Budget EEVA - some thoughts ...

[vlaiv]

I have an idea that I would like to share. It is far from usable, but with a bit of effort it could turn out to be EEVA on budget for masses.

I already fiddled with this idea in a different form - using afocal method for EEVA. In that instance I was calculating reduction factor for dedicated astronomy cameras, and I'm yet to test it out with ASI178 and Mak102 with 32mm eyepiece and 12mm C/CS mount lens.

In this instance - we would be using phone as EEVA imaging device. Adapters to attach phone to eyepiece are readily available and cheap - they can even be 3D printed.

Let's do some basic calculations to see what sort of results we might expect. I started with this image:

 

This is hand held Astro Essentials F/4 32mm guide scope with 32mm GSO plossl and Xiaomi A1 phone (phone also hand held). Image is of course neighbor's brick wall - but this image prompted me to think of focal length of lens in phone cameras. What sort of focal length / sensor size and pixel size we are talking about and how does that translate into arc seconds per pixel?

It turns out that things match pretty well. Simple Plossl eyepieces have AFOV of about 50-52°. Most phone cameras are going to be made to match ~35mm lens - which is general wider field lens for photography. That lens gives about 63° FOV. Here is first tool - FOV calculator:

https://www.pointsinfocus.com/tools/depth-of-field-and-equivalent-lens-calculator

That means that phone FOV will be pretty good match of plossl eyepiece AFOV - which is good thing, we will be able to utilize most of the field of view.

Next step is to find out camera specs for your particular phone model. Mine current is Xiaomi A1. At gsm arena - https://www.gsmarena.com/ you can find specs for your model.

This model has two sensors - one is 26mm lens equivalent with 1.25µm pixel size and 1/2.9" format, other is 50mm equivalent with 1.0µm size pixel (no mention of sensor format).

We are for the time being interested only in equivalent focal length. This is not true focal length as 26mm with 1/2.9" sensor would give extremely small FOV. According to above calculator of FOV, 26mm lens will have 79.5° - that is larger than 50° of plossl so there will be some vignetting - and it shows in above cropped image. Here is what image at eyepiece looks in full format when phone is carefully placed (above is at some distance so it is smaller):

And it looks like eyepiece FOV is about two thirds of camera FOV (by diagonal). That matches pretty good with 79.5° : 50° ratio that calculation gave us.

I just measured size of eyepiece FOV in original image and diameter is ~3070px. Camera is 4000x3000 so diagonal is 5000px. 80°:50° = 1.6 and 5000:3070 = 1.63. We have excellent match between the two.

This gives us way of determining sampling rate. At the moment, sampling rate is 50° / 3070px = 0.01628°/px, but remember we don't have magnification yet as eyepiece is not placed on telescope.

I'll be using Mak102 to test this out and with 1300 mm / 32mm  plossl it will give me = x40.625 magnification.

So we need to account for that in above formula and we need to divide above sampling rate with magnification. 0.01628 / 40.625 = 0.0004°/px = 0.024'/px = 1.44"/px - and usable 3000px in diameter (which can be furthered binned x2 or x3).

What actual FOV will it give? Surprisingly nice, like this:

almost 1.3 degrees of sky.

Compare that to something like 130PDS and ASI224 - often used as EEVA platform:

with ~1.2"/px sampling rate.

Only thing that is left to do is to see how to use phone for EEVA stacking. I can't speak of IPhone line but Android phones have something called Camera2API that has been around since 2015. It allows for raw camera data to be captured - exactly what we need for EEVA. Problem with this is - phone manufacturers decided that it is excellent selling point and they don't have it available on lower priced models - often reserved for flagship models - although Google specification meant it as generally available API. In fact - most phones are capable of doing this but are purposely disabled in software. My phone is similar in that it is capable of supporting Camera2API but it is disabled and I need to root my phone in order to enable it. Luckily I don't have to recompile or install patch to system itself so I get to test it. This is really annoying as I got this model because it is part of Android One initiative - it has stock android and should support most of android features - it is used by developers because of best compatibility.

I think that best course of action would be to develop small client server system. ASCOM driver (or INDI) that would work on computer and accept network connections and small client app on mobile phone that would operate camera and stream result over network to server application on computer that would emulate physical camera device and let ASCOM / INDI compatible software use resulting images.

Well - that is the idea

 

[Vulisha]

Hi Vlaiv

 

I did something similar with my phone OnePlus 5T it supports camera2api but not fully. There is also apo currently in development DeepSkyCamera thst should5 be able to take longer exposure images, bit my phone does not support exposute longer than 1 second outside main camera app for something reason. 

So capturing was solved using Mini Mouse Macro software to repeat clicks with teamviewer on PC + teamviewer quick support on phone. 

You could additionally add sharpcap to stack from directory (pro version supports) andtry using DeepSkyCamera to bypass teamviewer hack. 

There is pro version planned ad well, and it looks promising:

Quote

 

DEEPSKYCAMERA PRO VERSION
I've started the development of the Pro version. The Pro version will contain lots of advanced feature for astrophotography:

• Switching between camera sensors on the back of the phone (if phone has two or more camera sensors on the back)
• Live stacking
• Postprocessing of images (curves, saturation etc.)
• Lucky Imaging: Recording of videos, stacking and processing
• Timelapse videos
• Starttrails
• Anti startrails
• animated Startrails
• Upload of the images to the Cloud (Dropbox, Google Drive usw.)
• Upload of the images to your own webserver (if you have one)
• Support of PHDGuiding
• Profile management
• Remote control
   

 

 

 

Here is one result. 

 

Edited January 13, 2021 by Vulisha

[Jonk]

I have an old but brilliant Fujinon 25mm f0.85 CCTV lens, and with a 3D printed adaptor, managed to fit to a ZWO ASI290MM. The combination is 23.5"pp, 12.6 deg x 7.15 deg FOV according to the platesolved image.

It was placed on a tripod, set to decent focus and using live stacking in Sharpcap was very pleased to see the results. I had to stop down to f2.8 to reduce coma and as you can see in this screenshot, a 5 minute livestack shows the flame nebula.

Conditions were terrible, poor transparency but I couldn't resist the experiment. I was very impressed! Budget? Camera and tripod which I already own, and the lens was about to be binned so I saved it.

Sorry this diverts from your post Vlaiv, but it's another example of EEVA on a budget (but not as low as yours probably).