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Afocal EEVA consideration - or why smart phone won't work as a good EEVA device


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Premise is rather simple - take your smart phone, attach it to the telescope and eyepiece and let it take image of what you see - stack those images. Perfect way for novice astronomers to get into EEVA and possibly imaging.

So I got myself phone adapter to test this idea out - and idea itself really seems to work - It is quite easy to attach a phone and take a picture of what is seen in eyepiece.

Question is - is it good for EEVA and if so - will it work for novice astronomer?

I'll outline my case and point out interesting parts.

Equipment that I initially planned to use was: SkyMax 102 Maksutov from SW. That is 102mm F/13 scope. 32mm GSO Plossl. My phone is Xiaomi Mi A1.

It has 3.8mm FL F/2.2 lens (equivalent of 29mm regular lens), 1.25µm pixel size, 4000x3000 resolution and 5mm x 3.75mm size

That lens covers about 80° on diagonal, and the EP that I'll be using has 50° FOV - it will fit entirely on to the chip with some room to spare. In fact - this is what it looks like:

frame.jpg.204337e608e2da8a4661f8cc8bfb8810.jpg

What would be equivalent speed of such "compound lens system"? We have base speed of F/13 and we have 32mm / 3.8mm = ~ x8.421 reduction.

That would make it F/1.54 - amazing!

But then I realized something important - 3.8mm and F/2.2 system has effective aperture of only ~1.73mm. Our telescope + eyepiece combination is giving exit pupil of 32 / 13 = ~ 2.462mm.

If we use this combination - it will actually stop our aperture to 71.7mm.

Lesson 1. - We should try to match telescope exit pupil to aperture size of phone lens. Phones have very small aperture on their lens - this pretty much rules out any fast scopes for this as most of them will give small exit pupil only with very short focal length eyepieces - and that won't cover much of the sky. For example F/5 scope will need 8.5mm eyepiece to match exit pupil with my phone.

We can try to mitigate above problem by being smart and using - wider field of view eyepiece. These have smaller exit pupil for same field stop / area of sky covered.

Best candidate that I've found for this and my setup is 70° 20mm WA modified Erfle EP (yes there are other eyepieces like 24/82 - but those tend to be much more expensive and we are talking here about budget option / novice astronomers).

image.png.33a83593c55c020aa5e6f29de1109d27.png

Notice that exit pupil is 1.57mm with that eyepiece - less than aperture of phone lens - so that is good.

What is not the best is the fact that we are using 70° eyepiece and that things get very warped at those angles. There will be some pincushion distortion and we must take care of that - stacking software needs to be aware that image has been non linearly deformed.

Here comes the fun part - we seem to have it all working, but my adapter does not have "height adjustment". You can easily center phone camera onto eyepiece - but there is no way to adjust the distance between the two.

Why is that important and how easy will it be?

Here is handy diagram to explain what is going on:

image.png.be34b0fc60470a8ca8845cbae66d0079.png

Since we have 17mm of eye relief and 70° AFOV and such a small difference between exit pupil and entrance pupil - there is really not much room for error - phone camera must be very precisely positioned. Even small movement away from ideal position will result in serious vignetting. Less than 1mm sideways can vignette edges by more than 50%. Few millimeters up and down can completely cut light from the edges of the FOV.

Lesson 2 - not every phone adapter is good enough to provide you with accurate alignment of phone camera and at the moment - I can't imagine "protocol" that I would utilize to accurately place the phone. If you plan on doing EEVA with different lens and camera - bare in mind that you'll need to position them accurately and that often eyerelief quoted for eyepiece is not accurate - so you'll need to devise some method of making sure you are spot on.

BTW, above image that you see - is taken with F/4 finder/guider scope and 32mm plossl - giving 8mm exit pupil. Even with such large exit pupil - I think left side has more vignetting than right since I did not center lens at exact sensor center - and I feel that phone was too close to the EP. 32mm Plossl has large eye relief.

I still think that afocal method is viable option for EEVA - but not with phone camera. Dedicated astronomy camera like ASI178 and matching c-mount lens could be an option.

With 32mm Plossl and 12mm c-mount lens - this gives nice combination and lens only needs to operate on F/4 to be able to let in complete exit pupil.

@Thalestris24 - this is related to afocal method - not imaging per se, but you might find it interesting - exit pupil matching is not something that we immediately think of when considering afocal method.

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I'm pretty sure all the uncropped microscope images I've seen taken with a smartphone via the eyepiece show much vignetting. It's probably unavoidable - but always worth a fiddle! Maybe experiment to find the best distance and do something with the phone holder? The holder I ordered is this style but I won't get it until next week.

