctly uses a DC motor that doesn't even give absolute positioning - which is why they provide a secondary digital position readout. My SharpSky kit produces sharp 'V'-curves which leave no room for guessing the correct focus position.

Note that SGPro absolutely nails the focus position - and that this is focusing using the OIII filter (not using LUM with a calculated offset!)

 

The Pacman image data I show above was captured using my old Losmandy G11 mount which was good, but not a patch on my current 10-Micron GM2000HPS. The stars are clearly not perfectly round by any means but I hope to do better later this year.

ChrisH

[Rodd]

9 minutes ago, ChrisLX200 said:

Nobody said it was easy :-)  However, focussing happens to be one of the easiest to fix - just use an autofocuser because manual focussing simply won't hack it, and yes - I've tried both. You just _have_ to get perfect focussing or don't bother with the next step (which is to capture an image). No amount of post-processing will correct for poor focus. The Televue series are not too bad for rigging up a DIY stepper motor, and it's a darned sight cheaper to do that than buy the Televue focuser which if I remember correctly uses a DC motor that doesn't even give absolute positioning - which is why they provide a secondary digital position readout. My SharpSky kit produces sharp 'V'-curves which leave no room for guessing the correct focus position.

Note that SGPro absolutely nails the focus position - and that this is focusing using the OIII filter (not using LUM with a calculated offset!)

 

The Pacman image data I show above was captured using my old Losmandy G11 mount which was good, but not a patch on my current 10-Micron GM2000HPS. The stars are clearly not perfectly round by any means but I hope to do better later this year.

ChrisH

OK--first off--I use Maxim and I simply cannot live through making a change again to a different system like SGP--which I do own by the way.  I like Maxim, it works for many people and I am committed to make it work for me--at least until I have my sea legs.  I have been playing musical chairs with software and at some point you just have to say that's enough.  Starlight instruments makes a focuser for TV np101is (and 127 is).  You have to call Televue to order it.  I already own the Boss II kit--I got it along with a stepper motor for the Celestron Edge.  I got it mostly for planetary/lunar work so I could focus without touching the scope.  It works ok--but the focus travel is so big for the Celestron, I have to disengage the motor and move it by hand if I change FL (like going from F10 to F25 for lunar).  It becomes a real pain if I have to switch to an eyepiece--like to collimate--then the focus positions are so far off between the eyepiece and the camera that it is exhausting making the change).  Manual focus for the Moon is the way to go.   I simply will not do a DIY anything--I spent allot of money on good gear and believe that I should be able to obtain state oif teh art gear to focus--I just don't know what model to get.

[Rodd]

28 minutes ago, ChrisLX200 said:

Nobody said it was easy :-)  However, focussing happens to be one of the easiest to fix - just use an autofocuser because manual focussing simply won't hack it, and yes - I've tried both. You just _have_ to get perfect focussing or don't bother with the next step (which is to capture an image). No amount of post-processing will correct for poor focus. The Televue series are not too bad for rigging up a DIY stepper motor, and it's a darned sight cheaper to do that than buy the Televue focuser which if I remember correctly uses a DC motor that doesn't even give absolute positioning - which is why they provide a secondary digital position readout. My SharpSky kit produces sharp 'V'-curves which leave no room for guessing the correct focus position.

Note that SGPro absolutely nails the focus position - and that this is focusing using the OIII filter (not using LUM with a calculated offset!)

 

The Pacman image data I show above was captured using my old Losmandy G11 mount which was good, but not a patch on my current 10-Micron GM2000HPS. The stars are clearly not perfectly round by any means but I hope to do better later this year.

ChrisH

The other thing I don't get is if you have to check focus every 30 minutes or so--(or after every sub ideally), how can you program in a 20 sub run and use dither for drizzle integration?  If you have to move to a star to focus after every couple of subs--you might as well do it 1 sub at a time, which means why have a software package that can be programmed to capture full imaging runs?