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Which f ratio to use for planetary

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#1 gfunkernaught

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Posted 03 July 2020 - 11:22 AM

My focal length is 2032mm f/10. I need help figuring out which f ratio to use with different cameras. I know about the 5x pixel size formula. The thing is the numbers I get leave me in the middle of this rule. My canon eos ra pixel size is 5.36 microns, so that would require f/25, so a 2x barlow would have to be used. Even the bottom end of the formula I've seen 3xpixel size, would require f/15. My asi290mm pixel size is 3 microns, so again f/15. If I need to use f/15, do I round up or down in this situation?

#2 kbev

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Posted 03 July 2020 - 12:24 PM

You can purchase barlows from Siebert Optical that will get you in the range you need, and there are some other options as well.  Ultimately I would err on the side of running a shorter focal length, so f/10 for the 290MM and f/20 for the EOS.  Also keep in mind that the stated magnification on the barlow can be affected by how the optical train is set up, so you may wind up getting more like 2.2-2.3x when everything is assembled. 


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#3 gfunkernaught

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Posted 03 July 2020 - 03:08 PM

My image train is as follows for each camera:

2" visual back>2x barlow>t-adapter>t-ring>canon eos ra

2" visual back>2x barlow>asi290mm

 

I just got a ZWO ADC so I will be adding that to the mix, I'm assuming the best place for it is right after the visual back.  I also ordered a Celestron 2" to 1.25" adapter so I can use my Baader 2" click lock as my visual back, it is just more secure and easier to work with than celestron's stock visual back that came with my SCT.  Last night my AVX ran into a cable snag and made some discerning noise while slewing afterwards.  I hope I didn't damage anything.



#4 Sunspot

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Posted 03 July 2020 - 05:34 PM

I use this barlow with my Mewlon. It is designed to work between 1.2x and 2x. I use it at 1.6x on my system. It is expensive, but high quality.

 

https://www.highpoin...2x-1-25-mc-vipb

 

Brandon also makes less than 2x barlows, here is a link to the 1.5x version. They also make a 1.25x version, which I use.

 

https://agenaastro.c...50xbrandon.html

 

All these barlows offer flexibility so with the other suggestions, you have plenty to choose from.


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#5 gfunkernaught

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Posted 03 July 2020 - 08:26 PM

I use this barlow with my Mewlon. It is designed to work between 1.2x and 2x. I use it at 1.6x on my system. It is expensive, but high quality.

 

https://www.highpoin...2x-1-25-mc-vipb

That barlow is adjustable?  



#6 kbev

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Posted 03 July 2020 - 09:39 PM

That barlow is adjustable?  

Looks to be.  Here is a link to the manufacturer's site, on there you can download a PDF that goes into detail about how to adjust the magnification: VIP 2x modular barlow lens



#7 Tulloch

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Posted 04 July 2020 - 07:37 PM

That barlow is adjustable?  

Well, most barlows are "adjustable" up to a point, it just depends on the distance between the front lens element and the eyepiece/sensor. See the graph on the Tele Vue site here

http://www.televue.c...d=52&Tab=_photo

 

Andrew



#8 SupernovaDust

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Posted 17 July 2020 - 05:23 PM

2" visual back>2x barlow>asi290mm

 

I just got a ZWO ADC so I will be adding that to the mix, I'm assuming the best place for it is right after the visual back.

This thread and others regarding ADC and barlows makes me question if I made the right choice for my planetary imaging endevours. I've a similar setup but with a 1.25" back, celestron X-Cel 2xbarlow, ADC and ASI290MC in my NexStar Evolution 6" (1500mm f-length). I know that technically the best f-ratio for the camera would be 14.5 (when calculating by 5, 20.3 when calculating by 7 as others suggest).

 

In this thread John Boudreau suggest that first comes the barlow, then the ADC:

https://www.cloudyni...c/#entry7914331

 

This is also consistent with various other sources I've found regarding barlows and ADC.

 

I've ordered the 2x barlow because my thinking was that it's just about right when calculating by 7 and I'd like to have a bigger picture. But then I didn't realize yet that the ADC comes after the barlow, which will add another 30mm distance. And the X-Cel is a shorty, can it be estimated what my effective f-ratio will be with the chosen setup? The ADC will be delivered next week and I'll make some star-measurements then but it'd be nice to know beforehand. 

 

With my old 2x barlow from the celestron eyepiece kit there is a trick I've seen for the ASI290MC (2.9um pixelsize), the lense can be unscrewed from the barlow body and attached directly to the nose-adapter of the camera, resulting in a 1.5x magnification. Does anyone know if this works with the X-Cel barlow too? The front lense can also be unscrewed, and I think I can swap the nosepiece (threaded) from the camera with the one of the ADC (the later seems to be unthreaded from pictures on google). Could I attach the barlow lense then to the nosepiece of the ADC directly, which would reduce the distance by about 35mm to the chip compared to ADC in barlow body? If I could get into focus this way I would be slightly under f.20 (5mm less distance) even with the ADC, right?

