Planetary and Lunar

[Simon Pepper]

I initially added this to Imaging planets section but wondering if that was for actual images now so adding here in case anyone can assist. Thanks 

 

Hi all,

 

Couple of newbie planetary/lunar questions coming up here. 

• Barlows how do I know which type to get? 1.25 inch or 2 inch? 
• How do they fit in the OTA imaging train is a diagonal required?
• If using FF I assume you remove it?
• Mono or colour? I shoot mono for DSO, but not sure how this compares to solar system stuff?
• ASI 174mm seems to be the best shout according to other forums is there an equivalent OSC that's a good shout?

 

Thanks 

[vlaiv]

11 minutes ago, Simon Pepper said:

Barlows how do I know which type to get? 1.25 inch or 2 inch? 

1.25"

Planetary and lunar uses rather small field and there is simply no need to illuminate 2" field.

12 minutes ago, Simon Pepper said:

How do they fit in the OTA imaging train is a diagonal required?

Don't use diagonal.

Setup is rather simple: OTA - barlow - camera (with spacers where needed to get optimum configuration).

13 minutes ago, Simon Pepper said:

If using FF I assume you remove it?

Yes. You don't need extra glass in optical path, it will only interfere with image quality.

As already mentioned - lunar and planetary use very small central part of the field of view where definition is the best (no aberrations or minimal aberrations), so no need to introduce flattening to outer parts of the field of view that won't be used anyway.

14 minutes ago, Simon Pepper said:

Mono or colour? I shoot mono for DSO, but not sure how this compares to solar system stuff?

OSC is much simpler for planetary and there is not much of a difference in most cases. Mono is beneficial in some special cases - like if you plan using Solar Ha with dedicated solar scope or using narrow band or specialty filters (Ha lunar, CH4 for Jupiter or UV for Venus for example).

16 minutes ago, Simon Pepper said:

ASI 174mm seems to be the best shout according to other forums is there an equivalent OSC that's a good shout?

ASI174 is far from best planetary camera. It has rather high read noise.

You want fast camera (high fps), high QE and as low read noise as possible for planetary.

Size of sensor is beneficial in some cases (lunar / solar) but even smallest sensor will easily capture planets with room to spare.

Here is good one:

https://www.firstlightoptics.com/zwo-cameras/zwo-asi-662mc-usb-3-camera.html

In the end - depending on what scope you'll be using camera on - you want F/ratio to be around x4-x5 of pixel size.

This camera has 2.9um pixel size so you want to be between F/11.6 and F/14.5. Best way to get that is to get barlow with detachable barlow element and to adjust barlow element / sensor distance until you get exact F/ratio you want (larger distance - higher magnification and smaller distance - less magnification from barlow element).

[Simon Pepper]

@vlaiv you star ⭐️ thanks for this clear simple and precise! 

[Simon Pepper]

Quote

@vlaiv one other question on the pixel size vs f stop. The scope is ES 127 and ideally I would get a 3x barlow to get close into Jupiter and Saturn, however this would put me at F20 and ideal f ratio using your calculation here is F15. Can I take the risk and go with the x3 and maybe bin 2x2 or 3x3 to get the ideal sampling or would you recommend sticking with 2x barlow and cropping? I l know my refractor is not ideal for planetary but its all I got atm. I was looking at the ES focal extenders, but is there one you would recommend?  Thanks 

 

Edited February 5, 2023 by Simon Pepper

[vlaiv]

@Simon Pepper

Physics is clear on this one. You won't get any additional detail if you go above optimum sampling rate which is x5 pixel size for RGB type imaging (400-700nm visual wavelength range).

When you over sample - you get poorer SNR per sub. Binning is not the best idea for planetary as you increase read noise - and read noise is very important factor as many short subs are stacked and each stacked sub introduces read noise - so low read noise is quite important.

Having said that - many planetary imagers over sample. They explain that its easier for them to process the data that way and they simply prefer larger images and say they get better images that way (but this is all anecdotal evidence and I've yet not seen any comparison that shows this).

While it is clear that over sampling is bad - with modern sensitive and low read noise cameras - it might not be as bad (people who over sample still produce excellent images).

This is pretty much all the advice that I can give you and leave choice to you. I personally would aim to be within that F/15, but I don't think there will be that much harm if you choose convenience factor and go for F/20 for example.