Imaging with Achromatic Refractor and Mono Camera

[jeffwjz]

I'm currently using a newtonian and a very nice mono ccd camera, and am quite satisfied with the experience so far. However I'm still thinking of moving to astrophotography with refractors, for better contrast and supposedly more crystal details. But I was again and again put off by the price for high quality Apos, for the fact that even a "small" 80mm ED will cost more than twice the price of my 150mm Newt, let alone the higher end triplets with larger apertures.

Recently I was thinking of the feasibility of using Achromatic refractors for astrophotography. I understand that achromatics are prone to chromatic abberation as colors don't usually focus on one point. However this can be countered by the use of mono cameras, as what I have now. So all I need to do is to refocus for RGB channel and ideally I have have quality monochrome images for each channel at a much lower cost than a Apo (although Lum frames may be affected as it contains information of all channels).

I'm not an experienced refractor user so I'm not sure how plausible this idea is. Has anyone had any similar experience or even had a trial on this? Any suggestions will be helpful.

If it is a doable idea, are there any suggestions for the "good" achromatics for astrophotography on the market?

Thanks in advance.

[Gina]

I've used cheap scopes with narrow band imaging but haven't tried with RGB.  For galaxies I use LRGB and you certainly can't get away with the Luminance channel without an APO.  The SW ST80 is quite a good little scope and cheap.  I've used those in a triple imaging rig for narrow band in the past but the trouble with NB is the high cost of filters, though there is a bigger choice of manufacturers now than when I started.

[ollypenrice]

It won't be as effective as you hope, I'm afraid. As Gina said, the luminance channel will not be parfocal with itself across the spectrum it covers so stars, in particular, will be bloated and soft. Also, your thinking would probably be sound so far as red and green are concerned but blue would almost certainly be a disaster. The short wavelengths of the blue end of the spectrum are the hardest for the opticians to control, the more so as they head towards violet. The bandpass of a blue filter is wide enough to pass a range of wavelengths coming to very significantly different focal points so the blue channel would be soft and bloated, probably with large blue halos.

Again, as Gina said, you might find that your theory would work for narrowband imaging because the NB filters are more nearly monochromatic and so effectively exclude all but parfocal rays. In theory, at least, you might also need to use a program which not only aligned your three channels but which resized two of them to match a third because a major refocus might have a perceptible effect on image scale. Whether it would be significant or not I don't know. I began imaging with a very early TV Genesis corrected for visual use in the pre-CCD era. It gave predictably bloated blue and L stars but was excellent in NB where it was as tight as the Tak FSQ which replaced it. Also Ha/OIII/SII imaging keeps well away from the problem blue end of the spectrum.

Olly

[jeffwjz]

Thanks very much for the insights. It's a very good point - that if I were moving to narrowband imaging, e.g. using Ha/OIII/SII pallet, achromatic refractor could actually be a low budget choice. 

It is a bit disappointing that both luminance and blue channels would be affected - so it looks achromatic is certainly a show stopper for LRGB imaging. I'll probably just focus on finding a apo that suits me. 

[ollypenrice]

On 21/05/2019 at 08:04, jeffwjz said:

Thanks very much for the insights. It's a very good point - that if I were moving to narrowband imaging, e.g. using Ha/OIII/SII pallet, achromatic refractor could actually be a low budget choice. 

It is a bit disappointing that both luminance and blue channels would be affected - so it looks achromatic is certainly a show stopper for LRGB imaging. I'll probably just focus on finding a apo that suits me. 

There is nothing wrong with a well collimated Newt, though... An apo will probably make life easier and lose diffraction spikes but it won't necessarily produce a better image.

Olly

[gorann]

I think Olly is right in saying that there is nothing wrong with a well collimated Newton, and it is likely to outperform an achromat. Have a look at what Jens Zippel produces with his 10" Newton (and a 6" Newton would not be very far behind in the right hands). He gets a Top Pick or Image of the Day recognition for every second image he posts on Astrobin:

https://www.astrobin.com/users/Jedi2014/

I think a good idea would be to stick to your Newton until you saved up for an apo, if you decide you really want a refractor. Buying an achromat now would make it take longer to save up for an apo.

[Ags]

Bach to achro RGB-filtered imaging...

I understand that blue would be an issue because of the violet component, but what about a blue filter stacked with a minus violet filter? Where there is a will there is a way!

As for luminance, ditch the luminance and spend that time gathering more RGB? If you are imaging with a cheap achro, you are clearly prepared to make compromises...

Edited May 22, 2019 by Ags

[ollypenrice]

42 minutes ago, Ags said:

Bach to achro RGB-filtered imaging...

I understand that blue would be an issue because of the violet component, but what about a blue filter stacked with a minus violet filter? Where there is a will there is a way!

