DIY newt with raspberry pi and camera

[Some Dude With A Mak- Cass]

I am building a newtonian reflector telescope for the first time, and I have a few questions about things like focal ratio and identifying how long the telescope needs to be. I understand the basics of a focal ratio, a smaller number like an f/4 gets me a brighter image at the cost of clarity and field of view, while i higher number like an f/6 is dimmer but clearer and wider. I am planning on an 8" (203.2mm) primary mirror, and as an added complication (but definite benefit) in place of a secondary mirror I am adding a raspberry pi and a raspberry pi camera that will stream images to my laptop. I am going to 3d-print a holder for the electronics, does anybody know if there are print files for it I could use? The mirror will be 8" (203.2mm), but I am going to put it all in a 10" (254mm) wide Sonotube rather than an 8" (203.2mm) because it is more (comparatively) affordable and would allow me to upgrade to a larger mirror in the future if I wanted. My problems are that I can't choose a focal ratio I like, the telescope I have now is an f/5 and I am pretty happy with it, but I want a clearer image. However, I don't want to have to give up brightness for it. It wouldn't be such a big deal if I could use eyepieces to counteract some of that, but using the camera instead makes that impossible. I will probably end up going for a f/5, but I wanted to get your thoughts incase you know something about this I don't, because I am certain you do. My biggest issue, bigger than the f/r dilemma, is that I can only get 48" (1,219.2mm) long Sonotube where I live, and that may not be long enough to fit the focal length in. Let's say I go with my gut and get a f/5 mirror with a focal length of 39.4 inches, (1000mm) would I need it to be any longer than the 39.4 inches or could I just cut it precisely to 48" (1,219.2mm) and be done with it? I assume I would need a little extra space for the camera and the pi, so maybe it would end up closer to 42"(1,066.8mm), bit I wanted to make sure I understood it before I even buy the tube. My last question is about the mount. I am probably going to build my own, I have been thinking about a cabinet sort of thing, like a youtuber called StormTheCastle did, but I don't like that it isn't very stable. I was going to widen the base for stability, but then portability becomes an issue. What are your thoughts about mounting options? I want to build one if I can, but I have no problems with buying one from Orion if I have too. I think that is all my questions, thank you for helping.

[CraigT82]

35 minutes ago, Some Dude With A Mak- Cass said:

a smaller number like an f/4 gets me a brighter image at the cost of clarity and field of view, while i higher number like an f/6 is dimmer but clearer and wider

You've got this wrong... for a given aperture - the smaller the F number, the wider the view and vice versa. And generally the F number will have nothing to do with the clarity or sharpness of the image, that's mainly due to the atmospheric conditions but also down to the quality of the figure on the mirror. The brightness of the image is a function of exit pupil and magnification.

Mounting a raspberry pi at the focus is an interesting idea, not something I've heard of before.  People do talk about mounting a camera at the focus but usually give up on it as it difficult and doesn't really offer any benefits over using a diagonal mirror and mounting a camera conventionally outside the tube.

 

[Astrobits]

I assume that you will be making a classical Newtonian with a secondary mirror and focusser. If that is correct then an 8"f/6 will fit into a 48" long tube easily. When I made 8" f/6 mirrors they were all installed in 48" long tubes.

Nigel

[lenscap]

If you put the RasberryPi Cam in the main tube as suggested you will be building a scope that can really only be used for imaging.

If you build a conventional Newt with a diagonal secondary mirror and focuser you can pop the Pi in the eyepiece holder any time you want to image but you will also have a nice big scope for visual observing.

 

[Some Dude With A Mak- Cass]

2 hours ago, CraigT82 said:

You've got this wrong... for a given aperture - the smaller the F number, the wider the view and vice versa. And generally the F number will have nothing to do with the clarity or sharpness of the image, that's mainly due to the atmospheric conditions but also down to the quality of the figure on the mirror. The brightness of the image is a function of exit pupil and magnification.

Mounting a raspberry pi at the focus is an interesting idea, not something I've heard of before.  People do talk about mounting a camera at the focus but usually give up on it as it difficult and doesn't really offer any benefits over using a diagonal mirror and mounting a camera conventionally outside the tube.

 

Are you sure about the F ratio? I looked it up yesterday because I didn't know what it was, and that was what I found. You are right that mounting a raspberry pi doesn't have very many advantages, but for me the biggest are that it is way cheaper than a good camera, and I can use the raspberry pi for other things as well, like eventually I want to set it up with a stellarium-guided PushTo system, and the raspberry pi could be used for that as well. There are a lot of things you can do with a raspberry pi, which is why I am giving up some things, like eyepieces, for the potential improvements I could make.

