Tips needed for Neptune imaging

[Maideneer]

Opposition is coming up in a few days (14 Sep), and I have never imaged Neptune before or seen it for that matter through my scope.  See my sig for my equipment - I've become quite comfortable with capturing Jupiter and Saturn so I'm wondering if there is a vast difference between those and Neptune.  I use Sharpcap so I need to understand gain and exposure settings.  I also have to figure out how to add my 3x barlow to my image train, so I'm going to work on that tonight.

I'm going to try visual only the next few days so I can prep, now that I've gotten my brand new Polemaster tested and working I'm happy to report my alignment is better than ever, what a relief!

[Cosmic Geoff]

A flip mirror diagonal is a great help for imahing faint objects like Neptune.  In Sharpcap you may find that the Histogram tool does not work and you have to set the exposure by trial and error.  It is easy to over-expose the planet so that it burns out during processing.   If you deliberately over-expose by a lot, you may capture one or two of the moons.  I have my gain setting for this camera (ASI224MC)  permanently at about 350.

 

BTW, don't wait for the opposition date. 🙂

[vlaiv]

Yes, there is significant difference between Jupiter / Saturn and Neptune.

Jupiter for example has magnitude of about -2.8 (-2.88 this opposition), while Neptune has 7.82.

That is 7.82 - (- 2.88) = 7.82 + 2.88 = 10.7 magnitudes of difference in brightness, or about x19000 fainter

Neptune is x19000 fainter than Jupiter.

Next, Jupiter's apparent size in opposition is 49.12" in diameter while Neptune has 2.36" diameter.

With 9.25 EdgeHD - if you sample at critical sampling rate, you'll be sampling at 0.22"/px - this means that your pixel covers 0.22" x 0.22" = 0.0484"2 of sky.

Area of Jupiter is 49.12" ^ 2 * pi / 4 = ~1894.9886"2 / 0.0484"2 = 39152px

Area of Neptune is 2.36" ^2 * pi / 4 = ~ 4.3744"2 / 0.0484"2 = ~90.4px

Jupiter has x433 more pixels than Neptune if you sample at critical resolution - which means that on average each pixel receives 19000 / 433 = x43.88 less light.

There fore, if you image Jupiter with 5ms exposure - you need to image for x43.88 longer per exposure to get same SNR - or about 220ms (if my calculation above is right of course - please do check it again just to make sure).

Problem with doing 220ms exposures is that you won't freeze the seeing and you need very good seeing conditions and long video - many many frames in order to collect needed number to get decent stack.

With regular seeing - it can happen that you don't get single good / not blurred sub with 220ms exposures.

In the end - best you can hope for is blue dot about 11px in diameter

 

[Maideneer]

38 minutes ago, Cosmic Geoff said:

A flip mirror diagonal is a great help for imahing faint objects like Neptune.  In Sharpcap you may find that the Histogram tool does not work and you have to set the exposure by trial and error.  It is easy to over-expose the planet so that it burns out during processing.   If you deliberately over-expose by a lot, you may capture one or two of the moons.  I have my gain setting for this camera (ASI224MC)  permanently at about 350.

 

BTW, don't wait for the opposition date. 🙂

Oh I didn't wait, I tried last night.  Not for long though, I developed a massive headache and had to call it early...but I did learn the ropes of the Polemaster at least.  Giving it another go later on, I'll be able to focus (myself) better and really dig into things.  My biggest obstacle really was removing my EP train and replacing with my image train...I have the auto-focuser in the way so that really makes for tight quarters for my fingers.

Cranked up the gain to max for fun, and thought I spotted it but alas I was wrong after capturing 100 frames, for some reason I kept my exposure time to 5 seconds since it's the only way I could see much of anything but I have to play around more. Getting the focus will be trial and error too, so I'm thinking visual first and then replace with image train.  Gonna try to add my 3x barlow to the mix too, that should change things.

[Maideneer]

47 minutes ago, vlaiv said:

Yes, there is significant difference between Jupiter / Saturn and Neptune.

Jupiter for example has magnitude of about -2.8 (-2.88 this opposition), while Neptune has 7.82.

That is 7.82 - (- 2.88) = 7.82 + 2.88 = 10.7 magnitudes of difference in brightness, or about x19000 fainter

Neptune is x19000 fainter than Jupiter.

Next, Jupiter's apparent size in opposition is 49.12" in diameter while Neptune has 2.36" diameter.

With 9.25 EdgeHD - if you sample at critical sampling rate, you'll be sampling at 0.22"/px - this means that your pixel covers 0.22" x 0.22" = 0.0484"2 of sky.

Area of Jupiter is 49.12" ^ 2 * pi / 4 = ~1894.9886"2 / 0.0484"2 = 39152px

Area of Neptune is 2.36" ^2 * pi / 4 = ~ 4.3744"2 / 0.0484"2 = ~90.4px

Jupiter has x433 more pixels than Neptune if you sample at critical resolution - which means that on average each pixel receives 19000 / 433 = x43.88 less light.

There fore, if you image Jupiter with 5ms exposure - you need to image for x43.88 longer per exposure to get same SNR - or about 220ms (if my calculation above is right of course - please do check it again just to make sure).

Problem with doing 220ms exposures is that you won't freeze the seeing and you need very good seeing conditions and long video - many many frames in order to collect needed number to get decent stack.

