41 replies to this topic

[orbman]

I want to photograph Pluto for the first time, and I understand that it will only show up as a point source of light using any of my lenses or my MAK 102mm, so I plan to use a 400mm camera lens. My question is: what focal length is required to photograph Charon as separate point light source next to Pluto?

 

Thanks!

[GamesForOne]

Probably about an 8 meters objective with adaptive optics.

 

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Michael Mc

[Cotts]

Probably about an 8 meters objective with adaptive optics.

 

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Michael Mc

To put some numbers on it, Pluto is about 14.5 mag these days and Charon is about mag 17.  Their maximum angular separation is 0.9 arc seconds - this occurs about every 3 days. Minimum separation is about 0.1" so you would have to know when it is at max separation.

 

Video lucky imaging with a looong focal length, say 5m or more, using a 12-inch or larger telescope might, just might be able to detect them both.  

 

Gain and exposure length would have to be able to reach 18th magnitude, for Charon. 

 

Huge focal length and large aperture to put Charon enough pixels away from Pluto to be seen as a separate point of light.

 

Paging Damian Peach!!  If anyone can do it, he can.

 

Dave

Edited by Cotts, 25 August 2023 - 08:47 AM.

[kathyastro]

It was discovered with a 1.5 m telescope, but the discovery images do not split the pair.  I don't know what focal length you'd need, but you will need one heck of an aperture.  Be happy with seeing the pair as a tiny dot.  Take the same image on two different nights, since the only way to verify the image is by its movement.

[mikemarotta]

"In the winter of 1977, working for the US Naval Observatory (USNO) in Washington, DC, Christy went to his supervisor, Robert Harrington, with a suggestion that they request photographs of Pluto from the 60-inch reflector at Flagstaff, Arizona, which at the time was imaging Uranus and Neptune. It would be a good opportunity, Christy figured, to record all three planets. Harrington agreed." 

James Christy Discovers Charon

https://aas.org/post...story-june-2020

 

As kathyastro pointed out, that 1.5-meter telescope did not split the pair. Charon was imaged as a lump on the planet.

 

But that was 1977 and we amateurs have come a long way.

 

Thanks,

Mike M.

[Redbetter]

The primary consideration, even with lucky imaging, would be picking a location with excellent seeing, and Pluto high in the sky.  After that you will want aperture to provide separation and to shorten the exposure time needed to show Charon (back to the lucky imaging stacking.)

 

Allan Wade has been trying for it visually with a 32" from pristine skies in Australia, but even at max separation his seeing and aperture have not been up to it.   Charon is dim enough relative to Pluto that even in a 32" I wouldn't expect it to be direct vision, maybe a 48" might work, and that is what one will need for such a close pair to have a chance.   My experience with a 20" in less than stable seeing (norm here even on the best nights) is that other very close pairs without the magnitude difference become extremely difficult at the threshold of direct vision.  The image simply isn't bright enough to see more than some fuzzy elongation, and anything less than perfect seeing makes it much worse.  

 

While imaging can do more with it, you still need the seeing to make the captures work.  It should shorten the time of each capture making it more likely that the better moments will filter through.  But I am not an imager.

[Cotts]

This should  a 'holy grail' target for amateur planetary  imagers.  Visual is probably out of the question...

 

Those with smoothly driven large dobs  - 20, 30,40 inch class,  get the focal length up beyond 5000mm with an appropriate Barlow.

 

Video lucky imaging may work.  Can you reach 18th magnitude?

 

What may work better is speckle imaging - sub-arc second pairs can be imaged this way with a 'small' an aperture of 11 inches...

 

The Journal of Double Star Observations is a good source for potential methodologies... 

 

And with Pluto's declination these days around -23º folks in the southern hemisphere are in an advantageous position for the next 100 years or so...

 

I'll start making the popcorn.

 

Dave

Edited by Cotts, 25 August 2023 - 02:04 PM.

[555aaa]

Suggest using filters also because atmospheric dispersion is larger than the separation.

[Alnitak2009]

You would have to be located in a area in the world where the "seeing" is rock steady and imaging with a very large cooled telescope and at high magnifications. At best though I would think Pluto and Charon would look like a elongated star. One being slightly brighter than the other. Also I think that guiding is in order to display nicely round stars in the fov.

 

I may try imaging this area in the C14 and ZWO224 and also with a barlow.

