Is a top notch APO really needed if using mono camera?

[AndrewRrrrrr]

I was wondering about this......

You buy a fab APO so you don't get colour disortion (blue fringes etc) when doing visual. And this is because the different colours have slightly different focus points. i.e. when the red is in focus, the blue is slightly out of focus.

(this maybe completely wrong in which case the rest of this is misled)

So if I'm using a mono camera with automated focussing, can I get a not-so-amazing scope (maybe trade some more aperture for less colour correction) and as long as I set the image aquistion software to focus on change of filter, I should be all good?

(thinking I could sell my AA Wave80 Super ED triplet and get a AA 125 EDF as these seem to go for the same (ish)  and be none-the-worse off?

Any thoughts?

Andrew

[symmetal]

Yes, your thinking is correct as far as RGB and especially narrowband filters are concerned. The only issue is if you also want to take luminance frames where the APO scope will give sharper results.

Alan

[carastro]

I have always used doublets, apart from a few months when I bought a triplet.  To be honest I didn't see any difference, but then I do image from LP skies and do a lot of narrowband.  Perhaps the Triplet wins on broadband.  Anyway, since I found no difference and it was very heavy and didn't allow for dual rig on my mount, I re-sold it.

You may or may not like the images I produce, but I think my website is in my signature.

Carole  

Edited August 4, 2023 by carastro

[AndrewRrrrrr]

5 hours ago, carastro said:

I have always used doublets, apart from a few months when I bought a triplet.  To be honest I didn't see any difference, but then I do image from LP skies and do a lot of narrowband.  Perhaps the Triplet wins on broadband.  Anyway, since I found no difference and it was very heavy and didn't allow for dual rig on my mount, I re-sold it.

You may or may not like the images I produce, but I think my website is in my signature.

Carole  

your images are a tour-de-force! maybe I will go doublet.......

[AndrewRrrrrr]

5 hours ago, symmetal said:

Yes, your thinking is correct as far as RGB and especially narrowband filters are concerned. The only issue is if you also want to take luminance frames where the APO scope will give sharper results.

Alan

Yes Luminance frames. Are they really that necessary? You say "if" you want to take them, I've sort of assumed that you had to, but maybe not? Have to admit occaisonally I've captured LRGBHS and O for an object and the combined image is better without the L. 

 

[symmetal]

3 hours ago, AndrewRrrrrr said:

Yes Luminance frames. Are they really that necessary? You say "if" you want to take them, I've sort of assumed that you had to, but maybe not? Have to admit occaisonally I've captured LRGBHS and O for an object and the combined image is better without the L. 

If you spend for example, 4 hours in total capturing your target, you can either spend 1 hour each of LRGB of 1 hr 20 min each of RGB. The result with luminance will have a better overall S/N as luminance gathers photons around 3 x quicker than the individual colour filters do.

However, if your scope isn't well colour corrected adding luminance will likely give a slightly softer result and also with more bloated stars, which is what you might have experienced. If you're happy to spend the longer total imaging time with just RGB to achieve a similar S/N as you would get if you captured luminance as well, then RGB alone will give you a better result.

Many people use OSC cameras nowadays with excellent results and they have no luminance added of course, so it's not really necessary nowadays as you say. Before CMOS came along with more efficient, and lower read noise sensors, adding luminance would certainly have been the better choice. 🙂

If using a OSC camera though, an APO is preferable, as you can't refocus for the different colours.

Alan

Edited August 5, 2023 by symmetal

[Adam J]

9 hours ago, AndrewRrrrrr said:

I was wondering about this......

You buy a fab APO so you don't get colour disortion (blue fringes etc) when doing visual. And this is because the different colours have slightly different focus points. i.e. when the red is in focus, the blue is slightly out of focus.

(this maybe completely wrong in which case the rest of this is misled)

So if I'm using a mono camera with automated focussing, can I get a not-so-amazing scope (maybe trade some more aperture for less colour correction) and as long as I set the image aquistion software to focus on change of filter, I should be all good?

(thinking I could sell my AA Wave80 Super ED triplet and get a AA 125 EDF as these seem to go for the same (ish)  and be none-the-worse off?

Any thoughts?

Andrew

It doesn't quite work as simply as that, it's not just about the focal point being different it's about spherical error changing with wavelength. Hence although you can focus for RGB at best focus B and sometimes R will not be as sharp as G and that leads to blue bloat or purple halo irrespective of you refocusing. Longitudinal chromatic error and spherochromatism are traded against each other in all lens designs so if you want to bring all colours to focus at the same point you normal end up having to trade against a reduction in focus quality for some wavelengths normally blue. Also as others point out you can't do this for Lum anyway. To avoid this design compromise you need a triplet design slower than F6, a quad design will do better and doublet worse. 

In most cases.modern processing will fix things for you but for galaxy imaging. your going to inevitably loose some resolution. 