As for luminance, ditch the luminance and spend that time gathering more RGB? If you are imaging with a cheap achro, you are clearly prepared to make compromises...

I think you'll end up chasing your tail. I'd just stick with the Newt till a good apo turned up.

Olly

[alacant]

Hi. A cheap wratten #8 takes care of most of the blue without altering the rest of the colour too much. This is with a €100 ar80. Worth a go?

HTH

[rl]

Chromatic aberration is not the only issue with cheap short-focus crown/flint refractors. A lot of them suffer significant amounts of spherical aberration as well, and the field curvature might be a lot less predictable/ correctable. Alacant's suggestion with a Wratten #8 is worth a punt but that's about as far as I'd go. A least from the theoretical point of view, your Newt is a known quantity...any coma corrector to suit the f-ratio should be giving very acceptable results out of the box  assuming the spacing is correct. 

We are taking it as read that you are looking to do deep-sky work here..I don't think it makes it clear in the initial post. A decent long-focus achromat is capable of very reasonable results on planets/ moon with nothing more than a cheap UV/IR filter. 

Are there any other ways you could optimise your results from the newt, at minimal expense? Are you using a Bahtinov mask to focus? Is the CC spacing optimised? Would a cheap LPR filter help? Processing?

If you're determined to go the refractor route, the old blue Skywatcher ED80s often change hands for about £200 secondhand...mine did! They are a very good start, if a bit slow. The focal reducer is a very worthwhile addition. 

I've been doing astrophotography for 40 years and I'm still amazed at my own talent to mess it up completely either by clumsiness or forgetfulness. Given the infrequent good nights, anything that makes the data aquisition simpler and more reliable has to be a good thing. A small ED frac can be a big step in this direction providing it suits your chosen targets (wide field);  easier to mount (half the weight, half the focal length, less than half the wind sail area), don't go out of collimation. They are about the closest thing in astronomy to kit which "just works". It might be worth saving up....optimise the return on those precious hours spent outside. 

RL

[vlaiv]

Idea for using mono / filters with achromat will work to an extent. It is much better to do it on ED doublet with some residual color than on full achromat.

Although you would hope that filters will limit CA problems - it is only true to an extent as even in each of R, G and B bands there is significant shift in focal position. Blue part of spectrum will probably be affected the most.

There are some "crazy" ideas that you can try to image with achromat. For example you can try to get close to ED80 scope for a bit less money. One might wonder why would you do such a thing if it is not saving you considerable amount of money, but let's go with the idea just for argument sake - maybe you have readily available all components to give it a try (I do and plan to test this ).

Take for example 4" F/10 achromat. Stop it down further - let's say to 80mm making it F/12.5.

Is this going to be used for imaging you may ask? I know, crazy, but bare with me ... Above will make CA very controllable - it will not remove it fully, but you'll be able to filter it out.

Next, take focal reducer like CCD47 - which is similar in specs to AP CCDT67 (so similar that one might suspect a copy ). This can be bought rather cheaply online - I've seen it listed for $100 on AliExpress once. It will reduce focal length to about 670mm. This focal reducer works best if field is close to flat - and at F/12.5 it is going to be fairly flat already.

See those specs? 80mm aperture and 670mm FL - almost as ED80 which has 80mm aperture and 600mm FL. If you increase sensor / reducer distance - you'll get even larger reduction factor to get closer to 600mm FL.

Use wratten #8 for luminance to cut most offending part of blue spectrum. Shoot color with regular R, G and B filters. Maybe couple wratten #8 with blue filter as well - but it might not be necessary.

This technique works btw - stopping down achromat and using yellow filter - I've tested it successfully.

Here is a shot with OSC camera (so no filter refocusing here) - done with ST102 stopped down (to 66mm I think) and use of wratten #8 filter.

[Ags]

1 hour ago, vlaiv said:

and at F/12.5 it is going to be fairly flat already.

See those specs? 80mm aperture an

Slow focal ratio does not flatten the field, it simply increases depth of focus and makes field curvature less visible.

[vlaiv]

1 minute ago, Ags said:

Slow focal ratio does not flatten the field, it simply increases depth of focus and makes field curvature less visible.

Quite right - however original F/10 lens is going to have pretty flat field as is? right?

[Ags]

I think the general rule is a refractor has a curvature radius of 1/3 of the focal length. So a 1000mm FL frac has a curvature radius of about 330mm, compared to a 1000mm FL newt, which has a curvature radius of about 1000mm.

[vlaiv]

1 hour ago, Ags said:

I think the general rule is a refractor has a curvature radius of 1/3 of the focal length. So a 1000mm FL frac has a curvature radius of about 330mm, compared to a 1000mm FL newt, which has a curvature radius of about 1000mm.

I think that it also depends on type of glass. I know that fast triplets need field flattening and have rather curved field?