152mm Frac CA but will it be an issue ?

[Ewan]

Hi, as some will know by now the 152mm refractor has arrived, this was mainly purchased for Solar imaging but how will it fair doing NB & RGB imaging ?

What I mean is I guess if I used a OSC ccd & did say a Nebula CA would be evident but as I use a mono 460EX & RGB / NB filters won't CA not be an issue ?

I am more than happy if I just stuck to mono images with this set up as solar is the main purpose but if 'normal' imaging without that blue edge is possible it would be a bonus.

ATB

Ewan

[ronin]

If narrow band and the RGB filters are not overly wide then not a real problem. It is the multi wavelength aspect that is the problem and causes he CA - you cannot tie enough of them down in a narrow depth for it to be undetectable.

The focal length will alter, if Red and Blue are the same then Green will be a bit longer, depends on the characteristics on the lens and what the design criteria were, and on that all important thing - how it turned out.

I would get the RGB filter graphs and see what they pass, without getting to the extreme of narrow band it is a case of the narrower the better. The 600nm at the edge of Red will focus a little different to the 750nm at the near IR edge of Red. Both Red but focal plane a little different.

[Ewan]

I think I follow you Ronin, I just thought as I would be using the usual 3 filter set up CA wouldn't play a part at all as I would be processing each wavelength separately.

[happy-kat]

Member Gina had done some tests using NR with the ST80 was am interesting thread.

[Merlin66]

We also find in some solar telescope applications that although CA can be minimised by the very narrow bandwidths (ie H alpha at 1A) being used, that the residual spherical aberrations (SA) (caused by spherochromatism where the SA varies with wavelength)can cause some loss of definition.

[Ewan]

Time will tell then gents I feel

[ollypenrice]

Narrowband will propbably work pretty well and you can refocus between colour filters to help matters. I suspect the big difficulty will be from hot blue stars which are unlikely to be well controlled. The camera tends to 'see' deeper into the violet than the eye so lenses are corrected accordingly.

Mono and colour is not the issue. For example, although luminance gives a mono result it is, in fact, the most susceptible to imperfect colour correction because it is capturing all colours at once and cannot be focused on each one indvidually.

If you try RGB with refocus between colours it might be best to combine them in a programme which doesn't just align the colour channels but resizes them as well. Registar does this. I don't know whether the improvement would be visible or not though!

Olly

[Merlin66]

Olly,

I accept the concept that narrow band imaging minimises the CA impact, but unfortunately doesn't guarantee that the Quality of the resulting image being formed still doesn't suffer from spherical chromatic aberration.

I honestly hadn't even be aware of the possible issue until recently.......

[Stu]

I used a 150mm f5 Skywatcher with a Quark for a while. Whilst it's a nice scope for deep sky, widefield use, I found that the solar views at higher power were not as sharp or detailed as through my 120ED, despite being narrowband Ha. Rightly or wrongly I put this down to spherical aberration in the optics.

Hopefully with the longer f ratio and perhaps a better figured objective you will have some good results. Looking forward to seeing the first images.

[Ewan]

I used a 150mm f5 Skywatcher with a Quark for a while. Whilst it's a nice scope for deep sky, widefield use, I found that the solar views at higher power were not as sharp or detailed as through my 120ED, despite being narrowband Ha. Rightly or wrongly I put this down to spherical aberration in the optics.

Hopefully with the longer f ratio and perhaps a better figured objective you will have some good results. Looking forward to seeing the first images.

This taken with a 150mm Skywatcher very similar to what i'll be using , http://solarchat.natca.net/download/file.php?id=17875&mode=view  so I am hoping I will get equally good images.

Thanks for all the feedback & informative posts, it will be quite interesting as to what happens here when I do get a chance to try it out.

Olly yes I would always refocus between filters, as you say a resize of the image should work but I will have to watch how much by otherwise I could end up with the edges of stars a very odd colour but still sortable in PS.

I really don't think there will be a problem with this scope in doing either as a little bit of processing can sort most problems.

As I said mono Neb & Solar work will be the name of the game here, all I need now is an Obsy.

[Stu]

Well that doesn't look too shabby Ewan, bet you will be pleased if you can replicate similar results

[John]

I used a 150mm f5 Skywatcher with a Quark for a while. Whilst it's a nice scope for deep sky, widefield use, I found that the solar views at higher power were not as sharp or detailed as through my 120ED, despite being narrowband Ha. Rightly or wrongly I put this down to spherical aberration in the optics.

Hopefully with the longer f ratio and perhaps a better figured objective you will have some good results. Looking forward to seeing the first images.

I hope and expect that Ewan's scope will have a well figured objective lens.

I agree with your views on the short tube achromats Stu. Apart from the CA, which is to be expected, even the F/8's that I've owned had quite a bit of SA.

The F/5 ones are fun for deep sky, widefield though.

I'm looking forward to Ewan's images too

[ollypenrice]

Olly,

I accept the concept that narrow band imaging minimises the CA impact, but unfortunately doesn't guarantee that the Quality of the resulting image being formed still doesn't suffer from spherical chromatic aberration.

I honestly hadn't even be aware of the possible issue until recently.......

Don't get me wrong, I entirely agree with this, 100%.

Olly

[Moonshane]

This perhaps also explains why white light views with my 80ed are more detailed than I used to get with my 80ed f11 vixen and as far as I recall my 120 ST

[Ewan]

Maybe a Semi Apo filter or flattner will be a good addition as well judging by the answers.

This has become more interesting than I had imagined guys, great input so far many thanks.

[Merlin66]

The SA is inherent in the lens design. You would need additional figuring of the lens surfaces (similar to the differences between a spherical mirror and a parabolic mirror).

Adding filters and/ or working in narrowband (similar thing) only isolates the image formed at the transmission wavelength.

I like many others had "assumed" that the quality of the image produced by refocussing to compensate for the chromatic aberration "shift" would give a perfect Airy Disk and high Strehl ratio at every wavelength. Unfortunately this doesn't seem to be the case.....

[Ewan]

The SA is inherent in the lens design. You would need additional figuring of the lens surfaces (similar to the differences between a spherical mirror and a parabolic mirror).

Adding filters and/ or working in narrowband (similar thing) only isolates the image formed at the transmission wavelength.

I like many others had "assumed" that the quality of the image produced by refocussing to compensate for the chromatic aberration "shift" would give a perfect Airy Disk and high Strehl ratio at every wavelength. Unfortunately this doesn't seem to be the case.....

I am not expecting APO like performance from a 'cheaper' doublet, I am not that unrealistic.

I have read that a Semi Apo filter helps to reduce the effect of CA, NOT eliminate it.

At the end of the day the frac was ONLY bought for solar as there are some amazing images captured with this kind of set up & nothing more.

This is the reason why I bought an expensive Baader D-ERF full aperture filter, to give the scope the best chance to deliver the goods. 

The ccd can wait for an upgrade as I am sure the DMK21 will be more than enough to do the job.

Like I said time will tell & the images I do manage to achieve I am sure will be more than adequate for the cost of the scope.

[Merlin66]

Ewan,

Don't misread me.. I use a TS102 achromat doublet with both my Baader Wedge and my PST mod.

It performs very well, and I'm sure you will get the same satisfaction from your 152mm.

The CA and SA are only issues when you want them to become issues.