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My image sampling crisis... opinions?

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#1 AstroEthan

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Posted 05 February 2018 - 02:07 AM

As said in a previous post, I've recently become concerned over image sampling because of a recent experiment in response to poor seeing. My setup has contained a 2.5x or 3x barlow for most of my 4.5 years of planetary imaging. I've always used a barlow of those magnifications because my 2x and 5x barlows gave inferior images. However, I've realized for a while that the 2.5x and 3x barlows, by the numbers, are over-sampling, especially with the tiny pixels of the ASI290MM, but they've given me excellent results when conditions permit. Also, the 2x barlow I have on hand is a cheap model that usually come in beginner kits, so I've been starting the think the inferior images from that is due to the quality of the barlow instead of under-sampling like I once thought.

The way I see it, less magnification than what I have now should give a brighter image, which would mean shorter exposures, less gain, less noise, and more frames per second, all without losing details and perhaps recovering details in poor seeing, so it would be smart for me to get a lesser magnification barlow if it's a big deal.

I currently get about f/27.5 with my 2.5x Powermate. Based on the numbers I get from the Useful Formulae web page from Wilmslow Astro, the most optimal focal ratio for my setup is F/14, so it looks like I would be best served by a 1.25x barlow. The only one I know of is the 1.25x Magic Dakon Barlow from Brandon.

 

I think it's important for me to hear about personal experiences from other planetary imagers before I make further decisions, so I would be glad to get some opinions on this!

 

BTW, here's my imaging train as of this post: C14 EdgeHD -> MoonLite Focuser -> USB Filter Wheel -> 2.5x Powermate -> ASI290MM.


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#2 Bart Declercq

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Posted 05 February 2018 - 03:20 AM

You might want to look at this glass-path corrector - https://www.teleskop...-Binocular.html - it's supposed to serve to extend the focus out for use with a bino-viewer, but it provides 1.25x magnification so it would serve for your purpose as well - you may need some kind of adapter to fix it onto the camera and telescope though and I haven't used it, so I can't comment on its optical quality.


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#3 Bart Declercq

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Posted 05 February 2018 - 03:30 AM

And thinking about it further - for planetary imaging, why not just use the 2.5x barlow and 2x2 binning? The 290MM has more than enough resolution that in most cases, the planetary image should fit in the field of view easily this way. For Lunar work I usually prefer staying a little under the theoretical limits, so you'd probably be fine using full resolution without a barlow and using drizzling or resampling during stacking to recover the little bit you lost by not enlarging 1.25x (say use 3x resample, then downsampling the final stacked image 2.5x in postprocessing)


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#4 CPellier

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Posted 05 February 2018 - 03:30 AM

The lens bloc of the Q-Barlow Baader is supposed to magnify 1,33x.



#5 CPellier

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Posted 05 February 2018 - 03:31 AM

Bart is right. With my former F/32 Gregorian I was using the ASI290MM in binning 2x and it worked perfectly. However it will not work with a color CMOS, only b&w.



#6 happylimpet

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Posted 05 February 2018 - 05:06 AM

As I'm sure you realise, using 2x2 binning with a large image scale will giveyou 2x the read noise relative to using half the focal length and no binning.  However Im not really sure how much of a diference this will make in practise.



#7 RedLionNJ

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Posted 05 February 2018 - 10:27 AM

Unless you have super-excellent seeing, I would give it a couple mornings' try with no barlow at all.  If the C14 is native f/10, then you may not be so badly off with the ASI290MM.

 

The other option (other than those already mentioned) is to get a custom barlow from somebody like Siebert Optics (http://www.siebertop...s-barlows1.html).

 

If you give him a call (or email) and let him know what spacing you expect between your barlow and your sensor, and what amplification (e.g. 1.4x) you seek, he can likely rustle you up something which fits the bill (for a price, of course!).