[Some Dude With A Mak- Cass]

1 hour ago, lenscap said:

If you put the RasberryPi Cam in the main tube as suggested you will be building a scope that can really only be used for imaging.

If you build a conventional Newt with a diagonal secondary mirror and focuser you can pop the Pi in the eyepiece holder any time you want to image but you will also have a nice big scope for visual observing.

 

That is very true, but it adds other possibilities. I could connect the raspberry pi to some motors and a laptop with tellurium for a PushTo system, for instance. Don't like giving up the versatility of eyepieces, but I have a pretty nice mak that is more than strong enough for visual observing most objects visible in my area, so I think it is worth it.

[lenscap]

Another consideration is magnification/field of view.

If you build say an F5 scope the Pi cam (with lens removed) will give a fixed Mag of about X200 and a fov of about 0.5 degrees.

If you put the Pi in the focuser you can easily insert a Barlow or Focal Reducer to get the Mag or fov you need.

[coatesg]

Interesting project! Assume you're thinking about lunar/planetary here? The max exposure of the pi cam is quite small (6sec from memory?) and I don't suppose the read noise is that low - you may struggle with deepsky.

[Gina]

ZWO have some pretty low cost cameras and they're very good.

[Some Dude With A Mak- Cass]

7 hours ago, coatesg said:

Interesting project! Assume you're thinking about lunar/planetary here? The max exposure of the pi cam is quite small (6sec from memory?) and I don't suppose the read noise is that low - you may struggle with deepsky.

I might experiment with some of the nearest objects like the Andromeda galaxy and some binary stars, but I am planning pretty much exclusively on this being a scope for the moon and planets, because a raspberry pi camera is, as you said, not built for this. I might try stacking exposures, but I have more interest in the moon and planets right now.

[Some Dude With A Mak- Cass]

4 hours ago, Gina said:

ZWO have some pretty low cost cameras and they're very good.

I will have to check that out, thanks for the help.

[Gina]

FLO sell them - best place I've found

[coatesg]

1 hour ago, Some Dude With A Mak- Cass said:

 I might try stacking exposures, but I have more interest in the moon and planets right now.

I reckon this may work nicely - shoot the video with raspivid (with as little compression as possible), and then take the video off to be processed with something like Autostakkert.  You might find a ZWO gives a slightly better quality stream, but if you designed the scope so it could take either (eg managed to get a 1.25" adapter for the pi cam) then it wouldn't matter ?

[CraigT82]

22 hours ago, Some Dude With A Mak- Cass said:

Are you sure about the F ratio? I looked it up yesterday because I didn't know what it was, and that was what I found. You are right that mounting a raspberry pi doesn't have very many advantages, but for me the biggest are that it is way cheaper than a good camera, and I can use the raspberry pi for other things as well, like eventually I want to set it up with a stellarium-guided PushTo system, and the raspberry pi could be used for that as well. There are a lot of things you can do with a raspberry pi, which is why I am giving up some things, like eyepieces, for the potential improvements I could make.

Quite sure!

For a given aperture and a given eyepiece (or camera chip in your case) a scope with a shorter focal length (smaller F number) will have a wider field of view.

 

Here is a useful link for determining your field of view with any scope and any camera.  you just need to input the specs of each (scope aperture and FL, camera resolution and pixel size). https://astronomy.tools/calculators/field_of_view/

 

In this image I've loaded two different field of views on Jupiter for a 200mm aperture scope and a Raspberry Pi V2 camera module:

Red- 800mm FL (F4)

Yellow - 1200mm FL (F6)

[rl]

If you put the Raspberry Pi and the camera in the tube at the prime focus..don't forget you will have a permanent heat source in the optical path which may take the edge off your images. I guess its no worse than a dew strap but at least you can turn those off for the duration of the exposure. 

[Some Dude With A Mak- Cass]

7 hours ago, rl said:

If you put the Raspberry Pi and the camera in the tube at the prime focus..don't forget you will have a permanent heat source in the optical path which may take the edge off your images. I guess its no worse than a dew strap but at least you can turn those off for the duration of the exposure. 

That is something I hadn't thought of. A raspberry pi doesn't make much heat, but it might make enough to have an effect. Thanks for the tip!