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Apochromat vs Achromat for narrowband imaging.


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Hey all,

I'm new to this forum and currently on the brink of breaking into the hobby again after almost 15 years of break.

I used to take photographs on 35mm film with a 15" f/4.6 Lomo Newton on a 200pound (excluding scope, counterweights or tripod) beast of GEM (ALT-7 ) - with all the fun of manual off axis guiding in -15 degrees under Bartel 3/4 skies.
Now I live in London, don't have the strength to lift 200 pounds any more and never dreamed of trying to do astrophotography here.
But then I accidentally stumbled over some of the images taken in not much better conditions with relatively small Optics and suddenly I am thinking about buying a telescope setup again.

After a couple of weeks of reading it seems that a 100/120 Apo with something like an ZWO1600MM on an EQ6-R Pro could get half decent results even under London skies and is still small enough to fit in the car for a trip to Wales (if we're ever allowed out again).

Then I wondered if I even need apochromatic optics if I have to use narrow band filters anyway?
Registration might be a problem if the different filtered components end up with slightly different focal lengths, but that might be solvable with relatively simple scaling?

I have found a few posts discussing this, but I couldn't quickly find any comparative examples or photos taken with achromats and narrowband filters, so I figured perhaps you guys have any thoughts about it?

Cheers,

Daniel

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Get at least ED doublet for NB imaging.

There are a few reasons why you would want to avoid achromat scopes for this:

1. There is not much of a price difference between good achromat with all the extras and ED doublet. If you try to image with say F/5 wide field achromat - first thing that you'll notice is poor focuser. That will cause tilt issues and poor corner stars. If you calculate in price of focuser upgrade - you'll see how quickly you'll be reaching ED doublet prices.

2. Although you are right about basic principle of operation for narrow band imaging and that you could even use well figured singlet lens (btw, proper alignment routine has no trouble dealing with scaling - different FLs), problem is that optical performance of mass produced doublet scopes is not quite good. They are produced to be fast wide field visual scopes. There is significant amount of spherochromatism and often astigmatism.

I owned ST102 - that is Skywatcher 102mm F/5 wide field scope and it showed quite a bit of astigmatism in red part of spectrum (eggy red component).

In overall price of the setup, scope really does not take up much of the budget. Getting ED doublet with good reputation simply makes more sense.

Say you want to get 100mm scope for imaging. You have couple of options:

https://www.firstlightoptics.com/startravel/skywatcher-startravel-102t-ota.html

for about £200 (currently out of stock)

versus something like this:

https://www.altairastro.com/starwave-ascent-102ed-f7-refractor-telescope-geared-focuser-468-p.asp

(It is ED doublet with FPL-51 glass and shows some color even in visual) that costs £495.

Second scope has better fit and finish, dual speed 2.5" R&P focuser that is rather decent, retractable dew shield for ease of transport, etc and costs only £300 more which will be less than 10% in overall budget (scope, mount, mono camera, filter wheel and NB filters).

Fitting dual speed focuser on first scope will set you back £170 (https://www.firstlightoptics.com/skywatcher-focusers/dual-speed-2-crayford-focuser-for-sky-watcher-refractors.html), and that makes price difference even smaller.

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Oh my ... no idea how I could have missed your replies!

Thanks a lot for taking the time!
I'll check that video out, @Marc1964 thanks for that!


@vlaiv - thanks for your reply!
That makes a lot of sense! I was initially thinking about getting a 100 or 130 APO like the Esprit 100

 http://www.opticalvision.co.uk/astronomical_telescopes-sky-watcher-ed_refractors/esprit-100ed_professional_super_apo_triplet_refractor.htm

but then stumbled over these for example and thought, do I really need an Apo for narrowband imaging? The extra aperture probably can't hurt with all the filters.
https://www.rothervalleyoptics.co.uk/explore-scientific-ar152-152mm-f65-air-spaced-doublet-refractor-ota.html

But I am now leaning towards something along the lines of this
https://www.rothervalleyoptics.co.uk/skywatcher-evostar-120-ed-ds-pro-optical-tube-assembly.html

 

Too many options (although it's hard to get ones hands on any of these at the moment, so maybe I'll just have to take what I can get for now- haha)

Do you have any thoughts on the options above?

