Narrowband imaging with achromatic refractors.

[RobH]

Hello folks,

I read somewhere that achromats, while not being the best for RGB imaging due to CA, are fine when using narrowband filters.

I have an old Helios 120 F5 scope and now have the possibility of piggybacking it onto my SCT.

As it has a good FOV, and quite a bit more light gathering power than my 80mm, if this is the case it should be a good scope for NB imaging.

Does anyone have any thought on this?

Cheers

Rob

[Paxo]

I guess this should be the case Rob as you are only talking about a max bandwidth of 13nm (max) or so as compared to several hundred for a standard visible spectrum, keep us posted on how this proceeds.

Steve..

[Kaptain Klevtsov]

I've used an ST80 for Ha stuff and its fine. The thing to watch out for is refocusing when you move the filter wheel if you're doing more than one filter.

Kaptain Klevtsov

[RobH]

Thanks KK,

Theoretically.... ....the Astronomik filters I have shouldn't need refocussing.

We'll see however.

Will let you know the results.

Cheers

Rob

[bern]

Theoretically.... ....the Astronomik filters I have shouldn't need refocussing.

The filters won't but the achromat will.

bern

[FLO]

I read somewhere that achromats, while not being the best for RGB imaging due to CA, are fine when using narrowband filters.

That is my understanding too, that when narrowband imaging an achromat's false-colour is not seen. Neither is light pollution, making narrowband imaging an ideal solution when imaging under light polluted skies.

[GazOC]

Yep, the fact its narrowband means the colourful shortcomings of achromats don't come into play. Obviously you'll still get any other aberations the f5 lens has.

[RobH]

Bern, I was wondering about that....makes sense.

Gaz, I'm hoping that with the chip on the 16HR being pretty small, edge of field distortion won't be too bad....the proof will be in the pudding though as they say!

Looking forward to giving it a go.

Cheers

Rob

[narrowbandpaul]

rob, dont know if you caught that...even though the CA wont be noticed due to the restricted wavelengths, and even though the filters may be parfocal with each other, the scope will need refocussed when changing filters. this is caused by the fact that the CA is an inherent property of the scope. The O[iII] and S[iI] wavelengths are focuused at a completely different point.

Sorry if you already realised that.

No point in collecting data that turns out, out of focus

Paul

[themos]

Wouldn't you expect the various narrowband images not to coincide in the spatial sense? You'd probably have to be prepared to stretch some more than others when you put the final image together.

[MartinB]

Will be interested to see the results Rob. One problem could be the overall optical quality of the objective. Obviously it isn't just about CA. Spherical aberration and astigmatism could be an issue, maybe less of a problem with the smaller chip like you say.

I think you might also be able to do monochrome imaging using green filter but at F5 the CA might still make itself felt.

F5 sounds good though doesn't it!

Wouldn't you expect the various narrowband images not to coincide in the spatial sense? You'd probably have to be prepared to stretch some more than others when you put the final image together.

That's a good point. Fortunately decent stacking software should be able to automatically resize.