 

But there are also pictures on google where the barlow comes after the ADC, would it work this way too or why are most people suggesting to have the barlow first, ADC second?

 

Lastly, how severe is the impact of oversampling planets and the moon really? As I understand, it will introduce more noise to the signal but if not too extrem, it would also provide more detail and SNR can be improved with bigger stacks.


Edited by SupernovaDust, 17 July 2020 - 05:26 PM.


#9 Tulloch

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Posted 17 July 2020 - 05:35 PM

This thread and others regarding ADC and barlows makes me question if I made the right choice for my planetary imaging endevours. I've a similar setup but with a 1.25" back, celestron X-Cel 2xbarlow, ADC and ASI290MC in my NexStar Evolution 6" (1500mm f-length). I know that technically the best f-ratio for the camera would be 14.5 (when calculating by 5, 20.3 when calculating by 7 as others suggest).

 

With my old 2x barlow from the celestron eyepiece kit there is a trick I've seen for the ASI290MC (2.9um pixelsize), the lense can be unscrewed from the barlow body and attached directly to the nose-adapter of the camera, resulting in a 1.5x magnification. Does anyone know if this works with the X-Cel barlow too? The front lense can also be unscrewed, and I think I can swap the nosepiece (threaded) from the camera with the one of the ADC (the later seems to be unthreaded from pictures on google). Could I attach the barlow lense then to the nosepiece of the ADC directly, which would reduce the distance by about 35mm to the chip compared to ADC in barlow body? If I could get into focus this way I would be slightly under f.20 (5mm less distance) even with the ADC, right?

 

Lastly, how severe is the impact of oversampling planets and the moon really? As I understand, it will introduce more noise to the signal but if not too extrem, it would also provide more detail and SNR can be improved with bigger stacks.

Darren (aka DMach) produces some of the best (if not the best) images of the planets with his 11" SCT, ASI290MC and 2x Barlow, sampling at around 7x the pixel size (check out his work if you haven't seen it already). He sometimes comments that he possibly bought the wrong camera as his f/num is too large, but I reckon his results speak for themselves. In excellent seeing, having the extra length is really working for him. Would his results still be applicable in poor seeing? Who knows?

 

Having said that, you should be able to unscrew the lens off the Barlow and put it directly into the ADC like a "filter" - it should work, well worth trying. The focus point won't be an issue, SCTs have a huge focus range.

 

Severe oversampling leads to lower SNR and you need higher gain levels to get the signal strength up - more gain = more noise. How severe is it? Hard to know. But the point is that you wont be getting any more resolution from your system and the noise levels will be higher.

 

Andrew


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#10 SupernovaDust

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Posted 17 July 2020 - 05:45 PM

Darren (aka DMach) produces some of the best (if not the best) images of the planets with his 11" SCT, ASI290MC and 2x Barlow, sampling at around 7x the pixel size (check out his work if you haven't seen it already).

Thanks Andrew for the detailed reply, I'm a bit less worried now about my choices laugh.gif And yes Darren's pictures are stunning and according to astrobin he's also using an ADC. Is it known if he puts the ADC before or after the barlow?


Edited by SupernovaDust, 17 July 2020 - 05:46 PM.


#11 Tom Glenn

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Posted 17 July 2020 - 06:45 PM

Thanks Andrew for the detailed reply, I'm a bit less worried now about my choices laugh.gif And yes Darren's pictures are stunning and according to astrobin he's also using an ADC. Is it known if he puts the ADC before or after the barlow?

Please keep in mind that Darren's images represent seeing conditions the likes of which you will almost certainly not encounter from your location.  He is on the equator, surrounded by a tropic ocean climate, with the planets high in the sky.  Basically the opposite of where you (and most) people are located.  The difference is enormous, and in those conditions, you can use some imaging parameters that wouldn't be advisable elsewhere.  This includes large oversampling, slower shutter speeds, lower frame rates, etc. 


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#12 SupernovaDust

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Posted 17 July 2020 - 07:52 PM

Please keep in mind that Darren's images represent seeing conditions the likes of which you will almost certainly not encounter from your location.  He is on the equator, surrounded by a tropic ocean climate, with the planets high in the sky.  Basically the opposite of where you (and most) people are located.  The difference is enormous, and in those conditions, you can use some imaging parameters that wouldn't be advisable elsewhere.  This includes large oversampling, slower shutter speeds, lower frame rates, etc. 

Yes I'm aware of that which is why I do not want to increase the magnification of the already 2x barlow.

 

I've also read in a german manual why the barlow should be positioned before the ADC (page 6):

https://www.teleskop...hop/pdf/adc.pdf

 

Which google translates into (quite well I think):

 

ADCs are designed to work best at high f-ratios (> f / 10). With short f-ratios or with large corrections due to the very low level of the object, aberrations can occur which lead to a reduction in the ADC effect. To reduce aberrations with a short focal length ratio, it is best to place the ADC directly behind a Barlow lens and at an appropriate distance from the camera. If you move the ADC closer to the Barlow and thereby extend the distance to the camera, the ADC correction is increased. For longer focal length ratios, such as with SCTs, arranging the ADC is less problematic

 

So although I should be free to chose to place the ADC before or after the barlow with my f/10 I will probably get slightly better results when I attach the barlow lense to the ADC.