Louise 

ps my phone isn't good with low light - but probably fine with a microscope!

Edited by Thalestris24
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2 minutes ago, Thalestris24 said:

I'm pretty sure all the uncropped microscope images I've seen taken with a smartphone via the eyepiece show much vignetting. It's probably unavoidable - but always worth a fiddle! Maybe experiment to find the best distance and do something with the phone holder? The holder I ordered is this style but I won't get it until next week.

Louise 

That one looks like it might be adjustable for distance - at least you can slide it up and down EP barrel. I've got one of these:

image.png.9c754ad6062cfc8ba7b4f5e5d49097ee.png

It is attached to the top of EP and does not have height adjustment.

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8 minutes ago, vlaiv said:

That one looks like it might be adjustable for distance - at least you can slide it up and down EP barrel. I've got one of these:

image.png.9c754ad6062cfc8ba7b4f5e5d49097ee.png

It is attached to the top of EP and does not have height adjustment.

I've never had, or used, one before - I'll let you know what it's like. I should get it by, hopefully, early next week :)

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The adapter actually came today! I had a quick  daytime go on the frac - the adapter seemed ok, generally. The only trouble is my phone has a soft case and I don't really want to have to remove it. Still, I was able to get it in position. I had to set it up to use remotely via a desktop. Of course, any small vibration would be disastrous! Even so, I think there may still have been some vibration as the image wasn't as sharp as I'd hoped. It may have been just that I took the image through window glass else it might have been to do with the autofocus. On first switching on the camera there was just a relatively small, bright, round circle. But I realised I had to expand it with finger and thumb on the phone touch screen :) . Much better! I'm not sure about using it at night except on the moon. I don't know how long an exposure the phone can take but it's probably not long enough for anything faint. I'll try it on the microscope when I can. Anyway, here's my first go:

Pylon50pc.thumb.jpg.2be2bb73dd3b718c824c18c5d8795039.jpg

 

Just a distant pylon :) (much reduced in size. There is some shadowing on each side - I'm not sure if that may just be caused by the soft cover which does get in the way a bit). I'll try some more shots when I can. 

Louise

 

Edited by Thalestris24
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Interesting topic @vlaiv. Have you looked at the Celestron NeXYZ mount? Whilst not without its foibles, it does allow adjustment in the Z axis which really helps you get positioned correctly.

I do Facebook Live broadcasts occasionally to my group of the Moon and Sun, and it work well. Controlling exposure is always an issue as there is no manual control in FB, but so long as the target fills most of the screen it works ok.

In the last two days I took one minute videos of sun and moon in 4K at 60fps. Just need to process them now and see what happens.

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9 minutes ago, Stu said:

Have you looked at the Celestron NeXYZ mount? Whilst not without its foibles, it does allow adjustment in the Z axis which really helps you get positioned correctly.

No, I did not look at that one - but yes, it does have what it takes and I like the idea it has "fine" adjustment in all axis - using screws for fine positioning.

 

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On 27/02/2021 at 17:12, Thalestris24 said:

The adapter actually came today! I had a quick  daytime go on the frac - the adapter seemed ok, generally. The only trouble is my phone has a soft case and I don't really want to have to remove it. Still, I was able to get it in position. I had to set it up to use remotely via a desktop. Of course, any small vibration would be disastrous! Even so, I think there may still have been some vibration as the image wasn't as sharp as I'd hoped. It may have been just that I took the image through window glass else it might have been to do with the autofocus. On first switching on the camera there was just a relatively small, bright, round circle. But I realised I had to expand it with finger and thumb on the phone touch screen :) . Much better! I'm not sure about using it at night except on the moon. I don't know how long an exposure the phone can take but it's probably not long enough for anything faint. I'll try it on the microscope when I can. Anyway, here's my first go:

 

 

Just a distant pylon :) (much reduced in size. There is some shadowing on each side - I'm not sure if that may just be caused by the soft cover which does get in the way a bit). I'll try some more shots when I can. 

Louise

 

 

Have downloaded 'Open Camera' and taken a few more phone pics. Open Camera lets you operate the shutter with just a noise, which is handy. However, images are still unsharp and I've come to the conclusion it's because of thermals. Nothing I can do about that. A short focal length would help. My F4 300mm Pentax lens would probably work quite well but I'd need an EP (T2-1.25") adapter for it. I could make one :)

Louise

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I took this one last week without an adapter but just holding my iPhone close to a 26mm Plossl eye piece. I just wanted to see if it could be done.

It's hardly brilliant, but I was amazed at just how easy it was for my phone to 'see' the image from my scope!

 

Moon 28 Feb 2021.jpg

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