Imaging with an Achromat

[petevasey]

This might of course have been covered before, in which case my apologies.  But I wonder if anyone has had success long exposure imaging with a 'fast' achromat, which of course will be subject to Chromatic Aberration.  It strikes me that while unfiltered Luminance frames could have bloated stars due to the CA, perhaps the filtered RGB subs will not be badly affected, particularly if individually focused.  And of course narrow band subs should be as good as if taken with a quality apo.

Cheers,

Peter.

[vlaiv]

I played around with fast achromat and some OSC imaging long time ago.

This is probably "worst" kind of imaging with achromat, but it was relatively speaking a success. My technique was to stop down aperture making it slower but also to use wratten #8 filter to block most offending blue light. I used SW Startravel 102 F/5 achromat stopped down to 66mm.

If you are going to attempt such imaging, then I suggest you try luminance with #8 wratten or Baader long pass 495 filter. Long pass is a bit more expensive but it has better transmission curve - more than 90%. #8 is absorption filter and has transmission probably around 70-80%.

You are right that separating R, G and B is going to improve performance, but I think that while G and R might be ok on their own, B is probably not going to be good enough without additional filtering or stopping down aperture (second option might give you a bit larger stars anyway).

To see why this is the case, take a look at this diagram:

This is pretty much how achromats are optimized. If we roughly divide RGB ranges like 400-500, 500-600 and 600-700 you can see that green has very small total focal shift - in case of this diagram about 0.01 (of some units or others). Red will have quite a bit more total shift - about 0.05 (of same units), but blue is going to be rather problematic as it will have total of more than 0.25 units of shift. My feeling is that this is going to be way much than critical focus zone - so star bloat.

I'm planning to do some more achromat test imaging but using different approach this time. I swapped ST102 for Evostar 102 F/10 achro in the mean time, and since long focal length instrument should have much less CA, I'm planning to use it with focal reducer designed for slow scopes (with minimum field curvature - at F/10 I guess there should be very small FC). I'm going to use this one:

https://www.teleskop-express.de/shop/product_info.php/info/p8932_TS-Optics-Optics-2--CCD-Reducer-0-67x-for-RC---flatfield-telescopes-ab-F-8.html

I'll probably stop Evostar to 80mm to further reduce CA. Since that reducer is x0.67 but can be pushed to further with appropriate spacing, I wonder for LRGB imaging if such combination will be able to compare with ED80 - very popular choice for starting in AP, but a bit more expensive as both setups will be 80mm and around 600mm FL. I'll also test this with OSC camera and various filters - like Baader Contrast Booster and Wratten #8 (with and without mask).

And to address last part of your post - yes, best results will certainly be with narrowband filters, but don't expect results as good as high quality apo or ed doublet, just because there will be no chromatic aberration. There is more to image quality than control of CA. While doing tests with ST102, I noticed that there was quite a bit of astigmatism in red for that lens - so optical figure will also play part in how good images you get with narrowband, and let's face it, mass produced fast achromats don't necessarily have good optical figures nor spherical correction across the spectrum.

 

 

[petevasey]

Many thanks - the shift curve is particularly informative.  To explain, I do in fact have some apos as you will see from my footnote.  But I keep getting tempted by low price fast 6" achromats such as the f5 Skywatcher Startravel 150 to have as a Star Party/ outreach telescope.  Easy setup and handling, not  expensive so suitable for outreach - I wouldn't be happy letting strangers handle a 6" apo!  Plenty light grasp for many targets and a decently wide field.  Indeed there are good reports of crisp clean views without CA of fainter objects, including clusters.  But if an imaging opportunity arose....

A little bit more money (and a longer focal length) does anyone have any experience with the Altair Astro Starwave 152 V3 "Red Tube"?

Of course I'd have to get it past the lady of the house ?

Cheers,

Peter

 

[SkyGibbon]

Narrowband would be a suggestion. I have shot many targets with an ST-80 that way from a red/white zone.

[Hallingskies]

I have used a Vixen ED114 for imaging for a long time, certainly before the current crop of good quality reasonably priced apos came along.   The lens figure is first class and shows no sign of any geometric aberrations, at least, not across an APS-sized chip.  For RGB imaging, I have to resort to image processing to shrink the noticeably bloated stars in the blue stack before colour combination (although I quite like blue halos from an aesthetic perspective).  I don't see any problems with the green or red stacks, stars are sharp, as are those in narrowband stacks.  Olll images are always a bit ropey in my hands anyway, I think that’s as much down to the filters as the scope(s) I have used.

I think you could get very good imaging results with the scope you mentioned, but you would probably have to resort to a fair bit of processing along the way if you were after colour...

[parallaxerr]

As previously suggested, narrowband is you best shot with an achro although RGB has been done successfully too, you just have different image scales due to the re-focusing between filters.

Vlaiv make a very valid point about lens figure, though my attempts with a 120mm F5 achro came out OK. This is 6hrs of 600s subs in Ha...

[ollypenrice]

Hi Peter. I think there are other issues with the Startravels as well. I don't want to knock them because they have their place and I use two of them here for guiding. They are good for outreach but the other issues are the primitive focusers, which are prone to sag, and the very small corrected circle. Off axis the curvature is pretty extreme. I solve the sag by locking everything down brutally and Aralditing the extenders together! In the past people have posted colour images from them but I know you and I know your standards. I don't think you'd like them. On the other hand I think I remember Gina finding them pretty effective in NB for the reasons you suggest and, in fact, I remember your making this point once in conversation.

Star bloat in Luminance is pretty extreme and there is considerable focus shift between colours so using Registar to co-register them might well give a better result than a regular 'x-y and rotate' routine. Registar, of course, will resize as well.

Wouldn't a small reflector be more suited to your needs? The was a great thread on here devoted to imaging with the 130PDS. Some results were truly remarkable.

Cheers,

Olly

[petevasey]

Thanks for the heads up, Olly.

Yeah, maybe I'm being a bit too hopeful about an achromat.  It's just every time I see a 6", that expanse of glass grabs me!  Part of it is the frustration of our lousy weather - one of the worst seasons yet, so far anyway, and a new toy would be nice.  I remember being impressed by a 6" f5 some years ago, but my eyesight has deteriorated since then, so I guess there's some wishful thinking involved.  At my age I probably should be selling off gear, not lusting after some shiny new plaything.   (Yes, I'm talking about telescopes ? )

To be fair, my TMB 105 has superb optics and focuser, and my EQ3-2 GOTO mount carries it ok for visual, so a decent outreach setup.

Cheers,

Peter