Amount of vignetting with focal reducers

[Leedsgreen]

Dear All,

I have recently bought a Skymax 127 OTA and would like to try some deep sky imaging. With this in mind I have looked at various focal reducers. With regard the f3.3 type, I understand these are just for imaging purposes, not visual, and best used with small chip CCDs. However, would anyone be able to tell me how severe the vignetting would be on a DSLR APS-size sensor (1.6x focal length) or even a micro four-thirds sensor (2x focal length)? Would the image quality be any good?

If I were to attach my 1.25” diagonal to the image train and use my Canon Powershot afocally (with 32mm EP), would there be even less vignetting?

Finally, are f6.3 reducers much more forgiving with regard to the above?

Thanks very much for any help,

Nick

[Merlin66]

The Skymax MC at f12 is not really designed for DSO imaging.

I assume you're thinking...Ahh with x0.33 reducer it becomes an f4 system ( which would be OK)

Unfortunately a couple of things go against it:

The baffle tube on the Skymax is only 24mm inside diameter so that already will vignette any wide FOV and limit imaging to the smaller chips.

Adding the x0.33 reduced behind this restriction won't give you any better results, probably worse.

The x0.63 reducer to give f7.5 would work, albeit again with some vignetting from the baffle.

You'll also need a SW to SCT threaded adaptor for the rear cell.

[Leedsgreen]

Merlin,

Thanks for response. Yes, that was just what I was thinking! I have already tried 1.25" 0.5x focal reducer with limited effect (hard to achieve focus and plenty of vignetting) and had hoped larger reducers might help. That said, when I add up the cost of reducer, SCT thread, prime focus adapter etc the cost is getting close to that of basic f/5 Skywatcher refractor, which will probably suite me better and is, therefore, probably the route I will take.

Thanks again,

Nick

[Ant]

Merlin,

Thanks for response. Yes, that was just what I was thinking! I have already tried 1.25" 0.5x focal reducer with limited effect (hard to achieve focus and plenty of vignetting) and had hoped larger reducers might help. That said, when I add up the cost of reducer, SCT thread, prime focus adapter etc the cost is getting close to that of basic f/5 Skywatcher refractor, which will probably suite me better and is, therefore, probably the route I will take.

Thanks again,

Nick

Whatever you decide on getting, if you want to use it with a dSLR - make sure that it has a 2" focuser?

A second hand ED80 would be a good way to go with a dSLR

Ant

[philj]

Ive imaged with a Mak 127 with 6.3 reducer and a canon 300d just to try it, I neverbothered again.

Vignetting was quite bad and flat extraction did not really help. It messed the image up.

You must also make sure the camera chip is the right distance behind the reducer as well, if that is too long you will get vignetting from that as well. I use a short adaptot on the back of the reducer to which the dslr fits on with eos t ring

Philj

[Leedsgreen]

Thanks for further info everyone. It looks like refractor is the way to go.

Cheers,

Nick

[Macavity]

I never had much success with this or indeed a focal reducer! With a 1/2" chip, vignetting was clear. But I suspect this was (intentional and) at the remote end of the baffle tube, rather than the exit orifice! On the other hand the MAK127 works beautifully in eyepiece projection mode. Though I did have to invest in a Baader Maxbright diagonal and use Hyperion eyepieces in 2" mode. A 24mm Hyperion gives about the same image scale as the barefoot OTA. Of course, the field still reflects(!) that of a 1500mm primary.

This shows the sort of thing - with an ST102 for comparison: