Newby to planetary

[Simon Pepper]

Hi all,

Was looking for some pointers really after pointing my scope at the moon the other day for the first time in two years and seeing how majestic it was I feel I have failed to look around our solar system (I mainly go after DSOs), so I feel the time has come or at least to prepare now so I am ready. I have neglected our neighbours for too long. What I don't have is a planetary camera and a 2500fl scope with some serious aperture, however I am hoping the latter is optional lol. What I do have is 952mm ES 127 and a couple of ZWO pro cameras which I hope will do or I can buy a planetary, but I am sure with a Barlow I maybe in as my fl is lacking (hoping)?

• Can anyone recommend a good Barlow if so should I be looking for 2x 3x what's the cons here I assume like with a reducer where the F stop is quickened with a Barlow its slowed? Is this a massive issue if shooting in video?
• I have noticed when shooting a video via my 1600mm I have the option of formats. 360p up to 1080p and everything in between. Each option the picture scale size changes more zoom at lower format I assume this is pixel scale related? Is there an ideal resolution to shoot planetary? 

Thanks

[vlaiv]

You will probably need barlow with that scope.

With barlows magnification factor depends on distance between it and sensor. Increase the distance - increases magnification and decrease in distance - decreases magnification.

For that reason it is good to have "barlow element" - barlow which you can screw off the barlow body and use it on its own - so you can dial in needed magnification with extensions.

You'll need x2 barlow with your scope and ASI1600.

Optimum F/ratio is determined by pixel size and wavelength you will image at. For full spectrum (like RGB images) - we usually use 500nm wavelength as reference (rather than mid spectrum of 550nm or short end of visible spectrum at 400nm).

F/ratio = pixel_size * 2 / wavelength

For full spectrum and ASI1600 that would be F/ratio = 3.8µm * 2 / 0.5µm (500nm) = F/15.2 = F/15 or x2 barlow used with F/7.5 scope

As far as ROI (region of interest) is concerned - it serves dual purpose.

With planetary imaging, you want as high FPS as possible. First you want short exposure to freeze the seeing and that means exposure of 5ms or so (that alone gives you room for 200fps as exposure = 1s / fps), then you want USB connection to be able to transfer all that data - which is why you should use USB 3.0 connection. You want fast drive (like SSD) to be able to record all of that data.

In the end - if you transfer whole image instead just some part of it - you'll be transferring much more data then you might need, so it is better to use just central part of sensor.

Most planets fit well inside 640x480 ROI - with exception of Jupiter if you plan to image whole Jovian system with moons - then you'll need larger ROI.

The Moon is often larger than sensor and we need to utilize mosaic approach here if we want whole disk instead of shooting just certain feature. We still use ROI here as telescopes have diffraction limited performance only in center of the field. Further out, depending on design of telescope, aberrations start to increase - like coma in newtonian telescope or astigmatism in certain designs. Field curvature is present in APO triplets (for that reason flatteners are used for large sensors).

Although I'm not sure how large is aberration free field of your scope, I'd recommend using only central <10mm - and that would mean keeping ROI up to ~1900x1600 and not more for Lunar - even if that means more tiles for mosaic.

Do have a look around the youtube for planetary / lucky imaging tutorials to see how the capture is performed, and later, more important, how stacking and processing is performed.

 

[Simon Pepper]

2 hours ago, vlaiv said:

You will probably need barlow with that scope.

With barlows magnification factor depends on distance between it and sensor. Increase the distance - increases magnification and decrease in distance - decreases magnification.

For that reason it is good to have "barlow element" - barlow which you can screw off the barlow body and use it on its own - so you can dial in needed magnification with extensions.

You'll need x2 barlow with your scope and ASI1600.

Optimum F/ratio is determined by pixel size and wavelength you will image at. For full spectrum (like RGB images) - we usually use 500nm wavelength as reference (rather than mid spectrum of 550nm or short end of visible spectrum at 400nm).

F/ratio = pixel_size * 2 / wavelength

For full spectrum and ASI1600 that would be F/ratio = 3.8µm * 2 / 0.5µm (500nm) = F/15.2 = F/15 or x2 barlow used with F/7.5 scope

As far as ROI (region of interest) is concerned - it serves dual purpose.

With planetary imaging, you want as high FPS as possible. First you want short exposure to freeze the seeing and that means exposure of 5ms or so (that alone gives you room for 200fps as exposure = 1s / fps), then you want USB connection to be able to transfer all that data - which is why you should use USB 3.0 connection. You want fast drive (like SSD) to be able to record all of that data.

In the end - if you transfer whole image instead just some part of it - you'll be transferring much more data then you might need, so it is better to use just central part of sensor.

Most planets fit well inside 640x480 ROI - with exception of Jupiter if you plan to image whole Jovian system with moons - then you'll need larger ROI.

The Moon is often larger than sensor and we need to utilize mosaic approach here if we want whole disk instead of shooting just certain feature. We still use ROI here as telescopes have diffraction limited performance only in center of the field. Further out, depending on design of telescope, aberrations start to increase - like coma in newtonian telescope or astigmatism in certain designs. Field curvature is present in APO triplets (for that reason flatteners are used for large sensors).

Although I'm not sure how large is aberration free field of your scope, I'd recommend using only central <10mm - and that would mean keeping ROI up to ~1900x1600 and not more for Lunar - even if that means more tiles for mosaic.

Do have a look around the youtube for planetary / lucky imaging tutorials to see how the capture is performed, and later, more important, how stacking and processing is performed.

 

Thanks Vlaiv really informative and helpful as usual one last question would stick with the 2x Barlow or go with a 3x to get in closer? Thanks 

[vlaiv]

5 minutes ago, Simon Pepper said:

Thanks Vlaiv really informative and helpful as usual one last question would stick with the 2x Barlow or go with a 3x to get in closer? Thanks 

You can use x3 barlow to get in closer - but there is simply no point in doing so. Amount of detail available is limited by aperture of the scope and above formula for F/ratio versus pixel size takes that into account. Detail depends on aperture and focal length with pixel size determines how zoomed in you are - F/ratio is ratio of focal length and aperture - so everything is taken into account.

With stronger barlow - you would get more zoomed in image - but without detail - same effect as like imaging with x2 barlow and then resizing image in software. When you enlarge image in software - image is larger but no additional detail exists in the image.

On the other hand - using stronger barlow causes issues with signal to noise ratio because light is spread over more pixels and each pixel gets less light because of that (there is only so much light that is gathered by any scope). This means lower signal and in turn lower SNR.

With planetary imaging SNR is important because it lets you sharpen image without making it too noisy.

Using x3 barlow would therefore - make larger and more noisy image without additional detail and is not advisable.

[Simon Pepper]

22 minutes ago, vlaiv said:

You can use x3 barlow to get in closer - but there is simply no point in doing so. Amount of detail available is limited by aperture of the scope and above formula for F/ratio versus pixel size takes that into account. Detail depends on aperture and focal length with pixel size determines how zoomed in you are - F/ratio is ratio of focal length and aperture - so everything is taken into account.

With stronger barlow - you would get more zoomed in image - but without detail - same effect as like imaging with x2 barlow and then resizing image in software. When you enlarge image in software - image is larger but no additional detail exists in the image.

On the other hand - using stronger barlow causes issues with signal to noise ratio because light is spread over more pixels and each pixel gets less light because of that (there is only so much light that is gathered by any scope). This means lower signal and in turn lower SNR.

With planetary imaging SNR is important because it lets you sharpen image without making it too noisy.

Using x3 barlow would therefore - make larger and more noisy image without additional detail and is not advisable.

Ok perfect thank you again 2x it is!