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#8 sfugardi

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Posted 05 February 2018 - 12:38 PM

Ethan, I use the 1.25" 1.25x Magic Dakin Barlow with my C14 and ASI224MC which gives around f/16+/-. However, the thread in front of the glass is different from the back so when I use the RG610 it has to go in between the camera and barlow pushing the mag to f20. There are 2 versions of this camera thread to choose from so if you go this route, find out which one will allow standard filters to be added to either side. Good luck and let us know which direction you are going,

 

Regards,

Steve



#9 ToxMan

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Posted 05 February 2018 - 12:38 PM

Here is a "barlow" I have used to get 1.5x...and I'd try the other suggestions too. I ran into similar issues 6 years ago when I went up in aperture.

 

Link: https://starizona.co...w-P1317C80.aspx

 

You can thread it into a camera nose piece.


Edited by ToxMan, 05 February 2018 - 12:39 PM.

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#10 h2ologg

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Posted 05 February 2018 - 07:31 PM

Short of making a variable system where a barlow "head"  is inserted into a sleeve a the front end of the filter wheel,

Here are some thoughts that work fairly well for me and I can place the barlow where I want when the situation warrents it (i.e. adding an IR cut filter for my OSCs):

1.  Cut down the 1.25" T-2 camera adapter that came with your ZWO 290mm to move it closer to the filter wheel (brought down my F# from F18 to F14).IMG_3071.JPG

2.  I cut mine down with a hack saw leaving about 10mm extended to add the barlow, or an IR cut filter.IMG_3072.JPG

3. When I want to attach the barlow head directly to the camera I can use the ZWO T-2 to 1.25" camera adapter and screw the whole unit with barlow into the ZWO 290mm.  Surprisingly I even have room for the camera mounted into a ZWO EFW (the barlow is attached to the adapter and slides into the back end of the EFW).IMG_3073.JPG IMG_3074 (1).JPG

 

I love my TV 2 x Powermate and nice when I can handle ~ F20 however these expensive Dakin Magic Barlows hold their own in optical quality at $128 a pop and will reduce my F# to something the seeing will handle.

Best,

Dan


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#11 RickV

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Posted 05 February 2018 - 08:55 PM

Williams Optics also makes a 1.25" Barlow of 1.2X magnification for their binoviewer... works well.



#12 kbev

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Posted 06 February 2018 - 12:39 AM

1.  Cut down the 1.25" T-2 camera adapter that came with your ZWO 290mm to move it closer to the filter wheel (brought down my F# from F18 to F14).attachicon.gif IMG_3071.JPG

2.  I cut mine down with a hack saw leaving about 10mm extended to add the barlow, or an IR cut filter.attachicon.gif IMG_3072.JPG

Dan, I gather from this that the adapter has filter threads along the entire inside length?  



#13 AstroEthan

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Posted 06 February 2018 - 07:23 PM

Oh boy, tons of responses! I didn't expect so many! lol.gif
 

 
 

And thinking about it further - for planetary imaging, why not just use the 2.5x barlow and 2x2 binning? The 290MM has more than enough resolution that in most cases, the planetary image should fit in the field of view easily this way. For Lunar work I usually prefer staying a little under the theoretical limits, so you'd probably be fine using full resolution without a barlow and using drizzling or resampling during stacking to recover the little bit you lost by not enlarging 1.25x (say use 3x resample, then downsampling the final stacked image 2.5x in postprocessing)

Bart is right. With my former F/32 Gregorian I was using the ASI290MM in binning 2x and it worked perfectly. However it will not work with a color CMOS, only b&w.

Hmm... I didn't think about that. Perhaps it'll be a temporary solution until I do get a more optimal barlow, but not permanent because...
 

As I'm sure you realise, using 2x2 binning with a large image scale will giveyou 2x the read noise relative to using half the focal length and no binning.  However Im not really sure how much of a diference this will make in practise.

 

 

 

Unless you have super-excellent seeing, I would give it a couple mornings' try with no barlow at all.  If the C14 is native f/10, then you may not be so badly off with the ASI290MM.

Experimenting with it last month was why I started this thread in the first place! lol.gif
 

The other option (other than those already mentioned) is to get a custom barlow from somebody like Siebert Optics (http://www.siebertop...s-barlows1.html).
 
If you give him a call (or email) and let him know what spacing you expect between your barlow and your sensor, and what amplification (e.g. 1.4x) you seek, he can likely rustle you up something which fits the bill (for a price, of course!).

