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Need help understanding pixel size and barlows Please

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#1 CTerry

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Posted 18 September 2023 - 03:25 PM

Ok so im not really new to planetary so i understand the basics. and ive looked at the FAQ page which is nice and helpful but i have a few things that are still just being bounced around my head and i would like to discuss.

 

just and fyi to get it out of the way, My scope is a celestron 127mm mak and i use a asi585mc and i use an orion sirius (same as heq5) mount.   both scope and camera are fairly new to me and im learning from experience. I came from a big dob and had an 8" sct for a while.

 

Anyway ive been trying to figure out the best barlow to get for my scope (F/12.5) and camera (2.9 pixels) setup.  Ive read a bunch of info on google and these forums and i get the 5x pixel rule and i also get that im supposed to match the barlow and scope to get me close to that pixel rule.  so 2.9 x 5 = 14.5  my scope is native at 12.5 which is pretty close. a 1.5x barlow gets me around 18.75 and a 2x puts me at 25.

 

this is where i get sorta confused. I have looked and imaged at native f/12.5 and with a 2x barlow at f/25 and the barlow seems better in my eyes and in my images.  but in threory native should be better right? because its closer to the 5x rule of 14.5 for my camera, also ive looked at others who have imaged with the same scope and a 120mc (3.75pixels) using a 3x barlow (f/36) and got better results that i can get.  How and why (minus the obvious seeing and conditions)??  is it because the 5x rule on the 120mc puts it at 18.75 and thats pretty much half the F/36 they are pushing with the scope??  does that add up to being ok in some way?

 

I really wish there was a local telescope guru around my area that could just come make it all work for me, but im alone and rely on the internet for help.  so help me obi-wan telescopey  your my only hope. lol

 

Also im not very smart and math isnt my strongest area, so if possible keep it simple and use small words. lol

 

thanks



#2 Tapio

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Posted 18 September 2023 - 03:40 PM

The 5x 'rule' is not exact, it just gets you in right direction.

If the conditions, optics, collimation and seeing allows you can go 7x (like mentioned in FAQ).

In FAQ there is also theory behind, link:

https://www.cloudyni...b-which-barlow/

 

How many frames you can get and how you process the video makes also difference (how some can do better than others).



#3 Tulloch

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Posted 18 September 2023 - 04:05 PM

As noted above, the "5x rule" is just a guide for "good" conditions to tell you when you shouldn't expect to see any more resolution from your scope due to its aperture. In very good conditions you might expect to see a little more resolution at higher magnifications, however I suspect there are other factors at play with your comparisons, such as seeing and processing.

 

The problem with going too much above the "optimal" focal ratio is that in order to get enough light on the sensor at a higher f/num, you either need to increase the gain on your camera (and so increase the noise) or increase the exposure time (and so reduce the number of frames for stacking which increases the noise in the final image).

 

The best way (I've found) to learn how to get better is to show others where you are, so post your images here with and without the barlow, along with the log files from Firecapture/Sharpcap so we can see the settings you used during capture. This will give us a good way to provide some pointers to help you along. 

 

Andrew



#4 CTerry

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Posted 18 September 2023 - 04:44 PM

So numbers recap cause repetition is memory.

 

so my scope... 127x1500  F/12.5 (due to slighter under measured mirror)   and my camera  asi585  2.9pixels

 

Barlows  &  F- ratio of scope

 

1.5x     18.75

2x        25

2.5x     31.25

3x        37.5

 

Camera & seeing pixel ratios

 

might as well stay inside seeing  3 x 2.9       8.7

average seeing                            4 x 2.9     11.6

good seeing                                 5 x 2.9     14.5

better seeing                                6 x 2.9     17.4

best seeing                                  7 x 2.9     20.3

 

So then just from those rough numbers and guessing that on a goodish night of clear skys in my area im probably around a 4x on seeing scale. im in bortle 7 area also.  that would mean I should either not use a barlow at all or go with a 1.5x and only use it on very nice clear nights.

