Should/Could I Use A 3x Barlow For Imaging Jupiter/Saturn?

[wesdon1]

Hi all. So because the weather is still awful here in Liverpool UK, I have decided to try find a gap in clouds and grab a few minutes worth of video of Jupiter and/or Saturn and process their frames to hopefully get some nice final images. I will be using my ZWO 224MC Planetary Camera, my Newtonian Reflector 200/1000, obviously mounted on my big HEQ5 Mount. I'll be doing around 3 X 90 second vids on each planet, at a frame rate of around 150-200/sec.

My question is, would using a 3X or even 5X  Barlow be beneficial in terms of image quality? What I mean is, will increasing the FL of my 'scope so far with said Barlow's ( 3X/3000mm FL and 5X/5000mm Fl, respectively ) be "too zoomed in" and degrade the image quality? I want to use higher power Barlow's due to my Newtonians 1000mm FL not really being "zoomed in" enough, as say in comparison to a 8inch SCT with a native 2000mm FL which is easily increased to 4000mm FL with just a 2X Barlow.

Last year i imaged planets with a eyepiece Projection adaptor fitted with a 10mm EP and a DSLR camera and the results were poor. Very blurred images/frames.

Thanks in advance if anyone can give me any tips.

Wes.

[knobby]

Loosely speaking, for planets you want to be aiming for approx 5 x pixel size (3.75 for the 224) so around F19 would be good.

So a 3x Barlow will give you f15 on your scope,I'd say give it a go 🙂

Remember to cool the scope to surroundings as best as possible and check collimation.

[wesdon1]

12 minutes ago, knobby said:

Loosely speaking, for planets you want to be aiming for approx 5 x pixel size (3.75 for the 224) so around F19 would be good.

So a 3x Barlow will give you f15 on your scope,I'd say give it a go 🙂

Remember to cool the scope to surroundings as best as possible and check collimation.

@knobby Hi knobby, and thank you for your help and advice.

Yes i forgot to mention the pixel scale part of the equation, sorry! That was one of the other major reasons I wanted to use higher power Barlow's, to achieve better pixel scale and F-number. I will give the 3X barlow a shot with the 224MC and see how I fare, thanks. 

Just one other question, if I may, please Knobby? Would you recommend I decrease the ROI around the Planet so my images/frames are more "zoomed in" to the Planet, so to speak? Does decreasing the ROI until it's tight around the Planet increase contrast/detail/surface detail etc? 

[vlaiv]

26 minutes ago, wesdon1 said:

Does decreasing the ROI until it's tight around the Planet increase contrast/detail/surface detail etc? 

Not directly.

With planetary imaging it is important to freeze the seeing and for that reason exposure needs to be very short - often in 5-6ms range. Even if image looks under exposed.

When you image at those exposure lengths - you can expect to ideally achieve up to 200fps (1s / 5ms = 200fps). There is a limit to how many of those frames can be recorded and this limit is imposed by USB connection speed and speed of your disk drive (SSD/NVME can easily cope with needed speeds so its worth having those in your imaging rig).

USB link has limited bandwidth - it can achieve only certain amount of data transfer speed. Each frame you record contains some amount of data. If you increase FPS - you increase amount of data needed to be transferred over USB connection. At some point USB connection can become bottleneck in your recording.

When this happens - it is beneficial to reduce ROI as size of each frame determines how much data it contains - smaller ROI less data per frame - more frames per second can be transferred over USB.

You can achieve best contrast/detail/surface detail if you capture the most data and ROI can help with that - so in that sense it helps to increase those - but only for reasons of data transfer. After you hit max data rate / max FPS allowed by your exposure length - smaller ROI won't contribute anything.

BTW, ZWO publishes max theoretical FPS for every camera / ROI size combination and it is worth checking out.

Say that you work in 8bit format and you want to hit 200FPS because you are using 5ms exposure length - then you need to drop your ROI to at least 800x600.

One more note - exposure length should really be judged properly. It needs to be short enough to freeze the seeing - which really means that distortion of atmosphere is "static" in short period of single frame. If you don't do this you will have "cumulative effect" of two or more different distortions averaged - which is just "motion blur" of different distortions and is a bad thing.

