Why not use Achro for Astrophotography?

[originaltrilogy (Petr)]

It is seeming everybody is saying mono cameras are best for DSO and planets work.

If this is case would be best to get large aperture achro instead of many dollars on triplet APO like Takahasi and so on?

If imaging is through filter one colour at a time, why not just use achro scope and spend more money on filters and CCD?

Is there other reason if not using visual?

[Poita (Peter)]

Don't know on this one, I do know that I was recommended to get an achro scope when upgrading my PST as I was only going to do mono imaging, and that using an APO would be a waste of time.

[traveller (Bo)]

Interesting question. Remember the scope is only the lens part of the imaging rig. So no matter how good you camera is or what filter you use, if the scope/lens is sub standard, then there isn't a lot the camer can do.
So getting an achro over a APO is really not the point, the point is getting a scope/lens that can gather sufficient light AND project the image to the camera. Therefore a large achro will be good at gathering light, but still suffering from coma, while the smaller APO will have a good image but not sufficient light.
One variable is the mount or the exposure time. So a solid and accurate mount will allow smaller scopes gather more light, therefore producing superior images.
Bottom line, if budget is tight, get a decent APO and a solid and accurate mount. Aperature rule has its limitations when it comes to imaging.
My 5 cents worth.
Bo

[Moon (James)]

I guess an Achro would be ok with narrowband filters. But the images taken with a regular B or L filter would look ordinary.
I did some tests with my 'semi-apo' ED127 you can see here:
http://deepspaceplace.com/ed127.php
James

[LightningNZ (Cam)]

The biggest problem as I see it would be that you need to refocus for each colour. Given the focusing with a CCD or dSLR is annoyingly difficult anyway, spending lots of money on a heavy, computerised focuser that remembers which position it should be in for each colour and at what temperature is probably not worth it. Better to get an APO (or even better a mirror based scope) and not worry about focusing and filtering out chromatic aberration, then you have a great general purpose scope too.

Cheers,
Cam

[Alchemy (Clive)]

Originally Posted by Moon

I guess an Achro would be ok with narrowband filters. But the images taken with a regular B or L filter would look ordinary.
I did some tests with my 'semi-apo' ED127 you can see here:
http://deepspaceplace.com/ed127.php
James

Very interesting effect with the bhatinov mask.... Well done bit of sleuthing there.
................................

To the original question

You could use an achromatic for narrowband as suggested, but that would limit your targets. The luminance channel which most use for detail in an LRGB image would be blurry compared to an APO.

The other potential issue is that an achromatic scope is made to a budget, and you may have to replace the focuser if you have a heavy camera and want fine control.

As a general rule, planetary work is done with big ( read bigger is better) mirrored scopes. Short length ( up to 1000mm )deep space is refractors, above this spreads out into high end refractors, mirrors of all sorts. Once you are above 2000 mm you are unlikely to be using a refractor as once they are above 6 inch apertures they are brutally expensive.

Everything has it's pros and cons, if I had lots of scopes it would be fun to try and see just how one shapes up under narrowband 150 mm f5 would be a lot of fun

[originaltrilogy (Petr)]

Originally Posted by LightningNZ

The biggest problem as I see it would be that you need to refocus for each colour. Given the focusing with a CCD or dSLR is annoyingly difficult anyway, spending lots of money on a heavy, computerised focuser that remembers which position it should be in for each colour and at what temperature is probably not worth it. Better to get an APO (or even better a mirror based scope) and not worry about focusing and filtering out chromatic aberration, then you have a great general purpose scope too.

Cheers,
Cam

Most threads I read have users refocus for each colours anyway to get best results even for parfocal filters, I thought if refocus anyway maybe achro would do as good.
So people use mono camera and achro for sun or moon should probably use APO instead for mono work?

[LightningNZ (Cam)]

Originally Posted by originaltrilogy

Most threads I read have users refocus for each colours anyway to get best results even for parfocal filters, I thought if refocus anyway maybe achro would do as good.
So people use mono camera and achro for sun or moon should probably use APO instead for mono work?

I think it's fair to say that most achros are low-end instruments and have generally poor focussers, I know mine does. You'll end up paying almost as much for a completely square and repeatable focusser as you would for an ED scope anyway. The ED would also give you the flexibility of shooting in colour if you wanted. Can't comment more than that as I've never shot anything planetary other than the moon.

[Terry B]

I tried photographing through my 120mm achro that was my old guide scope. I found that I couldn't focus the blue image at all without a big halo.

[DavidU (Dave)]

Our own Bert has produced wonderful Ha images with an Achro.
http://www.iceinspace.com.au/forum/s...ad.php?t=52812

Here is another image done with an Achro with a minus violet filter
http://www.gamaelectronics.com.au/im...ge30617843.jpg