Image circle for full frame cameras

[OneCosmos]

Apologies if this has been done to death (although I quickly searched the forums for “full frame”).

I have a WO FLT 120 which I believe, with the P Flat 68iii flattener, may provide a 44mm image circle and camera manufacturers always seem to say this means it will work fine with full frame sensors. Everything I read, however, indicates full frame requires 46mm.

Is anyone using FF WITH THE FLT120?

Simple question then. Is it folly to buy full frame sensor leading to frustration and disappointment or should I go with APS-C e.g QHY 268 mono?

Second question does an APS-C chip really need 2” filters?

[Atmos]

Your best bet would be to check out Astrobin and see if you can find anyone using that combo and see what the image quality is like.
It’s rarely explicitly mentioned but in many instances the quoted image circle can be referring to “illuminated image circle” rather than how well the stars are corrected.

The difference between 44mm and 46mm is likely to be negligible as it’s only the extreme corners.

[AlexN]

WO FLT120 with the PFlat 68III will 100% work with a full frame sensor...

Sensor choice is a tough one.
I struggle with this daily, because a part of me always wants a smaller, more densely pixel packed sensor to get closer and more resolved, but the other part of me wants a bigger, more sensible pixel density sensor to capture larger area of sky...

If you're getting the WO FLT120, the world is your oyster really - it will cover full frame without an issue, however, you will really want to be using 50mm unmounted filters on a full frame sensor, which, if its colour and you're wanting to use dual narrowband filters, well, they don't exist.

That said, any of the IMX571 based cameras like the QHY268, ASI2600, Player One Posiedon etc are fantastic cameras, they are significantly cheaper than the full frame options, and, they have the advantage of working perfectly fine with 36mm unmounted filters. (Again, if you're going for a colour camera, you are going to need 2" mounted filters).

What do you like imaging the most? Do you want to capture large sprawling nebulae, or smaller targets like galaxies? also, are you going colour or mono camera?

If the cameras pixel size are the same, then you can basically do whatever you want...

Lets say you're comparing the IMX571 (APS-C) and the IMX455 (FF), and mono in both cases.. If you have the money to do it, I'd get the IMX455 and 50mm unmounted filters. as your resolution will be identical in either case, and you can just crop down to aps-c size for smaller targets, or even down to the size of the IMX533 for galaxies and still be working with a 9mp image...

The question of FF vs APS-C is so nuanced, as there are so many factors to consider.

From a price perspective alone, I'd say go the APS-C camera with 36mm unmounted filters for mono, or 2" mounted filters for colour, and you'll be VERY happy.

That said, if I could fine a full frame colour sensor for a reasonable price at the moment I'd be all over it, as my Askar 120APO is begging for more field of view.

[OneCosmos]

Hi Alex,

Thanks for the comprehensive response, although now a moot point because I did go full frame, mainly because a good deal on IIS came up and I took it. I do however only have 2" round mounted filters and it now sounds like that might cause me some grief. Down the line I suppose I can swap them out if I feel it is necessary.

On a related note, I see the ZWO 7 x 2" filter wheel can be bolted directly to the filter wheel - do you know the pros and cons of doing so/not doing so?

Chris

[JA]

Hi Chris,

I think you'll be OK.... barely/JUST, with 2 inch filters on a Fullframe sensor. If your camera is genuinely full frame at 36mm x 24mm, and not one of the larger 36x36mm cameras, then it has a 43.27mm diagonal.

As a guide, the screw-in 2 inch filters I have are M48 threaded and have a clear aperture diameter of 44mm which just exceeds the full frame sensor diagonal of 43.27mm, meaning that the filter should not restrict the illumination of the sensor, if properly centered/aligned. If anything it's more likely that the telescope optics might not fully or evenly illuminate the sensor. You can get a feel for the true image circle diameter by testing by projecting a daytime image at infinity (easier) on to a sheet at the focal plane and measuring the diameter of the illuminated circle. You can also get a feeling of image fall-off and distortion at the edges in so doing.

Best
JA

[AlexN]

Chris:

The beauty of bolting the CFW to the camera, and likewise, the OAG to the CFW, is that there is FAR less chance of flexture and tilt induced by the threads, but also the rotation is indexed. IE - your cameras sensor long axis will be horizontal, and the shorter axis will point vertically (with regard to the filterwheel)