Camera recommendation…

[Captain Scarlet]

I’m not an imager, nor can I see myself becoming a proper one. I’m too interested in Visual. However, I have been known to dabble especially for occasional wide-field MW style pictures, using my Canon 6D.

But for design purposes I do occasionally like to know and/or measure the exact focal lengths of my scopes, particularly Newtonians of which I’ve built two recently. I find that the >=44mm out-focus requirement for a DSLR is too much for my visual-only scopes.

So, my question: does there exist an inexpensive, say crop-frame sized, astro camera with a flange-to-focus distance of the order 15-20mm or less? It only needs to be good enough to capture focussed stars.

Thanks, Magnus

Edited December 13, 2022 by Captain Scarlet
typo

[happy-kat]

What sort of budget range. I wonder if a mirror less DSLR would fit in your flange-focus distance range. 

Edited December 13, 2022 by happy-kat
Edited as reread requirement

[Captain Scarlet]

16 minutes ago, happy-kat said:

What sort of budget range. I wonder if a mirror less DSLR would fit in your flange-focus distance range. 

Ideally budget will be much less than the cost of a mirrorless DSLR. I already have a 6D and a 7Dmk2, so I wouldn't get any extra use from something like that. I'm hoping something inexpensive and perhaps a generation or two old might fit the bill and be cheap too.

[Elp]

I think mirrorless is your only option, most DSLRs have been using the same format for years.

https://en.m.wikipedia.org/wiki/Flange_focal_distance

Edited December 13, 2022 by Elp

[Elp]

I would of thought a reducer lens may help with your short focal distance.

[Captain Scarlet]

I'm not looking for a DSLR, I'm looking for a (perhaps old) dedicated astro camera to screw or mount directly on to a focuser flange. The brand name ZWO or QHY occur to me?

[Captain Scarlet]

1 minute ago, Elp said:

I would of thought a reducer lens may help with your short focal distance.

Unfortunately that would defeat my purpose, which is to measure the distance on the sensor between two known stars at prime focus so I can exactly calculate the focal length of the scope.

[Elp]

You'd have to check their specs. Many of the mid range to more zwo are 6.5mm, the cheaper ones this distance is a bit longer. You can make up the difference needed with spacer rings.

Edited December 13, 2022 by Elp

[Captain Scarlet]

20 minutes ago, Elp said:

You'd have to check their specs. Many of the mid range to more zwo are 6.5mm, the cheaper ones this distance is a bit longer. You can make up the difference needed with spacer rings.

6.5mm would certainly work, probably anything up to 15mm.

[DaveS]

S/H ASI 1600 might fit the bill. A bit smaller than APS-C (I think it's closer to micro 4/3). The back focus is 6.5 mm as-is, but it *should* come with an 11 mm adaptor ring which takes the back focus to 17.5 mm, similar to a lot of old CCD cameras.

Possibly an ASI 071, which has a native 17.5 mm BF, but might be more, even S/H

[ONIKKINEN]

1 hour ago, Captain Scarlet said:

So, my question: does there exist an inexpensive, say crop-frame sized, astro camera with a flange-to-focus distance of the order 15-20mm or less? It only needs to be good enough to capture focussed stars.

Thanks, Magnus

Inexpensive probably not, APS-C and even Micro 4/3 sensors are on the large side for astrophotography.

Since the technical stuff is not important you may want to look at used mirrorless or used CCD cameras. Used CCD can be found for cheap since the market has mostly moved on from these. If platesolving the focal length is the target the sensor doesnt need to be as large as a crop frame one so you could find an old CCD for cheap enough to fit the purpose.

Most dedicated astrocams are either 12.5 or 17.5mm backfocal distance so should work. Some mini cameras are like eyepieces and have whatever distance you need. Those ones have small sensors though, but for just focal length measurements should do ok.

[Captain Scarlet]

Thanks for the replies people. The small sensor size for a given price level for these cameras seems to be a deal-breaker for me. Even using a full-frame sensor a standard error (say 5 pixels) in estimating the exact centre of a star off an image translates into a focal length uncertainty of the order of 6mm for my 1800mm FL scope. A smaller sensor and bigger pixels will only amplify this. I might see if I can borrow somebody's mirrorless camera...

Thanks, Magnus

Edited December 13, 2022 by Captain Scarlet

[vlaiv]

27 minutes ago, Captain Scarlet said:

Thanks for the replies people. The small sensor size for a given price level for these cameras seems to be a deal-breaker for me. Even using a full-frame sensor a standard error (say 5 pixels) in estimating the exact centre of a star off an image translates into a focal length uncertainty of the order of 6mm for my 1800mm FL scope. A smaller sensor and bigger pixels will only amplify this. I might see if I can borrow somebody's mirrorless camera...