#13 SupernovaDust

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Posted 19 July 2020 - 11:29 AM

Okay bad news (for me), although I can detach the lense from my Celestron 2x X-Cel Barlow it does not fit into a standard filter thread, meaning I will overshoot the optimal f-ratio quite drastically when I add the ADC after the barlow.

 

The good news is (for everyone interested in this topic), I found an article, actually two, which cover every aspect of ADCs in the most imaginable detail, including the difference of an ADC before and after the barlow and also at which heights and f-ratios the abberation is worse than the dispersion, e.g. when to use an ADC at all and how the airy pattern changes. What I also didn't realize, ADC correction shifts the image a bit (they present calculations by how much) and might need to be titled back, which causes a slight miscollimation.

 

General explaination: http://skyinspector....-corrector--adc

Indepth incl. computations made with optical modelling software: http://www.skyinspec...o.uk/adcs-part2

 

Bottom line for ADC before the barlow: it might increase abberation effects but the prism angle can be greatly reduced, leading to less abberation. In other words: test it out :D



#14 gfunkernaught

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Posted 27 July 2020 - 12:08 AM

Glad I stopped by this thread because I had no idea I could just unscrew the barlow lens and attach it directly to my asi290mm mini, the ADC, and also to my T-adapter that is attached to my canon eos ra.  Makes things interesting.  Thanks for continuing the convo!



#15 DMach

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Posted 27 July 2020 - 06:04 AM

Sorry, I missed this thread until now.

 

Here's how I have my imaging train set up:

 

planetary_imaging_train_C11_ASI290MC.jpg

 

I place the ADC after the barlow/PowerMate for this setup. As you've stated above, something to consider here in the impact on effective magnification and f-ratio. The magnification curve for the 2x PowerMate is effectively flat though, so I felt OK about this setup.

 

The choice of ordering for my is primarily driven by another practical concern: keeping dust out of the imaging train. Things get very dusty very quickly here, so I like the idea of using threaded connections as much as possible (which dictates the ordering somewhat) and I keep everything permanently assembled, including the ADC. This is of course often sub-optimal/unnecessary at my location (as the article you posted sates, plus you do lose some photons as they pass through the ADC) but I like the flexibility of being able to switch to low altitude targets e.g. to catch a particular feature, moon transit etc. Seems to work OK.

 

I would never have done this on my 6" scope as I needed all the signal I could get - when I tested with and without the ADC at high altitudes on my 6SE, it had a significant impact on the histogram. But the C11 pulls more photons in and the difference is much less.

 

And even given the conditions I'm spoiled with:

  • I'd still like to try at something close to 5x f-ratio one day and see whether I'm actually pulling more detail at the higher f-ratio - I suspect it has far more to do with the seeing. I'm thinking about ordering one of those Sieberts one day ...
     
  • I'm still not convinced about chasing high frame rates. This is not just based on my experience, but also conversations I've had with imagers at more "typical" latitudes and landlocked locations lol. Would be interesting to play with that ASI462 though, given the results Darryl is showing re: noise ...

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#16 gfunkernaught

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Posted 01 August 2020 - 07:42 AM

Just realized that I can still use my Canon for color planetary by getting a 2.5x barlow.  So my pixel size is 5.36 microns x 5 = 26.8

F/10x2.5=F/25

 

That should work right?  Or am I missing something?



#17 RedLionNJ

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Posted 01 August 2020 - 12:54 PM

Just realized that I can still use my Canon for color planetary by getting a 2.5x barlow.  So my pixel size is 5.36 microns x 5 = 26.8

F/10x2.5=F/25

 

That should work right?  Or am I missing something?

That would be fine, as long as you do truly get around 2.5x with whatever spacing you end up with.



#18 Mike Phillips

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Posted 01 August 2020 - 02:47 PM

Please keep in mind that Darren's images represent seeing conditions the likes of which you will almost certainly not encounter from your location.  He is on the equator, surrounded by a tropic ocean climate, with the planets high in the sky.  Basically the opposite of where you (and most) people are located.  The difference is enormous, and in those conditions, you can use some imaging parameters that wouldn't be advisable elsewhere.  This includes large oversampling, slower shutter speeds, lower frame rates, etc. 

Understated comment of the day right here!  Chris Go is in a similar situation.  Higher altitude planets each season in the tropics and fairly consistent weather patterns each night.



#19 gfunkernaught

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Posted 01 August 2020 - 03:34 PM

That would be fine, as long as you do truly get around 2.5x with whatever spacing you end up with.

I think the ADC adds some focal length because of its inherent magnification.  




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