 

that what im understanding here?

 

also im curious and this leads to another topic perhaps.  but when you image onto a camera sensor and the scale of the planet is X  then you barlow up that image say 2x you therefore double the image size across the sensor, while keeping the pixels the same size.  thus resulting in more pixels and better resolution across the planet. then if you were do downsize the image scale to match the non barlowed size... you would essentially have twice the resolution across the image.  right?



#5 Ittaku

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Posted 18 September 2023 - 05:01 PM

Yes, but half the light, limiting either how fast a frame rate you can use, or requiring more gain, pushing your signal to noise ratio down.


Edited by Ittaku, 18 September 2023 - 05:01 PM.


#6 Tulloch

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Posted 18 September 2023 - 06:17 PM

This video about barlows might help.

https://www.youtube....CEJVSkayYw&t=1s



#7 CTerry

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Posted 18 September 2023 - 06:53 PM

that is a good video. lots of information there.

 

 

Yes, but half the light, limiting either how fast a frame rate you can use, or requiring more gain, pushing your signal to noise ratio down.

what about binning? doesnt that counter the effect of loss light. also doubles the pixel size and allow for a better barlow match.

 

example...

 

2.9 x2binned = 5.8 pixel size.   5.8x 5 "good seeing" = 29.  thats closer to the 2x (25) or 2.5x (31.25) barlow and should in theory give me double the planet size and the same amount of light to the sensor. right?



#8 Ittaku

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Posted 18 September 2023 - 07:21 PM

that is a good video. lots of information there.

 

 

what about binning? doesnt that counter the effect of loss light. also doubles the pixel size and allow for a better barlow match.

 

example...

 

2.9 x2binned = 5.8 pixel size.   5.8x 5 "good seeing" = 29.  thats closer to the 2x (25) or 2.5x (31.25) barlow and should in theory give me double the planet size and the same amount of light to the sensor. right?

That will still severely limit your frame rate since the frame rate depends on the number of pixels used, even if it's then summated.



#9 CTerry

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Posted 18 September 2023 - 07:22 PM

im just overthinking this whole issue cause i want to understand it better. im also just trying to figure out how to get a better, bigger image into my little scope. lol  its fun to discuss and learn tho. 



#10 CTerry

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Posted 18 September 2023 - 07:26 PM

here is a without barlow native f/12 and a mid tier 2x barlow. lightly ran through the basic processing steps.

 

 

Attached Thumbnails

  • jup f12.png
  • jup f24.png


#11 Tapio

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Posted 18 September 2023 - 11:24 PM

Yes it's bigger but are there any more details?
Resize the first image to same size and compare.

The only way to get more details is to have excellent seeing and up to point getting bigger scope.
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#12 CTerry

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Posted 18 September 2023 - 11:32 PM

still think the barlowed version is better.   i also really need to learn how to process the image when im done. i do a really rough process. i know i could do more if i spent the time.  i could also use my laptop to do the image recording and get alot more frames in my short videos. that would help a lot im sure.

Attached Thumbnails

  • bigger crap.png

Edited by CTerry, 18 September 2023 - 11:35 PM.


#13 johnyj

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Posted 19 September 2023 - 03:51 AM

here is a without barlow native f/12 and a mid tier 2x barlow. lightly ran through the basic processing steps.

Judging from image size, that is more like 2.4x. I have the same problem, camera added some distance, so a 2x Barlow become 3.3x Barlow for me

 

And did you noticed the grainy feeling on the larger image? I regard that as a sign of the resolving power limit of the scope, where each adjacent pixel become less and less distinct

 

And you are right, always bin 2x2 pixels to 1 on Bayer sensor based cameras, since they require 4 pixels for full color information

 

To make a rough calculation, Jupiter is 50 arc second wide, and your 127mm aperture have a resolving power of 1 arc second. This means you would not get more than 50 distinct datapoint even in best seeing condition.  On Bayer sensor, any image beyond 100 pixels wide would not bring any more details. And your shot without Barlow is already 100 pixels wide, so you need to downsize that image to 50 pixels to make it look even better.