On the other hand - you don't want to have your exposure set any lower than that because it will hurt you final image (more noise then needed).

Reality is - that there is no single well defined exposure length, and it is trade off - some frames will be usable some won't as there will be motion blur. Longer the exposure length - more frames you'll need to discard and stack fewer good ones so it is a fine balance of finding good exposure length. Another thing is how bad the seeing is - in average seeing you will need exposure in range of 5-6ms. In really good seeing you might afford to have exposure set to 10ms or even 15ms. In really poor conditions you might need to go as low as 3-4ms.

Btw, Lunar imaging can often employ 3ms as standard because of amount of light - this even allows for narrowband filters to be used with lunar with longer exposures - but that is "advanced topic"

 

[knobby]

Yep, great answer from Vlaiv 😀

There is no point in having a large black area around the planet so crop in as close as your tracking will allow, it uses less data so allows faster write speed .

[wesdon1]

On 13/10/2023 at 15:13, vlaiv said:

Not directly.

With planetary imaging it is important to freeze the seeing and for that reason exposure needs to be very short - often in 5-6ms range. Even if image looks under exposed.

When you image at those exposure lengths - you can expect to ideally achieve up to 200fps (1s / 5ms = 200fps). There is a limit to how many of those frames can be recorded and this limit is imposed by USB connection speed and speed of your disk drive (SSD/NVME can easily cope with needed speeds so its worth having those in your imaging rig).

USB link has limited bandwidth - it can achieve only certain amount of data transfer speed. Each frame you record contains some amount of data. If you increase FPS - you increase amount of data needed to be transferred over USB connection. At some point USB connection can become bottleneck in your recording.

When this happens - it is beneficial to reduce ROI as size of each frame determines how much data it contains - smaller ROI less data per frame - more frames per second can be transferred over USB.

You can achieve best contrast/detail/surface detail if you capture the most data and ROI can help with that - so in that sense it helps to increase those - but only for reasons of data transfer. After you hit max data rate / max FPS allowed by your exposure length - smaller ROI won't contribute anything.

BTW, ZWO publishes max theoretical FPS for every camera / ROI size combination and it is worth checking out.

Say that you work in 8bit format and you want to hit 200FPS because you are using 5ms exposure length - then you need to drop your ROI to at least 800x600.

One more note - exposure length should really be judged properly. It needs to be short enough to freeze the seeing - which really means that distortion of atmosphere is "static" in short period of single frame. If you don't do this you will have "cumulative effect" of two or more different distortions averaged - which is just "motion blur" of different distortions and is a bad thing.

On the other hand - you don't want to have your exposure set any lower than that because it will hurt you final image (more noise then needed).

Reality is - that there is no single well defined exposure length, and it is trade off - some frames will be usable some won't as there will be motion blur. Longer the exposure length - more frames you'll need to discard and stack fewer good ones so it is a fine balance of finding good exposure length. Another thing is how bad the seeing is - in average seeing you will need exposure in range of 5-6ms. In really good seeing you might afford to have exposure set to 10ms or even 15ms. In really poor conditions you might need to go as low as 3-4ms.

Btw, Lunar imaging can often employ 3ms as standard because of amount of light - this even allows for narrowband filters to be used with lunar with longer exposures - but that is "advanced topic"

 

@vlaiv Hi again Vlaiv. Thank you so much for such a detailed and helpful response. Youy really know your craft! I will take on board everything you've educated me about and put that new knowledge into practice. Again thank you so much Vlaiv, I really do appreciate yours and everyone else's help and education, I'd be lost without these forums! 

As a side note, I do plan to invest in either a large mak' cass' with a minimum 150mm aperture, or a medium sized SCT of around 200mm to 250mm'ish aperture, specifically for my planetary work. I've seen incredible planetary works produced with these optical systems, they seem made for planetary and very small DSO's, and I've never actually own either type of optical system, so I very much look forward to getting one in the coming months.

Clear Skies to Vlaiv and all who contributed!

Edited October 16, 2023 by wesdon1
missed a bit/grammar mistakes