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Narrowband Imaging – Is it time to finally ditch the SCT?

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#1 jsquires80

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Posted 26 October 2020 - 02:35 PM

I have been imaging using a Celestron 11” XLT with a .63 focal reducer and L-Extreme filter, ASI294MC on a CGX mount using an off-axis guider/ASI224MC.  I’m in a severely light polluted city and would like to invest in a mono camera and narrowband filters to help deal with the light pollution.

My issue is that even with the off-axis guider, I am not able to get exposures longer than about 3 minutes without getting some wonky looking stars.  However, from what I read about narrowband, most people are getting their best results with longer exposure times of 5+ min.

I am getting “excellent” alignment using All Star Polar Align, and PHD2 is typically indicating guiding errors of about .65.  I am starting to think that 3 minute subs is just the best I am going to be able to pull off at a 1,750 mm focal length given my current equipment.  If anyone has experience with this type of setup that proves me wrong, please do share, but I am not looking to upgrade my mount at this time.

 

My question is, would I likely benefit by switching to a shorter focal length scope, APO or reflector, which in theory would be more forgiving and allow longer exposure times?

I know that it all “depends” and that there are mathematical formulas that can calculate ideal exposure times.  But I am not that sophisticated and am just looking for those who can perhaps share some general real-world experience.


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#2 Ron (Lubbock)

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Posted 26 October 2020 - 03:29 PM

It sounds like your main problems stem from mount limitations, so you can try shortening the required sub length.  You could try going to a smaller, lighter telescope that has a shorter focal length so that guiding isn't so critical.  To keep the subs very short, keep the f-ratio low.  I am currently using a reducer with my refractor that gets me down to f/3.6, and the longest subs I have bothered to acquire are of 4 minutes duration.  Anything longer will burn out the bright stars, so it's pointless. 

 

The image scale of 2.6" per pixel is so forgiving that I actually set up and did an entire imaging session a few nights ago without bothering to do polar alignment!  I pointed the mount at a star (Caph) and make crude manual adjustments to RA to get it close, but the rest of the error just guided out with PhD.  You won't see much of an effect in a 1-2 minute sub.



#3 DaveSD

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Posted 26 October 2020 - 04:15 PM

I image with a similar setup to yours, see my signature.  I have gone from taking 5 minute subs with an L-eNhance to 10 minute subs, since getting an L-eXtreme and still have round stars.  If guiding, I don't see a reason why the exposure duration change from 3 minutes to 10 minutes should make much difference (other than wind or some other external force).  If you are really getting a guiding error of .65" - that should be great and I would look for your problem elsewhere.


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#4 rkinnett

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Posted 26 October 2020 - 05:02 PM

A general rule of thumb for DSO work is you want your total payload weight to not exceed 50% of your mount's rated capacity.  I'm guessing with your scope plus imaging gear and mounting plates, your total payload weight comes in around 40 pounds, which is 73% of your mount's capacity.  It's okay to exceed that guideline with shorter focal lengths, given much less stringent autoguiding requirement as you noted, but with an SCT you're bound to get fuzzy stars.  I operate a 10" fast Newtonian on an EQ6R and am just under the rated payload capacity, but with only 1000mm focal length, I can get away with it.  Overrunning the 50% payload guideline means the setup will be more prone to vibration from wind, or through the ground, or ringing in response to tracking and autoguiding control inputs.  Which isn't to say it's not possible to get good results with your setup.. just challenging. 

 

As Dave noted.. if you're guiding 0.65" RMS consistently, then your 3-minute limitation is likely not due to the mount. 

 

Another common cause of blurry stars is imaging at higher resolution than seeing allows.  Our atmosphere typically scatters starlight on the order of several arcseconds.  With your long focal length, that equates to several pixel widths of blur, in addition to guiding error blur and other sources.  But this also should not limit you to 3 minutes.

 

In what way are your stars wonky?  Is it really a function of exposure duration?

 

Anyways, point being, it sounds like you may still have some potential for optimization, but understand you're fighting an uphill battle when imaging with long focal length.  It can be done, and there are clear benefits when you're imaging narrow objects.  Shorter focal length is indeed easier to handle, but for a difference class of targets.  Rather than framing this question as SCT vs refractor, consider getting a refractor or fast Newt for wide field targets, and keep working on optimizing your SCT setup for narrow field.


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#5 nimitz69

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Posted 26 October 2020 - 05:42 PM

Why do you think you need longer subs to get better images?  All things being equal it’s total integration time that trumps everything.  While you’re certainly going to need more total time in LP skies than a dark location people take amazing images with OSC cameras in Bortie 8 skies.  My skies are SQM 19.5 and for LRGB I only do 2min subs ....

 

BTW, not try to talk you out of mono since all things being equal mono will always produce a bette image than an OSC.  At extremely dark sites OSC s can approach a mono but make sure you’re switching for the right reasons 


Edited by nimitz69, 26 October 2020 - 05:46 PM.

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#6 bobzeq25

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Posted 26 October 2020 - 06:05 PM

I have been imaging using a Celestron 11” XLT with a .63 focal reducer and L-Extreme filter, ASI294MC on a CGX mount using an off-axis guider/ASI224MC.  I’m in a severely light polluted city and would like to invest in a mono camera and narrowband filters to help deal with the light pollution.