 

50 pixels may not sounds a lot, but the following image I just found here is only 33 pixels wide, just to see the potential 

post-33530-0-30792000-1676066448.png


Edited by johnyj, 19 September 2023 - 03:53 AM.


#14 RedLionNJ

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Posted 19 September 2023 - 08:29 AM

Judging from image size, that is more like 2.4x. I have the same problem, camera added some distance, so a 2x Barlow become 3.3x Barlow for me

 

To make a rough calculation, Jupiter is 50 arc second wide, and your 127mm aperture have a resolving power of 1 arc second. 

 

The first point may well be valid.

 

The second isn't. "Resolving power" as expressed above is not relevant. People routinely aim for 0.10 arcsec resolution with a C14. Scaling that down to 127mm indicates a resolution of 0.28 arcsec for the 5-inch Mak - almost four times better than the 1 arc second stated.  Take a look at some of the "small bore" threads for empirical evidence of this - there are some mighty fine festoons visible on some of the Jovian images, as well as the darker edges of some white ovals. They're all well under an arc second across, some around 0.25 arcsec.



#15 johnyj

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Posted 19 September 2023 - 09:38 AM

The first point may well be valid.

 

The second isn't. "Resolving power" as expressed above is not relevant. People routinely aim for 0.10 arcsec resolution with a C14. Scaling that down to 127mm indicates a resolution of 0.28 arcsec for the 5-inch Mak - almost four times better than the 1 arc second stated.  Take a look at some of the "small bore" threads for empirical evidence of this - there are some mighty fine festoons visible on some of the Jovian images, as well as the darker edges of some white ovals. They're all well under an arc second across, some around 0.25 arcsec.

If what you said is true, then you are defying the theoretical resolving limit of any optical instruments defined by Rayleigh criterion, why not collect more detailed results and submit them to scientific magazines/forums for serious discussion, if that criterion is totally off

 

Based on my own experience, I can not get better photo than OP with my 130mm reflector, and even a 200mm reflector would not help that much, due to average seeing condition

 

I do not really trust others photo here as a reality, so many commercial advertisement. For example, a guy in Hong Kong just published incredible detailed photo for Jupiter, how is that possible with that bad air in Hong Kong? They just had a typhoon season and typically they can not even see the sun clearly during afternoon (I visit there often)
 

I am indeed interested to see the potential of my telescope. A standard way to test optical instrument would be shooting resolution charts in a room and see how many lines per inch you could still separate them on the photo. But since the closest range I can get a focus is at least 100 meters, I have to shoot outdoor targets, that will be difficult for now, so I just trust the theoretical value


Edited by johnyj, 19 September 2023 - 09:41 AM.


#16 Tulloch

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Posted 19 September 2023 - 04:01 PM

If what you said is true, then you are defying the theoretical resolving limit of any optical instruments defined by Rayleigh criterion, why not collect more detailed results and submit them to scientific magazines/forums for serious discussion, if that criterion is totally off

 

Based on my own experience, I can not get better photo than OP with my 130mm reflector, and even a 200mm reflector would not help that much, due to average seeing condition

 

I do not really trust others photo here as a reality, so many commercial advertisement. For example, a guy in Hong Kong just published incredible detailed photo for Jupiter, how is that possible with that bad air in Hong Kong? They just had a typhoon season and typically they can not even see the sun clearly during afternoon (I visit there often)
 

I am indeed interested to see the potential of my telescope. A standard way to test optical instrument would be shooting resolution charts in a room and see how many lines per inch you could still separate them on the photo. But since the closest range I can get a focus is at least 100 meters, I have to shoot outdoor targets, that will be difficult for now, so I just trust the theoretical value

This topic is often discussed, you might want to read this thread.

https://www.cloudyni...ars-comparison/

 

Rayleigh limits are all about resolving two equally bright objects (such as stars), which is not what we are doing here. Our imaging and processing techniques allow us to do much better than Rayleigh to detect features..