Daniel

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1 hour ago, Daniel Karl said:

Do you have any thoughts on the options above?

Daniel

Now that you have mentioned that ES refractor - there are couple of fast refractors that could possibly be of suitable quality to allow for decent NB imaging. ES is one example.

I've read good things about this one as well:

https://www.teleskop-express.de/shop/product_info.php/info/p2229_TS-Optics-6--f-5-9-Refractor---2-5--R-P-Focuser---Ohara--Japan--Objective.html

However, I'm not sure how well they would do on NB imaging. For visual they are quite good and do show some chromatic aberration  - but they are also very sharp scopes according to reports.

This second TS scope has excellent focuser as well.

SW120 ED + matching flattener is going to be excellent scope for NB imaging and very good match for ASI1600

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Haha - vlaiv, I was just looking at that TS scope this very moment.

I actually remember Wolfgang Ransburg, who started this company back in the days when I was last active.

Now the question is if the bit of extra aperture of the TS 152 could give it an edge over the SW120ED, or if it's
just a little price advantage.
I'm planning on putting them on an CEM40 (or EQ6R), they should be within the payload?

The ASI1600 was also my "target".

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I had one of the TS 152 f/5.9 and found it was an excellent wide field scope for low to medium magnifications. Not for high mag use though.

I have one of these now which I chose over the SW120ED. Same optics and build quality as the WO126.

https://astrograph.net/epages/www_astrograph_net.sf/en_GB/?ObjectPath=/Shops/www_astrograph_net/Products/AGTEC125F78

Edited by johninderby
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39 minutes ago, Daniel Karl said:

Now the question is if the bit of extra aperture of the TS 152 could give it an edge over the SW120ED, or if it's
just a little price advantage.
I'm planning on putting them on an CEM40 (or EQ6R), they should be within the payload?

What will be your working resolution, do you have any idea?

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If you've been out of astrophotography for a long time you may well be greatly over-estimating the need for aperture. Even the need for focal length has been reduced by diminishing pixel sizes. 2 metres used to be a 'galaxy' focal length. Now a metre will do fine:

https://www.astrobin.com/full/omc9sk/0/

https://www.astrobin.com/full/393219/0/

https://www.astrobin.com/full/335042/0/

Two other things to note: firstly, not many chips are as big as the 35mm format film you may have used, so a metre FL will fit plenty of planetary nebulae onto modern chips but the classic emission nebulae are very large and you might find a much shorter FL, maybe 350-500mm, would suit you better.  We have 530mm and 1015mm focal lengths here and the overwhelming majority of emission nebulae are shot at the shorter length. Even with a full frame chip we often do mosaics with the 530mm FL instruments. (Actually yet another thing has changed in the last twenty years: the nebulae have got bigger. Well, not really, of course, but amateur imagers are going far deeper than before and finding countless extensions and interconnections which ask for a broader field of view.)

I'll just endorse what Vlaiv said, as well.  The Apos tend not only to be apos but to be imaging-specific instruments with regard to field curvature, tube length and focuser design.

Just in case you do go ahead with an achro for NB, the program Registar from Auriga Imaging will align and resize one image to fit another. It might cost more than the difference between the scopes, though... 

Olly

Edited by ollypenrice
typo
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Posted (edited)

@johninderby That one looks good as well - haha. I'm getting more and more options. I'm probably going to be more in the low and medium area. Did you use the TS photographically or visual?

@vlaiv With the ASI1600 I'd be looking at 16MP with 3,8 micrometer pixels I think. I suppose I'd need a 0.8 FF/reducer, so effectively I'd end up more like 700mm focal lenght.