With regular seeing - it can happen that you don't get single good / not blurred sub with 220ms exposures.

In the end - best you can hope for is blue dot about 11px in diameter

 

Thanks for this very detailed numerical review of things, I get the feeling you enjoy going down these number holes lol.

I'm going to keep working on this 11px target as my mission of sorts, everyday if I have to!  I love these sorts of personal challenges and once I start I make sure to see it through to the end no matter how long it takes.  If others can do it, I can too!  Will definitely use these guidelines to start in the right direction, cheers!

[vlaiv]

6 minutes ago, Maideneer said:

I get the feeling you enjoy going down these number holes lol.

I guess I do

It's quite useful tool - spend short time crunching numbers and it gives you good starting point and it lets you set your expectations as well.

[CraigT82]

I found a good 60mm finder, well aligned to the main OTA, essential in locating the the ice giants in my light polluted skies.

Goto will get you close but it won't get the camera on the chip. Once you're in the right neighborhood use the finder to spot the planet then guide the main scope to it using the hand controller. 

As for the capture, very high gain and short as possible exposures. Even if the histogram is only at 20% that will do as long as you keep the exposures short get loads and loads of frames. The image will look terribly noisy and dim on the screen but don't worry about that. Focusing on the planet will be tough so focus on a star first.

Capture for  at least 15 mins or even 30 mins if you can. You won't get any surface details so rotation isn't an issue like it is with Jupiter. 

As for filters, near IR longpass filters work well. The Baader 610nm longpass and 685nm longpass are good when used with a mono camera. The usual method is to spend a good while capturing the mono NIR data which will become your main image (I.e. luminance channel), and then get some shorter colour captures just to add the colour to it. Or you could scrap the colour captures and just false colourise it in post processing. 

The 3x barlow will give you f/30 which is pretty high. Not sure what camera you have? Maybe if you unscrewed the lens cell from the barlow (if poss) and put it on the nose of the camera it will give you less amplification and a better SNR.

[Maideneer]

9 minutes ago, CraigT82 said:

I found a good 60mm finder, well aligned to the main OTA, essential in locating the the ice giants in my light polluted skies.

Goto will get you close but it won't get the camera on the chip. Once you're in the right neighborhood use the finder to spot the planet then guide the main scope to it using the hand controller. 

As for the capture, very high gain and short as possible exposures. Even if the histogram is only at 20% that will do as long as you keep the exposures short get loads and loads of frames. The image will look terribly noisy and dim on the screen but don't worry about that. Focusing on the planet will be tough so focus on a star first.

Capture for  at least 15 mins or even 30 mins if you can. You won't get any surface details so rotation isn't an issue like it is with Jupiter. 

As for filters, near IR longpass filters work well. The Baader 610nm longpass and 685nm longpass are good when used with a mono camera. The usual method is to spend a good while capturing the mono NIR data which will become your main image (I.e. luminance channel), and then get some shorter colour captures just to add the colour to it. Or you could scrap the colour captures and just false colourise it in post processing. 

The 3x barlow will give you f/30 which is pretty high. Not sure what camera you have? Maybe if you unscrewed the lens cell from the barlow (if poss) and put it on the nose of the camera it will give you less amplification and a better SNR.

I have the 9x50 finderscope that came with the bundle, you think that would be sufficient?  I didn't realize my finder could even see that far.

I have the 224MC

[CraigT82]

18 minutes ago, Maideneer said:

I have the 9x50 finderscope that came with the bundle, you think that would be sufficient?  I didn't realize my finder could even see that far.

I have the 224MC

You may be ok, give it a try, it will just look like a dim star in the finder, so you will need to compare the view through the finder with a star chart to identify the planet.

With the 224c you could try a longer IR pass filter like the 850nm or 742nm and it should work quite well, if you have one of those. Failing that just go ahead and use the standard UV/IR cut. 

Good luck! 

[Maideneer]

42 minutes ago, CraigT82 said:

You may be ok, give it a try, it will just look like a dim star in the finder, so you will need to compare the view through the finder with a star chart to identify the planet.

With the 224c you could try a longer IR pass filter like the 850nm or 742nm and it should work quite well, if you have one of those. Failing that just go ahead and use the standard UV/IR cut. 

Good luck! 

Here's my rough guide on what to look for.  These two triangle formations of stars give me a good idea of where to look and/or I could use a box pattern to achieve the same result.  Once I get that I'm home free, just got to remember to set my motor speed really low because my tendency is to keep it at max speed and get annoyed with myself that I lost the formation lol.

[Maideneer]

On my way to Neptune I decided to try out Uranus, and got extremely lucky I guess. The focus was just right enough on my sensor (even with pretty poor seeing) and the blurry thing showed up for just long enough to grab 5,000 frames. No Barlow, filter or ADC on this.

I processed it to subconsciously look like Neptune lol so I have work to do there. My original data showed a lot bigger of a blurry dot and I stacked it down to this tiny thing. Trying again tonight, expecting seeing to be better.

[Pete Presland]

Congratulations on capturing Uranus, both the outer gas giants are very tricky targets. Uranus is slightly less tricky than Neptune, but neither are going to yeald much more than what @vlaiv said a small disc. Some hints of detail can be revealed on Uranus, such as the Polar region, or banding with bigger apertures. But this still needs some very good seeing conditions, are managed once myself with my C9.25, but only once though.