[bunyon]

It really is a combination of lucky imaging and DSO imaging. I've grabbed Triton (mag 13.5) solidly when imaging Neptune but on my unguided and not terribly smooth tracking 15" dob, it smears out significantly. So I think very accurate guiding, as in DSO imaging is needed. But also excellent seeing and short exposure. That's where aperture, as others have suggested, come in. However, in my experience, the larger the amateur scope the less well guided and smoothly tracked. 

 

On the other hand, we've come a long ways and this feels like it's probably in the realm of the possible with current amateur tech. It will just take a special night in a special place with special hands at the wheel (or keyboard).

[JMP]

Just for reference, I have a C14 and I can barely separate Miranda from Uranus, and Miranda is Mag 15. With your scope you might be able to get Neptune and Triton and be proud of the accomplishment!

[Redbetter]

The problem with Miranda is the magnitude difference, and that it is so dim as well (in the mid 16's, not 15.)  Being close to 11 magnitude dimmer than Uranus makes the moon a problem because of the glare of the planet. 

 

That is why I have only seen one visual report of it, and that one was not checked vs. the star field at the time, so it is doubtful.  With Uranus' moons the orientation of the moons vs. the planet frequently makes it easy to mistake a 14+ mag star for one of the moons, throwing off the whole pattern.  I have often searched for Miranda with the 20" on nights I could see the other four, but the seeing has never been excellent when I tried.

Edited by Redbetter, 25 August 2023 - 05:34 PM.

[555aaa]

Einstein’s cross is only 2 arc seconds across and amateurs have imaged it with its separate components so I think it’s doable

Edited by 555aaa, 25 August 2023 - 09:54 PM.

[cometcatcher]

Like JMP mentioned, practice on Neptune and Triton first, then try and get as many moons of Uranus as possible, then on to your goal. You may need some years to practice and a "small" scope upgrade in the meantime.   For comparison, this is Neptune and Triton last year with field stars and their magnitudes through a 12" Newtonian and ZWO 183mm camera. Which is no comparison at all because Pluto and Charon will be 1000X harder. Good luck. You're gonna need it.

 

[Kokatha man]

Triton is an easy target for a C14 in decent seeing, we've picked Triton up in unsharpened stack in good seeing along with BCW's. (storms)

 

The  set of 8 panels/images at the start of this page of our website https://momilika.net/WebPages/Neptune2015_2017Pics.htm 

shows that. (for some reason atm you cannot click on the composite image to get a larger version, but if you look carefully you'll see it - lower down than where it is in the top (sharpened) panels.)

 

Miranda is a completely different kettle of fish, exacerbated by its proximity to Uranus and requires excellent sky transparency to stretch it out from the raw stacks conclusively...

[Redbetter]

Triton shows up visually for me with a 125ED refractor; but I haven't succeeded with the 110ED, which is not visually apochromatic and shows a violet halo on bright white objects--makes the Pup a lot harder with that scope too.  I would think a 102 Mak like the OP has would be sufficient for Triton considering all of the magic folks do with planetary imaging.

[Marshalll]

According to the backyard astronomer's guide, 10" aperture is the hard minimum for visual.  But with a camera, that changes things.  As mentioned before it'd probably be best to use some combination of lucky imaging and normal DSO imaging. 

[555aaa]

Ideally you’d want to find when Pluto and a fairly bright star are very close together. Then you use lucky imaging on the bright star with the hope that it is close enough to Pluto that they get the same aberrations.

[orbman]

Thanks for all the input. Guess my 400mm camera lens isn't quite big enough!

[Redbetter]

According to the backyard astronomer's guide, 10" aperture is the hard minimum for visual.  But with a camera, that changes things.  As mentioned before it'd probably be best to use some combination of lucky imaging and normal DSO imaging. 

For what?  Triton? Pluto?  From what I can see my old 90's version of the The Backyard Astronomer's Guide didn't have any comment like that.  Only mentioning observing the disk of the planet with at 100 and 200x, that the disk is generally blue with a 6" and gray with small apertures (which matches my experience.)   It notes that Triton is 13 magnitude.  I have always found Triton easily with an 8" SCT, even the first time I found Neptune.

 

I don't recall wording like "hard minimum" anywhere in the book, so I would like to see an actual quote and what edition it is from.  Statements like "hard minimum" when it comes to visual are nonsense so I doubt that would be in the text.  Pluto keeps moving further away and dimming, yet I have observed it several times in recent years with a 110ED.  Only once have I failed on it with that scope, a night of poor seeing.

