Vignetting with a 2.5" focuser (in f/7 scope)?

[johngwheeler]

I've been struggling to find an answer to whether a 2.5" focuser is likely to cause vignetting on a full-frame (36x24mm) camera sensor.

This would be for an f/7 130mm scope, probably used in conjunction with a 3" 0.79x focal reducer (stated to have an illuminated image circle of 45mm).

Whilst the focal reducer should fully illuminate the sensor (having an M68 connector on the scope side and M48 on the camera side), it is not clear whether any "mechanical" vignetting might occur with a 2.5" focuser drawtube. 

I imagine this would depend on the backfocus of the scope - i.e. how far the focal plane is behind the end of the focuser.  If it's a long way back, I can visualize the light cone potentially being restricted when it enters the drawtube inside the OTA. Hopefully, the scope isn't designed this way, but it's hard to tell.

If there is no mechanical vignetting, it does raise the question of what the benefits of larger focusers are. Greater load capacity certainly; improved stability? maybe. But does it actually give a wider unvignetted image circle?

I'd like to apply some science to this question, rather than buying a bigger focuser "just in case"!

 

Thanks for any answers!

John.

[pete_l]

All I can suggest is drawing a ray diagram of the back end of the equipment.

It has been a frustration for me also, trying to get information from suppliers about the (fully) illuminated field of the devices they sell. I asked a similar question of Meade, regarding a £ 5-figure product of theirs. All they could say was

Quote

We have no spec for this as a “largest” however these work quite well with the modern DLSR type setups

and a suggestion that I look on forums for examples. "Quite well" ??? Ha!
Needless to say, I walked away!

[MarsG76]

I dare say that you will have some vignetting.... even if its just intensity fall off toward the edges.

I have vignetting like intensity fall off when I use my 40D with the reducer... nothing that can't be fixed with flats.

 

[billdan]

Hi John,

Your light cone expands at the rate of your F ratio. If you know the length of your drawtube you can work it out whether you will get vignetting.

So with a 44mm diag sensor at F7, the light cone will be 10mm wider (54mm) at a distance of 70mm (70mm / 7) away from the sensor, and 20mm wider (64mm) at 140mm away from the sensor. As 2.5inches is 63.5mm wide, if the distance from the sensor to end of the drawtube is less than 140mm ( 5.5")  you should be OK.

Cheers

Billl

[johngwheeler]

45 minutes ago, billdan said:

Hi John,

Your light cone expands at the rate of your F ratio. If you know the length of your drawtube you can work it out whether you will get vignetting.

So with a 44mm diag sensor at F7, the light cone will be 10mm wider (54mm) at a distance of 70mm (70mm / 7) away from the sensor, and 20mm wider (64mm) at 140mm away from the sensor. As 2.5inches is 63.5mm wide, if the distance from the sensor to end of the drawtube is less than 140mm ( 5.5")  you should be OK.

Cheers

Billl

Excellent answer Bill, and quite logical when one thinks about it!

I think the total distance would need to be from the focal plane to the rear of the draw tube plus the length of the draw tube.. This total could easily exceed 140mm I think.

My next question would be how a focal reducer affects this, but this should just be a question of the entrance and exit apertures being large enough for its image circle.

[billdan]

Not sure what the answer is regarding the focal reducer, you would have to see its spec's to see what the entrance pupil has to be. It's a pity in our hobby we can't rent before we buy, it would make life a lot easier.