#17 CTerry

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Posted 19 September 2023 - 04:38 PM

so im trying to understand all this.  but you all seem way more intelligent that me, so... yea. lol

 

given my equipment perhaps just throw me some advice on what i should do with it to get some great planetary images and ill start there. i learn better by doing anyway.

 

celestron 127mm mak f/12 ish  and an asi585 i control it all through an asiair plus and tablet.  i can however use a laptop to record the videos and get much higher frame rates if needed.  i have not tried that yet with this setup and i know it would help having more frames stacked up.  i also track with my orion sirius mount.

 

also mentioned the small bore challenge threads. i have looked and continue to look at those threads and thats what lead me here actually. just trying to make my stuff perform as well as others i have seen.  im also a bit skeptic on pictures posted. i know how easy it is to take a crap pic and photoshop and pretty one on top of it. anyway can fake a photo. lol  but i trust most people dont fake the pics i see on here.



#18 Tulloch

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Posted 19 September 2023 - 04:53 PM

given my equipment perhaps just throw me some advice on what i should do with it to get some great planetary images and ill start there. i learn better by doing anyway.

 

celestron 127mm mak f/12 ish  and an asi585 i control it all through an asiair plus and tablet.  i can however use a laptop to record the videos and get much higher frame rates if needed.  i have not tried that yet with this setup and i know it would help having more frames stacked up.  i also track with my orion sirius mount.

To start out, capture without a barlow with just the mak and the 585 (with an IR-cut filter which I assume you have). Connect a laptop to the camera and capture with Firecapture. Follow the instructions from section 13 onwards in the FAQ.

https://www.cloudyni...d-january-2023/

 

This will get you along the way, hope this helps :).

 

Andrew


Edited by Tulloch, 19 September 2023 - 04:53 PM.


#19 Ittaku

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Posted 19 September 2023 - 05:08 PM

I do not really trust others photo here as a reality, so many commercial advertisement.

That's really quite offensive given the time, effort, and money people invest in capturing and perfecting these planetary images. This is not that sort of forum, and any place on earth can have periods of good seeing.


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#20 CTerry

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Posted 19 September 2023 - 06:36 PM

here are 2 more just for fun.  same thing. saturn with and without 2x barlow. same night same time same process routine. etc etc.  just comparing with and without barlow.

Attached Thumbnails

  • saturn.png
  • saturn 2x.png


#21 t-ara-fan

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Posted 19 September 2023 - 07:44 PM

Yes, but half the light, limiting either how fast a frame rate you can use, or requiring more gain, pushing your signal to noise ratio down.


One quarter of the light on a given pixel.
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#22 t-ara-fan

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Posted 19 September 2023 - 07:49 PM

s and an asi585 i control it all through an asiair plus and tablet. i can however use a laptop to record the videos and get much higher frame rates if needed.


I get 200fps with that camera, a small Region Of Interest (ROI), and a USB3 laptop.

What is your latitude? Do you have an Atmospheric Dispersion Corrector?

#23 CTerry

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Posted 19 September 2023 - 07:54 PM

oh hey just had a thought. what is a good recommended distance from back of scope to camera.  i currently have it as shown, and only cause i slapped it on there and focused it and it worked ok. didnt ever think about changing it to a certain spacing.  i know this has been discussed a thousand times and im just too lazy to look it up.

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  • IMG20230919175024.jpg


#24 Tulloch

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Posted 19 September 2023 - 11:06 PM

I’m too lazy to answer
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#25 CTerry

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Posted 19 September 2023 - 11:17 PM

Lol well played. I was looking up info as i posted that cause my brain cant do only 1 thing at a time. But i figured it would have helped push the overall narrative further along and entice conversation... Guess not. Lol


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