[cometcatcher]

Thanks for all the input. Guess my 400mm camera lens isn't quite big enough!

You could image Pluto with it, just not split the moons. 

[RedLionNJ]

I want to photograph Pluto for the first time, and I understand that it will only show up as a point source of light using any of my lenses or my MAK 102mm, so I plan to use a 400mm camera lens. My question is: what focal length is required to photograph Charon as separate point light source next to Pluto?

 

Thanks!

You can't.

[RedLionNJ]

And instead of just leaving it as "you can't", in response to the original poster's question, here's why:

 

S&T issued a challenge MANY years ago to all imagers - record Charon as a bump on Pluto, commensurate with the predicted PA of Charon. It's been a lot of years and nobody has done it.

 

Right now, Pluto is buried in the densest part of the Milky Way, at a very low elevation for northern imagers. And it's at opposition in northern summer, when the skies are not entirely dark. This is definitely a task for a southern imager in, say, June.  Add to that Pluto is receding from the sun and is currently around mag 14.5 - about a magnitude fainter than it was in 1989 at its brightest.

 

Getting the "lucky imaging" necessary on such a faint target at a potentially low elevation (if you're northern) in order to even get Charon as a bump on Pluto's pinhead is just not practical, no matter if you have 12 inches or 32 inches. Maybe somewhere around 1.5-2 meters might be able to gather enough light to do the necessary, but I'm doubtful.

[N-1]

For what? Triton? Pluto? From what I can see my old 90's version of the The Backyard Astronomer's Guide didn't have any comment like that. Only mentioning observing the disk of the planet with at 100 and 200x, that the disk is generally blue with a 6" and gray with small apertures (which matches my experience.) It notes that Triton is 13 magnitude. I have always found Triton easily with an 8" SCT, even the first time I found Neptune.

I don't recall wording like "hard minimum" anywhere in the book, so I would like to see an actual quote and what edition it is from. Statements like "hard minimum" when it comes to visual are nonsense so I doubt that would be in the text. Pluto keeps moving further away and dimming, yet I have observed it several times in recent years with a 110ED. Only once have I failed on it with that scope, a night of poor seeing.

Hard limit ALWAYS means it's doable beyond it.

Edited by N-1, 27 August 2023 - 12:38 AM.

[Redbetter]

And instead of just leaving it as "you can't", in response to the original poster's question, here's why:

 

S&T issued a challenge MANY years ago to all imagers - record Charon as a bump on Pluto, commensurate with the predicted PA of Charon. It's been a lot of years and nobody has done it.

 

Right now, Pluto is buried in the densest part of the Milky Way, at a very low elevation for northern imagers. And it's at opposition in northern summer, when the skies are not entirely dark. This is definitely a task for a southern imager in, say, June.  Add to that Pluto is receding from the sun and is currently around mag 14.5 - about a magnitude fainter than it was in 1989 at its brightest.

 

Getting the "lucky imaging" necessary on such a faint target at a potentially low elevation (if you're northern) in order to even get Charon as a bump on Pluto's pinhead is just not practical, no matter if you have 12 inches or 32 inches. Maybe somewhere around 1.5-2 meters might be able to gather enough light to do the necessary, but I'm doubtful.

I am not convinced that it can't be done, but I don't know that anyone has been trying for it in recent years.  It's simply a tight double star type imaging problem at this point, with very faint sources.  The sky has been plenty dark at 37 N for Pluto for visual and Charon is about 2 magnitude dimmer. 

 

For a close double I don't know that the busy Milky Way field has any substantial impact--this field is not nearly as dense since the duo is moving further from the main Milky Way glow.   From having seen it many times in recent years, there is nearly always ample separation of Pluto from stars down into the 16 magnitude range.

 

Seeing however is a huge factor, particularly low in the sky.  So if a person can do their imaging in very stable skies they will have a big advantage, and if the skies happen to be southern (e.g. Florida) then they are going to get a greater fraction of useful images.  

 

So what sort of photon count per second will it take to have a shot of resolving ~14.5 and 16.5 mag spurious discs at something approaching 0.9 arc seconds separation? How many counts per frame will there need to be to stack together an image that will show Charon?  That will define the aperture required (assuming that the aperture is already sufficient for the separation distance.)