Thanks, Magnus

Centroid algorithms have sub pixel accuracy - you can measure star position with sub arc second precision with modern pixel sizes.

What are your requirements in terms of FOV / sensor size, and what sort of budget you have in mind?

How much DIY capable are you?

There are few options out there that might be better suited for your need.

How about RPI camera with RPI?

If budget is tight - how about something like C270 - web camera. All you need to do is remove front lens and add 1.25" nose piece (that can even be piece of 32mm PVC pipe slightly sanded down).

[vlaiv]

Btw, if you want to do simple astrometry to determine focal length of telescope - what about just using mobile phone camera with something like 10mm ortho eyepiece?

Mobile phone adapters are readily available (and cheap), and all you need is scope of known FL - which will help "calibrate" ortho eyepiece to exact FL.

 

[Captain Scarlet]

23 hours ago, vlaiv said:

Centroid algorithms have sub pixel accuracy - you can measure star position with sub arc second precision with modern pixel sizes.

What are your requirements in terms of FOV / sensor size, and what sort of budget you have in mind?

How much DIY capable are you?

There are few options out there that might be better suited for your need.

How about RPI camera with RPI?

If budget is tight - how about something like C270 - web camera. All you need to do is remove front lens and add 1.25" nose piece (that can even be piece of 32mm PVC pipe slightly sanded down).

Reasonably good DIY-wise.

Centroid algorithms … can they cope with Newtonian coma? I’d be using pictures taken with no CC, of course. And where would I find one? Either the code or an implementation.

Although I have a Raspberry Pi, a gift from someone who evidently wanted to waste a lot of my time (😈) I have not yet even got the bits out of the boxes … I haven’t dared.

[vlaiv]

21 minutes ago, Captain Scarlet said:

Centroid algorithms … can they cope with Newtonian coma?

No, but how wide do you want to go?

What is that you actually want to perform? Just measurement of focal length? You can use just central portion of the FOV for that - coma free region of focal plane.

Just find a way to get some images and leave rest to already existing software - like AstroImageJ - it will measure stars - do basic photometry as well, and if you plate solve - it will give you RA/DEC positions. All that will be in table ready to be copy/pasted to spreadsheet that you can then use to determine pixel scale (and from that focal length).

In fact - just uploading image to astrometry.net will do exact same thing - provide you with pixel scale.

As far as raspberry pi and camera go - just do google search on it and you'll get plethora of hits like these:

https://astronomynow.com/2022/02/25/raspberry-pi-high-quality-camera/

https://adambaskerville.github.io/tabs/astro/

https://www.amateurastrophotography.com/astrophotography-with-the-raspberry-pi

https://www.instructables.com/Long-Exposure-and-Astro-Photography-Using-Raspberr/

As long as you take exposure with it and you can fashion adapter to put it at prime focus of telescope - you can combine the two to produce an image.

 

[nfotis]

I think that the easiest way to get a large sensor (APS-C size) is to get a mirrorless used camera, like the Canon M-series (eg., M3), with 18mm flange distance or Sony E-mount 6xxx series (18mm). Especially the Canon M-series could use your Canon EF lenses via an adapter.

A dedicated astro camera with a large sensor will be VERY pricey, as the cost goes up with the size of the sensor. Maybe a used CCD camera could be useful?

N.F.

 

[Avocette]

I think the thread is rather centred on quite large sensor sizes, but if you’re really only trying to measure focal length, you don’t need a big sensor. Many astrophotography software will measure the accurate focal length of any scope/camera combination. They do this by platesolving an image. A large FoV is not necessary for this. In my case I have a mix of equipment, and for each configuration after any image is captured during polar alignment or guiding, my software immediately indicates the actual focal length of the combination. For instance my SkyMax150 with a simple visual back with a closely coupled camera measures 1922mm and when a so-called 2x Barlow is inserted the focal length measures 3445mm giving a measured Barlow ratio of 1.79x. My Sky-Watcher EvoGuide measures 253mm rather than the specified 240mm.

You might consider getting hold of (or borrowing) a ASI120MM-mini which has its sensor chip very close to the face of the camera (8.5mm according to the specification). This camera would be a good investment if you ever do get into imaging and in particular guiding!

Edited December 19, 2022 by Avocette