Doublet for mono imaging?

[MartinFransson]

Since I´m dreaming of going mono later this year (ASI 1600 or maybe a QHY 163 or similar), I was wondering about the scope requirements. For colour imaging a triplet is necessary to get away from CA, but what about with mono? Do I get away from the colour issue when refocusing between filters?

Might it even be a better choice to get for example a 5-600 Euro doublet than a 8-900 Euro triplet?  If so, any pointers or tips?

[geordie85]

1 hour ago, MartinFransson said:

For colour imaging a triplet is necessary to get away from CA, but what about with mono? Do I get away from the colour issue when refocusing between filters?

I do believe this is true

[GuLinux]

Well, unless you go for pure RGB (which for deep sky is not really recommended), with the L channel you'll still gonna experience CA...

If it's an ED doublet, though, might be still quite acceptable

[fireballxl5]

In my experience doing RGB imaging with an 'ED' doublet, colour was always difficult owing to bloated stars in the blue channel. I think that this results from the objective not being able to bring all light contained within the bandpass of the blue filter to the same point, even with careful focusing for each filter. In the end I bought a quad?

[HunterHarling]

Astro teck 65mm quadruplet looks good.

[ronin]

By "doublet" I assume that you still mead an ED doublet not an achromatic doublet, equally they are the same just one has some fancy glass in it.

The achro creates colour by bringing the wavelengths to different focal planes and this will still exist as your R, G and B will have an image formed at different positions on the optical axis, meaning that when you rotate to another filter then the focal plane alters and so the camera need to be repositioned or the focus changed. Not sure if an auto focuser will take care of this aspect.

The question is then what is the change in focal plane caused by an ED doublet.for the wavelengths that you are working in and for that you need probably more detail on the lens then may be available or supplied. Ultimately it should be small but should also be greater then taht from a triplet.

[MartinFransson]

Yes, I was thinking of an ED doublet. Didn't think of the lum, that might be a problem. For narrowband it might work but not for LRGB. 

My thought was that for example maybe a Skywatcher Evostar 80 ED could be a wiser choice than an Esprit 80 triplet considering economy as well as quality.

[Adam J]

16 minutes ago, MartinFransson said:

Yes, I was thinking of an ED doublet. Didn't think of the lum, that might be a problem. For narrowband it might work but not for LRGB. 

My thought was that for example maybe a Skywatcher Evostar 80 ED could be a wiser choice than an Esprit 80 triplet considering economy as well as quality.

I have seen someone image H-a with a ST80 + upgraded 2 inch focuser and get a good result, so if it is just for narrowband then I would say you are onto a winner. I have often considered purchasing some cheap scopes for a duel imaging rig and then just keeping the H-a and OIII on the same scope and never removing them. 

[ollypenrice]

Terminology is important here. We can present a monochrome image which was taken in broadband/polychromatic light or in (almost) monochromatic light. From the optical point of view what matters is whether the light being passed by the filter is broadband/polychromatic or nearly monochromatic.

The most truly polychromatic is luminance which, while resulting in a greyscale image, is fully polychromatic since it passes all colours. This does need excellent colour correction.

Blue, green and red are themselves broadband filters and will benefit from good colour correction, but at least the imager can refocus between them in the case of imperfect colour correction. Blue is the most likely to be a problem because the filter may pass short blue wavelengths which the optics cannot correct, causing inevitable bloat.

Narrowband filters pass light which is almost monochromatic and so colour correction is not all that important. (Very narrow bandpass solar scopes don't even need a doublet lens. A singlet is fine.)

There are some very good ED or flourite doublets out there. Don't discount them. There are also lots of good small imaging refractors of various kinds. The problem is often one of QC, so choice of dealer may matter more than choice of scope. FLO have a good returns policy and a pre-check service as well.

Olly

[Uranium235]

A note of caution, yes you may save some money using an achro or cheaper doublet (non-FPL53 glass) - but the field may be far from flat so it will limit the available useable field.

However, the humble 80ED doublet (FPL53 glass) does perform well in both NB and BB photography, and gives a fairly flat image when using the corrector.

Another question though... if you are using an ASI1600, then why not use the Canon lens in your sig? That camera and lens should give you quite a useable resolution.

[alexbb]

4 hours ago, Uranium235 said:

Another question though... if you are using an ASI1600, then why not use the Canon lens in your sig? That camera and lens should give you quite a useable resolution.

Indeed. You can attach them together and they have the same holder ring. With some effort you can align them well on a vixen dovetail (you might need to drill some holes), but make sure that you convince the lens and the camera to hold on each other well, as the ZWO Canon EF - T2 adapter is c...

Taken with ASI 1600 MMC and Canon 300 F4 L

Or astrobin for full resolution: http://www.astrobin.com/294290/B/?nc=user

Edit:

[MartinFransson]

9 hours ago, Uranium235 said:

A note of caution, yes you may save some money using an achro or cheaper doublet (non-FPL53 glass) - but the field may be far from flat so it will limit the available useable field.

However, the humble 80ED doublet (FPL53 glass) does perform well in both NB and BB photography, and gives a fairly flat image when using the corrector.

Another question though... if you are using an ASI1600, then why not use the Canon lens in your sig? That camera and lens should give you quite a useable resolution.