Adam

Edited August 5, 2023 by Adam J

[Louis D]

1 hour ago, Adam J said:

It doesn't quite work as simply as that, it's not just about the focal point being different it's about spherical error changing with wavelength. Hence although you can focus for RGB at best focus B and sometimes R will not be as sharp as G and that leads to blue bloat or purple halo irrespective of you refocusing. Longitudinal chromatic error and spherochromatism are traded against each other in all lens designs so if you want to bring all colours to focus at the same point you normal end up having to trade against a reduction in focus quality for some wavelengths normally blue. Also as others point out you can't do this for Lum anyway. To avoid this design compromise you need a triplet design slower than F6, a quad design will do better and doublet worse. 

In most cases.modern processing will fix things for you but for galaxy imaging. your going to inevitably loose some resolution. 

Adam

I can verify that when using my Meade RGB interference filters with my ST80 visually, I could get a sharp image in green, a reasonably sharp image in red with a bit of refocusing, and a somewhat fuzzy image in blue no matter how much refocusing I tried.  I wrote it off to violet focusing too far from medium blue to bring them both into focus at the same time.  However, spherical aberration in blue might have been what I was seeing as well.  Regardless, I can't imagine a camera not seeing this same blue-violet fuzziness.

[Coco]

@carastro images are superb with a Skywatcher ED 80 or 72, processing experience wins... 

[900SL]

The doublet may also change the image scale slightly for each filter, if there is significant CA. This can be addressed in registration though.

[tomato]

What about the OP’s suggestion of trading in the APO for a bigger doublet refractor? Would the increase in aperture trump some of the other issues wrt to detail resolution?

[Clarkey]

Assuming the same rules apply as for an achromat, the amount of CA will increase with aperture. Therefor the 125 will show relatively more CA than the 80mm scope. However, good doublets still give good results if well corrected. There is also the option of using something like an Astronomik L3 to reduce any bloating.

Another option would be the AA 115 Starwave. Slightly less aperture but an FPL-51 triplet with good correction.

[JuergenB]

I once owned a WO FPL-53 doublet - and yes, it showed blue colour fringes around brighter stars in LRGB images. It depends a lot on the L and B filter transmission on the blue side of the spectrum.

Nowadays, you can select beween different types of L filters, for example Astronomik offers three different ones, L1, L2 and L3. L3 opens the transmission window at higher wavelengths (>420 nm) than L2 or L1, which would be advantageous for a less than perfect ED lens.

Juergen

 

Edited August 5, 2023 by JuergenB
formatting

[Louis D]

2 hours ago, JuergenB said:

I once owned a WO FPL-53 doublet - and yes, it showed blue colour fringes around brighter stars in LRGB images. It depends a lot on the L and B filter transmission on the blue side of the spectrum.

Nowadays, you can select beween different types of L filters, for example Astronomik offers three different ones, L1, L2 and L3. L3 opens the transmission window at higher wavelengths (>420 nm) than L2 or L1, which would be advantageous for a less than perfect ED lens.

Juergen

 

If imaging with a fast achromatic doublet like an ST80 or KUO 152, I would think you'd want an L cutoff closer to 470nm like the Yellow #12A to eliminate enough violet blurring to be useful for luminance.  This would still leave the Hβ visible.

[vlaiv]

5 minutes ago, Louis D said:

If imaging with a fast achromatic doublet like an ST80 or KUO 152, I would think you'd want an L cutoff closer to 470nm like the Yellow #12A to eliminate enough violet blurring to be useful for luminance.  This would still leave the Hβ visible.

I still would not image with such scope and yellow filter without aperture mask.

Residual chromatic aberration is still present with fast scopes, even with yellow filter - but there is also issue of spherochromatism.

Only one wavelength will be free of spherical aberration - others will have more or less of it and with fast achromats - it is usually more

Take a look at spherical assessment of ST102 F/5 achromat:

Green is almost free of spherical - but other two colors suffer from under / over correction

In the end - it is a tradeoff - one can image even with fast achromatic scope - if one accepts to:

1. use filter to remove part of spectrum thus loosing some of the light

2. use aperture mask to reduce aberrations further - and again loosing some of the light

3. reduce sampling rate so that residual blur does not impact image sharpness as much

[Louis D]

Would any of that outweigh simply using a fast Newtonian with coma corrector instead of a fast achromat just to avoid a central obstruction?  I can't imagine ever getting better results regardless of achromat filtering.

[vlaiv]

1 minute ago, Louis D said:

Would any of that outweigh simply using a fast Newtonian with coma corrector instead of a fast achromat just to avoid a central obstruction?  I can't imagine ever getting better results regardless of achromat filtering.

Problem with newtonians is size and issues that come from speed.

You'll be hard pressed to find imaging newtonian in sub 500mm focal length, while that is quite easy thing to do with refractors.

As soon as you try to do something like 4" newtonian for imaging - you get crazy large central obstruction in terms of percentage - and you still have to couple that with 1.25" focuser instead of 2" one - because of draw tube size and issues with protrusion into light path as well as again - field illumination and size of secondary mirror.

5" is minimum usable and no wonder 130PDS is such a popular imaging scope. It is still over 600mm in FL (and does have issues with focuser draw tube).