 

Edited by Daniel Karl
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@ollypenrice Thanks for sharing your experience. It is really impressive, what people do with the "small" scopes today. That is the reason, why I am thinking about giving it a go again, because there's no way I'd wrangle anything like my 15" again. One of the most impressive images I have seen lately was actually shot on a 130mm lens - haha.
 

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29 minutes ago, Daniel Karl said:

@vlaiv With the ASI1600 I'd be looking at 16MP with 3,8 micrometer pixels I think. I suppose I'd need a 0.8 FF/reducer, so effectively I'd end up more like 700mm focal lenght.

I probably did not express myself well, so I'll expand on your example.

Say you are imaging with ASI1600 and 700mm focal length. You'll be sampling at 1.12"/px - which is very high resolution to work with. In fact - I would say that 1"/px is upper limit of what amateurs can achieve.

One can certainly image at higher sampling rates (resolutions - btw, word resolution is used in so many contexts that it is sometimes confusing) - and many people do, but they are oversampling at those sampling rates - their effective pixel size is smaller than it needs to be in order to capture all the detail available and they are settling for lower SNR because of that.

I would advise against going for such high sampling rate for number of reasons:

1. already mentioned over sampling - you'll need very steady skies and excellent mount performance in order to achieve detail that requires 1.12"/px (which equates to ~1.8" FWHM stars - seeing alone is often higher than that at 2" FWHM not to mention guiding error and telescope PSF that adds onto that)

2. 4656px x 1.12" will give you about a degree and a half. There are quite a few interesting narrow band targets that are larger than FOV provided at 700mm of focal length - but there are quite a few that are just right.

Maybe you'll be better served with 1.5"/px for example? I'm rather happy with 2"/px for wider field imaging. For 1.5"/px you'll need around 500mm of focal length.

Alternative to going shorter focal length would be to bin your pixels. This way you can get 1.5"/px from 1000mm of FL with use of 2x2 bin. Cmos cameras bin in software - which means you can bin after you finish imaging and calibrate your subs. Downside to this is that FOV is left unchanged - you'll get small FOV at 1000mm.

You could use 700mm scope and bin that 2x2 - that would give you better FOV than 1000mm (not as good as 500mm) but 2.24"/px - which is well suited sampling rate for NB imaging of larger targets. Down side is that you no longer produce 4656 x 3520 but rather 2328 x 1780 subs (if that is important to you - like for printing).

Hope this is helpful. Bottom line is - aim for 1.5"/px - 2"/px regardless if you opt for shorter FL or you decide to bin your data.

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@vlaiv Yeah, the oversampling is a problem, especially since my seeing conditions will most likely be bad most of the time. So really with the ASI1600 I want to be around 500mm focal length. But nearly all the scopes I have in my selection at the moment are natively around 900 (Evostar 120, TS 152, Tecnosky 125). I'll have to use a 0.8 reducer anyway but that's still too much. 

So is it then a question of choosing a camera with a larger pixel pitch, or getting a stronger focal reducer 0.6 or so, or going for a different scope altogether ... haha.

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1 hour ago, Daniel Karl said:

@ollypenrice Thanks for sharing your experience. It is really impressive, what people do with the "small" scopes today. That is the reason, why I am thinking about giving it a go again, because there's no way I'd wrangle anything like my 15" again. One of the most impressive images I have seen lately was actually shot on a 130mm lens - haha.
 

Exactly. I was trying to be brief so made no mention of the Samyang 135 lens which is doing such good stuff of late. 

Olly

 

 

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16 minutes ago, Daniel Karl said:

@vlaiv Yeah, the oversampling is a problem, especially since my seeing conditions will most likely be bad most of the time. So really with the ASI1600 I want to be around 500mm focal length. But nearly all the scopes I have in my selection at the moment are natively around 900 (Evostar 120, TS 152, Tecnosky 125). I'll have to use a 0.8 reducer anyway but that's still too much. 

