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Achromat and Mono imaging


nightster

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I have been reading the thread Olly posted with his diagram about Aperture and FL and reducers and such. Pretty much seals it for me that aperture wins most battles. But as we all know well corrected triples are worth their weight in fluorite.  Obviously big Achros are cheap by comparison but with purple fringes and such being an unacceptable consequence.  But that started me thinking about mono LRGB and narrowband filters.  It's my understanding that you get the color errors from the various frequencies not focusing at the same point.

I'm wondering if adding a stepper motor focuser and good filters could a scope like an ES AR152 be made to get good images? Or would those Achro artifacts find their way into the subs?   

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Firstly let's remind ourselves that both luminance and narrowband are 'mono' in the sense that they generate a greyscale image, but only NB isolates (roughly) monochromatic light. Even though a luminance filter generates a monochrome image it is a polychromatic filter and is badly affected by achromatic optics. Luminance will, therefore, never be any good in an achro. It is red and green and blue at the same time which is precisely what an achro cannot co-focus.

If you image in an achromat through RGB separately you will, as Michael says, get different focal points (no big deal because refocusing is routine at regular intervals) and also a different image scale. The latter would be a problem if you didn't register your R and G and B in a sophisticated programme but one which only tried to place each set of stars in the same place without resizing. This cannot work, but Registar and, I believe, Pixinsight will rescale while co-registering so that problem is not insurmountable.

The remaining problem, alas, is more severe. A broadband colour filter still passes a wide range of wavelengths and if they don't focus at the same point you don't have a sharp image. In blue, particularly, this will get you. Short wavelength blue light is very hard to control and even top end apochromats can struggle to bring the whole of the 'blue' spectrum to the same point. An achro propbably won't get near to doing so.

Conclusion: if you want to use an achro for three channel imaging it might well work for three narrowband filters because the bandpass of each is nearly monochromatic and the achro will focus the bandpass well enough. Registar or Pixisnsight will resize them well enough for a successful three channel image. But even within one colour filter, and especially blue, an achro is not sufficiently achromatic even within a single colour bandpass.

Such is my belief. The usual disclaimers apply!

Olly

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This is actually quite promising for the type of imaging that I'm interested in.  Pure Ha, Tri and Bi colors are what I'm striving for these days. I frequently go down the rabbit hole with Sara's gallery of images, just mesmerizing.  

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Its definitely doable for narrowband but the main problems I've seen with achros are (with the exception of a few)they arent well suited and by the time you've upgraded focusers and bits and bobs you could probably have grabbed a second hand apo.

That said im a sucker for going against the grain and using achros\lenses.

 

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12 hours ago, nightster said:

In that manner could my 80mm triple be used for L and B? Quite a different FL though, there would have to be a significant crop.

Yes. I often overlay long focal length data onto shorter FL widefield to enhance resolution in key areas. Registar does this in a trice.

Olly

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2 minutes ago, ollypenrice said:

Yes. I often overlay long focal length data onto shorter FL widefield to enhance resolution in key areas. Registar does this in a trice.

Olly

When you do this Olly, do you just need a 'Luminance' for the detail (the colour is provided by the shorter F/L data?).  The example I am thinking of is adding some luminance 'detail' of the Crescent Nebula into a wider field SADR mosaic.  

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12 minutes ago, gnomus said:

When you do this Olly, do you just need a 'Luminance' for the detail (the colour is provided by the shorter F/L data?).  The example I am thinking of is adding some luminance 'detail' of the Crescent Nebula into a wider field SADR mosaic.  

You certainly can just use L to enhance the L layer of the widefield. You need to extract the L from the wide image, co-register the high res to it, use the Crop and Pad function for ease and then take the Registar-prepared high res back into Photoshop. You can't just drop it onto the widefield in Registar because it will look very artificial. In Photoshop you can then paste the cropped and padded high res onto the widefield, erase the black padding and use a feathered eraser to blend the high res on gently round the edges. Full opacity may be too much even so.

I say you 'can' do it this way but generally I find I'm applying a full colour high res to a wide field just because I have both.

A little artistry is required when combining different focal lengths but it's enjoyable to do and very satisfying when you get it right.

Olly

Edit: this Sadr Crescent had a little help from the TEC140 as you discuss. A hawk-eyed French imager noted the O111 presence in the Crescent which is absent from the rest of the image. Touché!!

HARGB%20P5SCNR%20sRGB%20SADR%20CILOUR-X3

 

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5 hours ago, ollypenrice said:

You certainly can just use L to enhance the L layer of the widefield. You need to extract the L from the wide image, co-register the high res to it, use the Crop and Pad function for ease and then take the Registar-prepared high res back into Photoshop. You can't just drop it onto the widefield in Registar because it will look very artificial. In Photoshop you can then paste the cropped and padded high res onto the widefield, erase the black padding and use a feathered eraser to blend the high res on gently round the edges. Full opacity may be too much even so.

I say you 'can' do it this way but generally I find I'm applying a full colour high res to a wide field just because I have both.

A little artistry is required when combining different focal lengths but it's enjoyable to do and very satisfying when you get it right.

Olly

Edit: this Sadr Crescent had a little help from the TEC140 as you discuss. A hawk-eyed French imager noted the O111 presence in the Crescent which is absent from the rest of the image. Touché!!

HARGB%20P5SCNR%20sRGB%20SADR%20CILOUR-X3

 

Stunning image, Olly.

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