Large ACHROMAT for RGB imaging?

[JohnH]

I saw an article in a recent SAN reviewing 5" ACHROs and it got me thinking -the costs are not to high considering the appature so:

How about a large ACHRO teamed with a mono CCD and filters instead of the costly ED/APO glass? Are any ACHROs really suitable for AP or would you not get the build quality/focuser etc needed in that format?

Anybody doing this?

[Terry B]

I have a 120mm skywatcher 600mm fl achro as my guide scope and have tried to image through it with coloured filters and my mono CCD.
The red and green filters give lovely sharp images with pinpoint stars. The blue filter gives stars with big halos around them. I fiddled with the focusing for ages and still the halos remain. I don't have a minus violet filter to try but it might be worth giving it a go stacked on the blue filter.
Narrow pass filters would be a different option and shouldn't suffer from the same problem.

[gbeal]

What sort of cost John? Why I ask is this, I have an 8" f4 newt, and it requires a Baader MPCC, but there is no CA as you will understand. All up I guess it was significantly less that what I imagine even a decent achromat will cost? OK, in this I figure a top quality focuser as well, but this will be in the mix regardless, so the basic scope(s) should be compared.
Does this make sense?
Gary

[ballaratdragons (Ken)]

Bert already does this with great success!

[JohnH]

Originally Posted by gbeal

What sort of cost John? Why I ask is this, I have an 8" f4 newt, and it requires a Baader MPCC, but there is no CA as you will understand. All up I guess it was significantly less that what I imagine even a decent achromat will cost? OK, in this I figure a top quality focuser as well, but this will be in the mix regardless, so the basic scope(s) should be compared.
Does this make sense?
Gary

I suppose I should have mentioned that I already own an 8" reflector - A Vixen VC200L - and I use a Canon 20D with that....but I am thinking about dabbling in narrow bandand RGB imaging as I also own mono camera - the long fl of the Vixen is not really suitable for use with smallish chip in my camera and my small guidescope (66mm) would limit me to long(ish) exposures...so I am thinking about ota's that cost $3-500. That would also be more portable, have higher contrast and have few collimation issues than the reflector option....

[gbeal]

Yes, I see the Vixen in your sig. I still believe that a decent small reflector will easily out-perform the achromat. By all means try one, and let us know, as it is something I have longed to try, for quite a while.
I also take on board the caveats that you list for the reflector, but I put up with them for the image quality.
Good luck,
Gary

[montewilson (Monte)]

I saw quite a good Ha shot some time ago taken with a 5" chinese cheapie. The author said it was suitable for RGB if you were prepared to refocus for each colour.

There is one caveat. The focusers on these scopes are neither particularly accurate or ridgid so a heavy camera would run the risk of sagging the focuser assembly.

Apart from that they are a viable option IMO.

[Prickly]

Hi John,

Ive seen some good shots through some 120mm refractors with minus violet filters. A couple of cheaper refractors though stars were sharp in one spot and not others.

I have a vixen 140 neoachromat as they have a pretty flat field with sharp star images across. I have posted a quick snap of eta-carina previously with a Baader UHCS filter. This seems to eliminate pretty much all fringing for me. You can reduce the green channel a bit to come up with a fairly pleasing image.

Recently I've obtained a semi-apo filter which is very good for visual - lots of light throughput and dramatic reduction (but there is not complete elimination) of the purple finging.

I think the GSO 8f4 would be a great option too with a coma corrector. I got the neoachromat to experiment with for a bit for fun. It is a good telescope for deep sky for its aperture but still trying to sharpen it up a bit on planets. I was considering the GSO (arguably a better option).

Have you tried a focal reducer with the VC200L - an great unit.

There have been some good (amazing in fact) images through achromats using the HDR method in the ice in space deep sky photography forums.

Cheers
David