I'll have to take a deeper look at that later. I came across that website while writing the original post, but didn't dig very deep. They look nicely made!

 

 

 

Ethan, I use the 1.25" 1.25x Magic Dakin Barlow with my C14 and ASI224MC which gives around f/16+/-. However, the thread in front of the glass is different from the back so when I use the RG610 it has to go in between the camera and barlow pushing the mag to f20. There are 2 versions of this camera thread to choose from so if you go this route, find out which one will allow standard filters to be added to either side. Good luck and let us know which direction you are going,

I use my filters in a filter wheel, so I don't think there will be a problem with threading, and I'll definitely let everyone know what happens. Regardless, I'll probably do a comparison of all barlows (currently four, perhaps five soon) the next time I can. It's been a few years since I've done one!

 

 
Thank you to everyone that has contributed their thoughts on this thread so far! Even if I didn't respond to every reply on this post, I did read all of them. My front-runner is still a Magic Dakin Barlow, probably 1.25x, but the other possibilities presented are also interesting!


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#14 h2ologg

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Posted 06 February 2018 - 08:23 PM

 

1.  Cut down the 1.25" T-2 camera adapter that came with your ZWO 290mm to move it closer to the filter wheel (brought down my F# from F18 to F14).attachicon.gif IMG_3071.JPG

2.  I cut mine down with a hack saw leaving about 10mm extended to add the barlow, or an IR cut filter.attachicon.gif IMG_3072.JPG

Dan, I gather from this that the adapter has filter threads along the entire inside length?  

 

Kevin, Threads to the base of the ZWO adapter, so you can cut the length to your level of comfort.  I had extra adapters so I didn’t mind experimenting with one.

Dan


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#15 AstroEthan

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Posted 11 February 2018 - 04:27 PM

I did some more calculations and saw the optimal sampling depends on the filter. That's not new to me, but I was surprised to find that it ranged from F/10 at the low end of red, all the way to F/20 at the high end of blue. Before today, I assumed the range was much smaller so I only calculated green's focal ratio and based my thinking off that.

 

I have no idea if I should aim for the middle, which is what I was planning to do with a 1.25x barlow before this new info, or the high end of the range, where I wouldn't be under-sampling blue data, but at at the cost of dimmer data. Once I know which is more optimal, I think I can make a final decision! smile.gif



#16 troyt

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Posted 11 February 2018 - 10:10 PM

Sample for white light ಠ‿ಠ



#17 CPellier

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Posted 13 February 2018 - 04:50 PM

Yes: don't bother trying a special samling for blue light, usually many factors do limit effective resolution in practice, such as rayleigh scattering, loss of strehl ratio, lower seeing, and so on.

Only in IR light you can on some situation think about lowering the focal length. For example, CH4 imaging of the gas giants is efficiently made at a much shorter focal length.



#18 Guest_djhanson_*

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Posted 13 February 2018 - 08:26 PM

Hi Ethan, a few years ago I also was over-sampling with my C14 using a TMB 1.8x barlow.  But what made this worse was that my imaging train was amplifying my barlow.  Next, I migrated to my current CFF350 and decided to downsize to a 1.25x Brandon barlow. (courtesy of Paul's reports on it)  Great little barlow, but once again I was placing it in such a way to cause significant barlow amplification.  I just wasn't getting the desired frame rates and realized I was still over sampling.  So anyways, about a year ago I wised up and rebuilt my imaging train with various adapters and new EFW:

 

https://www.cloudyni...nt#entry7626555

 

I'm pretty happy with these new adapters and I rebuilt it so as to minimize any barlow amp.  (that is when I do use a barlow now - e.g. Mars - Jove I like imaging without a barlow since my CFF is f/17)

 

I will say the trickiest part to spec'ing out the adapters was just getting the diameter and thread jargon down :D  It's a bit confusing at first :D

 

cheers, DJ


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#19 gregj888

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Posted 14 February 2018 - 01:17 PM

I use eyepiece projection for speckle (double stars).  Similar requirements...

 

For bright objects, binning is fine.  I bounce off the noise floor, so no binning with CMOS cameras if I can help it.  Lots of discussion in earlier threads, CMOS camera bin mathematically, not by shifting charge.




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