So is it then a question of choosing a camera with a larger pixel pitch, or getting a stronger focal reducer 0.6 or so, or going for a different scope altogether ... haha.

Hm, I think I have a solution for your problem. At least - that is what I would do now.

I have ASI1600 which I got when it just came out (I have V2 model rather than later pro) but if this sensor was available back then - I'd go for it instead.

ASI294mm-pro

It has better QE than ASI1600 by quite a large margin:

%E5%AE%98%E7%BD%91%E5%B0%BA%E5%AF%B8%E2%

It has 2.3µm native pixel size although camera first appeared like 4.63µm pixel size camera. This is because it was "automatically" binned 2x2 by design, but in later model they decided to unlock small pixel size.

This is actually very good thing as you can now image at 2.3µm pixel size and then software bin it either 2x2 or 3x3 - depending on how you judge the quality of your sky on particular night. With ~700mm of FL that will give you 1.36"/px (still a bit on high side but better than 1.12"/px) and ~2"/px which I think you'll use most of the time.

Sensor is a bit larger than ASI1600 - ~23mm vs ~21mm and I think that it does not suffer from micro lensing issue with narrowband filters. ASI1600 does suffer from that on bright stars in some configurations. It depends on actual setup and optical elements involved - sometimes it is stronger and sometimes it is weaker. Many people don't like it.

Here it is on OIII data from my RC8" (note that this was taken at 1"/px - and it is oversampled although it was very good night):

image.png.216b391da5a2b156b212880145b4ea72.png

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@vlaiv  I have read about that one. Had not heard that they have unlocked the 1x1 binning. That does indeed sound like a good match to a 900/700 mm focal length. Which brings me back to choosing the scope - haha. 

@ollypenrice we're probably thinking of the same picture - haha. I still have some fast Canon L lenses, I can start to experiment with till I have found a scope. Although I don't have the mount yet, either 🤣🤪

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8 minutes ago, Daniel Karl said:

@vlaiv  I have read about that one. Had not heard that they have unlocked the 1x1 binning. That does indeed sound like a good match to a 900/700 mm focal length. Which brings me back to choosing the scope - haha. 

@ollypenrice we're probably thinking of the same picture - haha. I still have some fast Canon L lenses, I can start to experiment with till I have found a scope. Although I don't have the mount yet, either 🤣🤪

I had a spell with a Canon 200L lens. A good trick is to avid the diaphragm and make your own round front aperture mask.

spacer.png

Olly

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I agree with Vlaiv on the camera. The ASI1600 is now a bit outdated and the ASI294 would be my obvious choise for a camera with that sensor size. There are now lots of ASI1600 on sale on the used market - I think that people are upgrading to the more sensitive next generation of CMOS, like the 294, or the amazing 2600 with no amp glow.

Edited by gorann
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6 minutes ago, gorann said:

I agree with Vlaiv on the camera. The ASI1600 is now a bit outdated and the ASI294 would be my obvious choise for a camera with that sensor size. There are now lots of ASI1600 on sale on the used market - I think that people are upgrading to the more sensitive next generation of CMOS, like the 294, or the amazing 2600 with no amp glow.

If you're buying new this is very sensible advice.

If you're considering second hand, I've seen 2nd hand ASI1600s (or equivalent) for less than 1/2 the price of a new ASI294. Of course, a second hand 294 would also be a good shout, but I've seen very few for sale, whereas there's been a lot of 1600's on offer recently.

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On 26/03/2021 at 19:59, Daniel Karl said:

Hey all,

I'm new to this forum and currently on the brink of breaking into the hobby again after almost 15 years of break.

I used to take photographs on 35mm film with a 15" f/4.6 Lomo Newton on a 200pound (excluding scope, counterweights or tripod) beast of GEM (ALT-7 ) - with all the fun of manual off axis guiding in -15 degrees under Bartel 3/4 skies.
Now I live in London, don't have the strength to lift 200 pounds any more and never dreamed of trying to do astrophotography here.
But then I accidentally stumbled over some of the images taken in not much better conditions with relatively small Optics and suddenly I am thinking about buying a telescope setup again.