Speed brings in whole set of additional issues - like need for multi element coma correctors that often don't have enough back focus for all the things one would like to use (say filter wheel and oag) and sensitivity to tilt.

 

[carastro]

Agree with Vlaiv, an Acromat won't cut it, horrible chromatic aberration.  Needs to be an Apo. 

Edited August 5, 2023 by carastro

[ollypenrice]

16 hours ago, Adam J said:

It doesn't quite work as simply as that, it's not just about the focal point being different it's about spherical error changing with wavelength. Hence although you can focus for RGB at best focus B and sometimes R will not be as sharp as G and that leads to blue bloat or purple halo irrespective of you refocusing. Longitudinal chromatic error and spherochromatism are traded against each other in all lens designs so if you want to bring all colours to focus at the same point you normal end up having to trade against a reduction in focus quality for some wavelengths normally blue. Also as others point out you can't do this for Lum anyway. To avoid this design compromise you need a triplet design slower than F6, a quad design will do better and doublet worse. 

In most cases.modern processing will fix things for you but for galaxy imaging. your going to inevitably loose some resolution. 

Adam

I agree. You won't fix blue bloat with a bit of refocusing.  You can attack it in cosmetic post processing but it's better not to have to.

Olly

[GTom]

Besides Luminance frames, an APO with better BLUE that is OK for the old wider "ccd" B filters, could effectively add 10+% to your B channel compared to a lesser petforming one tgat needs to be filered from 420-450nm instead of 400.

Edited August 5, 2023 by GTom

[Louis D]

1 hour ago, vlaiv said:

5" is minimum usable and no wonder 130PDS is such a popular imaging scope. It is still over 600mm in FL (and does have issues with focuser draw tube).

I was thinking in terms of the GSO 6" and 8" imaging Newtonians available at various f-ratios versus the 152mm KUO achromat or even larger achromats (anyone know of any readily available?).  Weight-wise and length wise, I find the 6" f/5 GSO Newtonian much easier to mount than the 152mm f/5.9 KUO achromat.  Perhaps things change once you try to include a filter wheel and other imaging gear?

Also, I could see if you simply want to image wide fields, the refractor route handily wins out.

[Adam J]

5 hours ago, vlaiv said:

I still would not image with such scope and yellow filter without aperture mask.

Residual chromatic aberration is still present with fast scopes, even with yellow filter - but there is also issue of spherochromatism.

Only one wavelength will be free of spherical aberration - others will have more or less of it and with fast achromats - it is usually more

Take a look at spherical assessment of ST102 F/5 achromat:

Green is almost free of spherical - but other two colors suffer from under / over correction

In the end - it is a tradeoff - one can image even with fast achromatic scope - if one accepts to:

1. use filter to remove part of spectrum thus loosing some of the light

2. use aperture mask to reduce aberrations further - and again loosing some of the light

3. reduce sampling rate so that residual blur does not impact image sharpness as much

Got to be honest that is not its only issue, collimation is off and it has a touch of astigmatism too. Still might be hard to see hidden under the CA. 

Adam

[ONIKKINEN]

5 hours ago, Louis D said:

I was thinking in terms of the GSO 6" and 8" imaging Newtonians available at various f-ratios versus the 152mm KUO achromat or even larger achromats (anyone know of any readily available?).  Weight-wise and length wise, I find the 6" f/5 GSO Newtonian much easier to mount than the 152mm f/5.9 KUO achromat.  Perhaps things change once you try to include a filter wheel and other imaging gear?

Also, I could see if you simply want to image wide fields, the refractor route handily wins out.

There is also the new Quattro 150 from Skywatcher, and this also comes with a reducer that shortens the focal length to 518mm.

Still doesn't compete with the widest fracs, but its getting there in terms of what sorts of targets fit in the FOV.

But from what i have read, and seen in YouTube videos of this scope it is similar to the rest of the cheap Skywatchers so ok but not fantastic for imaging and likely requires some work to get going.

[GTom]

On 06/08/2023 at 01:39, ONIKKINEN said:

There is also the new Quattro 150 from Skywatcher, and this also comes with a reducer that shortens the focal length to 518mm.

Still doesn't compete with the widest fracs, but its getting there in terms of what sorts of targets fit in the FOV.

But from what i have read, and seen in YouTube videos of this scope it is similar to the rest of the cheap Skywatchers so ok but not fantastic for imaging and likely requires some work to get going.

Paired with a 533mm, maybe even 294mm camera, that should be a great scope. Would not go much larger with sensors though.

[Clarkey]

3 hours ago, GTom said:

Would not go much larger with sensors though.

Interesting that it quotes 'suitable for APS-C' but also says a 21.7mm image circle. Slight contradiction in my book. But I agree that larger sensors will not be good. I actually considered one of these for imaging but was put off by the small imaging circle (to pair with an IMX571). I do have a 150mm TS Photon F4 which I pair up with a 1600MM pro. I would certainly advise against this scope as it needs too many modifications to be acceptable for imaging - not least a new focuser.