Yes, I would absolutely use my Canon lenses. I would however like a dedicated refractor as well since I often use my lenses for other purposes and since I sometimes get weird star shapes with the 300/4L. Mostly when shooting at a steep angle. Might be some gravitational thing with the lens packages inside? That image stabilization must add a few extra pieces of glass that could cause effects...

[MartinFransson]

8 hours ago, moise212 said:

Indeed. You can attach them together and they have the same holder ring. With some effort you can align them well on a vixen dovetail (you might need to drill some holes), but make sure that you convince the lens and the camera to hold on each other well, as the ZWO Canon EF - T2 adapter is c...

Taken with ASI 1600 MMC and Canon 300 F4 L

Or astrobin for full resolution: http://www.astrobin.com/294290/B/?nc=user

Edit:

That is a really neat setup and that Elephants Trunk is spectacular! How do you like the ASI 1600? Satisfied? Considering the massive amount of subs I would expect the result to be virtually noise free but maybe you´ve been careful about heavy noise reduction? Don´t get me wrong, I think it´s really great as it is - just thought it would be even smoother.

[alexbb]

I can compare the ASI 1600 only with unmodded DSLRs and I think it's an unfair comparison. I couldn't get even close with the DSLR compared to what I can get with this camera on the faint emission targets. On the brighter ones, a DSLR gets closer, but the mono camera wins all rounds in the end. Cooling gets me even now to -10C when there are 25-30C in my balcony.

As for the elephant's trunk processing, I didn't do much noise reduction. I did some, but I applied back the no-noise reduced image with a low percentage to all the layers as it looked too artificial for my tastes. I also put the O3 and the S2 in the final luminance layer and that's another reason why it looks noisier. Definitely on the colours it's due to the lower amount of signal in the O3 and S2 which I stretched very much. Also, definitely I can make it look smother, but I have to restart the processing from the beginning and I was/am happy with the result. At 16 mp, it takes a lot and I'm lucky enough that usually I can gather more data than I can process.

I do get weird star shapes too with this combination, last night being the worst, but make sure that you secure the camera to the lens as tight as possible. without tilt and it's going to be better.

More on topic, with many doublets with slower F ratio, it will take much longer to get a similar bright image. The 300mm lens has 75mm of aperture, the SW 80 ED has 80, but the Canon lens has 300mm fl, while the reduced/corrected 80 ED has 510mm. Binning with the ASI is only software and the benefits are not that high in my opinion.

The 80 ED is a very good scope for the money, many people get very nice results with them and they hold their value well. Did you consider getting a second hand one? You can sell it without much loss if you want to upgrade to an Esprit maybe.

Did you try shooting with the 150 PDS I see in your signature?

At the end, if you still want to try your Canon - ASI combination, make sure you tight them together firmly and without tilt. Did I mention this earlier?

HTH,

Alex

[MartinFransson]

8 hours ago, moise212 said:

I can compare the ASI 1600 only with unmodded DSLRs and I think it's an unfair comparison. I couldn't get even close with the DSLR compared to what I can get with this camera on the faint emission targets. On the brighter ones, a DSLR gets closer, but the mono camera wins all rounds in the end. Cooling gets me even now to -10C when there are 25-30C in my balcony.

As for the elephant's trunk processing, I didn't do much noise reduction. I did some, but I applied back the no-noise reduced image with a low percentage to all the layers as it looked too artificial for my tastes. I also put the O3 and the S2 in the final luminance layer and that's another reason why it looks noisier. Definitely on the colours it's due to the lower amount of signal in the O3 and S2 which I stretched very much. Also, definitely I can make it look smother, but I have to restart the processing from the beginning and I was/am happy with the result. At 16 mp, it takes a lot and I'm lucky enough that usually I can gather more data than I can process.

I do get weird star shapes too with this combination, last night being the worst, but make sure that you secure the camera to the lens as tight as possible. without tilt and it's going to be better.

More on topic, with many doublets with slower F ratio, it will take much longer to get a similar bright image. The 300mm lens has 75mm of aperture, the SW 80 ED has 80, but the Canon lens has 300mm fl, while the reduced/corrected 80 ED has 510mm. Binning with the ASI is only software and the benefits are not that high in my opinion.

The 80 ED is a very good scope for the money, many people get very nice results with them and they hold their value well. Did you consider getting a second hand one? You can sell it without much loss if you want to upgrade to an Esprit maybe.

Did you try shooting with the 150 PDS I see in your signature?

At the end, if you still want to try your Canon - ASI combination, make sure you tight them together firmly and without tilt. Did I mention this earlier?

HTH,

Alex

Thanks Alex! Regarding the processing of your Elephants Trunk, that's what I suspected and it's probably how I would do it too since it looks great! I preferenser some noise to give it less of an artificial look.

Regarding the scope it was more of a wondering for the future. For now I would use the Canon and absolutely the 150-PDS. It's a great scope for the money but not as easy as a lens (or refractor I imagine). Wind and dew being the two biggest issues. Other than that I love it! No colour issues there

First steg will be to raise money for a camera, then to decide on ASI 1600 vs QHY 163M or similar. A while ago I had my mind set to a KAF-8300 chipset but they are too expensive and I'm not sure it would be that much better for me.