After a couple of weeks of reading it seems that a 100/120 Apo with something like an ZWO1600MM on an EQ6-R Pro could get half decent results even under London skies and is still small enough to fit in the car for a trip to Wales (if we're ever allowed out again).

Then I wondered if I even need apochromatic optics if I have to use narrow band filters anyway?
Registration might be a problem if the different filtered components end up with slightly different focal lengths, but that might be solvable with relatively simple scaling?

I have found a few posts discussing this, but I couldn't quickly find any comparative examples or photos taken with achromats and narrowband filters, so I figured perhaps you guys have any thoughts about it?

Cheers,

Daniel

If the kit you choose isn't too heavy, consider a HEQ5 mount over the EQ6. I've got an EQ6 and whilst you can certainly pack it up in the car, it's only worth the extra weight if you need the extra capacity. 

YMMV and there's loads of options for tuning, but I don't get the impression that guiding performance is significantly worse on an HEQ5. And it's a little cheaper.

 

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@rnobleeddy At the moment I am sort of thinking to start with the mount - depending on what I can get, too. My preference would be a CEM40 or CEM60 if I can find one.
I think performance is more important to me than weight at this point, because if the tracking is bad, I'm just wasting time and sleep - haha. I had my eyes on the HEQ-5 as well, but I don't want to risk having to change mount in a year if stuff gets too heavy. Although with the CEM40 that would probably be the case. Too many options and too few of them actually available at the moment - haha. Luckily it's not really the season, so there's not too much point in rushing it all.

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4 minutes ago, Daniel Karl said:

@rnobleeddy At the moment I am sort of thinking to start with the mount - depending on what I can get, too. My preference would be a CEM40 or CEM60 if I can find one.
I think performance is more important to me than weight at this point, because if the tracking is bad, I'm just wasting time and sleep - haha. I had my eyes on the HEQ-5 as well, but I don't want to risk having to change mount in a year if stuff gets too heavy. Although with the CEM40 that would probably be the case. Too many options and too few of them actually available at the moment - haha. Luckily it's not really the season, so there's not too much point in rushing it all.

Get CEM60 if you can find one.

I have HEQ5 and my upgrade path was going to be CEM60 / CEM120 depending on the budget. CEM60 is no longer made and is replaced with CEM70 which is much more expensive and I don't see justification for that. My personal belief is that iOptron realized that CEM60 is too good a deal in performance/$ so they decided to "upgrade" it - and of course increase their profits that way. From what I've read, experience with CEM60 has been really positive.

For me, CEM120 is still upgrade option.

HEQ5 is excellent mount once it is properly tuned / upgraded / modded. However, having done all the mods to mine myself (belt mod, changed bearings, changed saddle plate, upgraded tripod) - I now have a feeling it is somehow too fragile. I constantly worry that something will throw it out of perfect tune and it will stop performing the way it is performing now. Tuning it to perfect running order seems more luck than skill to be honest and part of me is scared if I'll be able to do it again. I managed to get my mount down to incredible 0.36" RMS guiding error on a very quiet night (which is funny because I maintained that it can't go below about 0.5" RMS as stepper precision is 0.17" per step and it is doing 64 micro steps - so it can't have much holding power to keep the scope centered in DEC against the wind for example).

Keep in mind that you'll feel most comfortable by mounting less than 2/3 of specified load capacity on a given mount. In one configuration I put about 11Kg on Heq5. I did load it with more weight - but it did not perform well. Similarly CEM40 is declared at 18Kg - so for imaging consider 12-13Kg to be upper limit on it.

CEM60 will carry up to 27Kg according to specs so you can really put 18-20Kg on it for imaging.

Btw, I think that CEM40 is similarly good product - but I have not read much on people's experiences with it.

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