My issue is that even with the off-axis guider, I am not able to get exposures longer than about 3 minutes without getting some wonky looking stars.  However, from what I read about narrowband, most people are getting their best results with longer exposure times of 5+ min.

I am getting “excellent” alignment using All Star Polar Align, and PHD2 is typically indicating guiding errors of about .65.  I am starting to think that 3 minute subs is just the best I am going to be able to pull off at a 1,750 mm focal length given my current equipment.  If anyone has experience with this type of setup that proves me wrong, please do share, but I am not looking to upgrade my mount at this time.

 

My question is, would I likely benefit by switching to a shorter focal length scope, APO or reflector, which in theory would be more forgiving and allow longer exposure times?

I know that it all “depends” and that there are mathematical formulas that can calculate ideal exposure times.  But I am not that sophisticated and am just looking for those who can perhaps share some general real-world experience.

Experience. 

 

You do need to understand just this much theory.  You want to shoot long enough subs that the read noise of all your subs (which adds with every sub) is negligible compared to other noise.  Read noise is something you can control.

 

In my Bortle 7 skies my proper sub length is about 10X as much with narrowband as with broadband, to properly reduce read noise, which ruins dim detail.  To put numbers on it (I have some different setups so they're just illustrative) 1 minute for broadband, 10 minutes for narrowband.

 

So you can just shorten the sub length, at the cost of some dim detail.  Or, you can get faster optics.  Or a better mount.  Or just accept slight blurring with longer subs.

 

There is no magic fix here.  There are only tradeoffs.


Edited by bobzeq25, 26 October 2020 - 06:10 PM.


#7 jsquires80

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Posted 26 October 2020 - 06:36 PM

Thank you all for the helpful replies.  My SCT has served me well over the years and I am certainly not looking to buy all new optics if I don't have to especially considering that many of my favorite nebula seem to be framed so nicely at the longer focal length.  It gives me some confidence that there are some who are able to successfully get longer exposures and round stars with similar setups.

 

For perspective here is a typical image taken from about 30 X 3 min subs.  I also tried 5 minute subs once and the star trails were noticeably worse.  But as some stated there could have been other factors such as wind that night so it sounds like I should give it another go.

 

Regarding total exposure time, I have heard that there could be a point of diminishing returns and that I would unlikely notice any improvement when stacking any more than about 40 or so exposures.  But I also see some examples of images online where way more frames were used, any general thoughts on this?

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#8 meegja

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Posted 26 October 2020 - 08:09 PM

I made this with an Meade LX65 8" f/10 (2032mm) ACF scope with exposure times of 160 x 60 and 80 x 120 seconds and no reducer. Not with the L-Extreme but with the L-Enhance. I did find that with those exposure times enough data is collected as long as you make enough exposures.
I am by far experienced, only started with all this around March this year. But I did find that a lot is possible with an SCT, even in my Bortle 7/8 skies. So really you should try at least once to aim for like 120 second exposures and then like around 100 of them and see what you get.

Don't have experience with guiding, all this is done with only tracking of the Ioptron CEM25EC mount.

 

The full resolution image and further info is on https://www.cloudyni...pelican-nebula/

 

hh555.jpg


Edited by meegja, 27 October 2020 - 03:16 AM.


#9 jgraham

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Posted 26 October 2020 - 09:01 PM

I use the same exposure times with my narrowband images as I do with my white light images, typically 4 minutes. Most narrowband targets are emission nebula of one sort or another and the emission lines are not dimmed by the filter; only the off-band, broadband light is dimmed (which is kinda the point). I have done a lot of narrowband imaging using really fast scopes like my 5.5" f/3.6 Comet Catcher and 6" f/3.6 Comet Tracker up to my 8" f/8 RC and the biggest difference between them is the field of view. Right now I'm luv'n my RC with it's wonderfully flat field. If you are having tracking issues then going with a smaller, lighter scope may help. A nice, wide field scope is a handy tool to have anyway.

 

Food for thought.


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#10 bobzeq25

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Posted 26 October 2020 - 11:36 PM

Thank you all for the helpful replies.  My SCT has served me well over the years and I am certainly not looking to buy all new optics if I don't have to especially considering that many of my favorite nebula seem to be framed so nicely at the longer focal length.  It gives me some confidence that there are some who are able to successfully get longer exposures and round stars with similar setups.

 

For perspective here is a typical image taken from about 30 X 3 min subs.  I also tried 5 minute subs once and the star trails were noticeably worse.  But as some stated there could have been other factors such as wind that night so it sounds like I should give it another go.

 

Regarding total exposure time, I have heard that there could be a point of diminishing returns and that I would unlikely notice any improvement when stacking any more than about 40 or so exposures.  But I also see some examples of images online where way more frames were used, any general thoughts on this?

There is no real "diminishing returns" point.  But signal to noise ratio increases only with the square root of the total imaging time.  So going from 2 hours to 4 hours gives you the same improvement as going from 1 to 2 hours.

 

Practical bottom line.  Do not worry about "diminishing returns" until you're over 10 hours total imaging time. And imagers who do more than that on one target are generally informed enough to make their own decisions.

 

So, not an issue.  Shoot more subs.  <smile>  The actual number of subs is pretty irrelevant, the important parameter is total imaging time.   With my C8 RASA in my Bortle 2 skies, my subs are very short, and I shoot hundreds of them.  I guarantee you, it's worthwhile.


Edited by bobzeq25, 26 October 2020 - 11:38 PM.

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