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Will I be able to tell the difference between an Achromatic and an APO?


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I have the itch for a refractor scope which will be for pure visual, I don't plan to go down the AP rabbit hole.

 My first and only scope is a MAK 102 and so I have nothing to compare it to, I've enjoyed getting what use I can out of it, dodging the clouds and the freezing weather. It's given me some great views of the Moon, Saturn and Jupiter Conjunction and a few days ago I bagged my first view of the Orion Nebula. My first proper look at a DSO has given me thirst for more and I realise that the Mak is not necessarily the best tool for this job, at this stage I'm not interested in chasing down every faint object possible, I just want to enjoy the more prominent DSO's. So no Dob required.

I've discovered that I like seeing objects in context, the Moon suspended in black space rather than the ultimate in magnification for instance is what gives me pleasure. What I would hope to get from a refractor scope are wide field views with a nice contrasty image and maybe some colour in the nebulae. I've obviously done some surfing around looking at "fracs" and with no particular budget in mind but was aware that inch for inch they are going to cost a lot more than a reflector. Or are they?

I was initially thinking medium to longer term for acquiring one and whilst a Tak may be a bit of a stretch I was attracted to the Stellamira 80 by it's gorgeous build quality, this would involve a new mount and tripod (iOptron and Berlebach were drooled over) so the cost was beginning to mount up. I then realised that at 900mm FL it was probably not ideal for deep sky  so started looking at 80mm ED scopes, this one for instance https://www.teleskop-express.de/shop/product_info.php/info/p7169_TS-Optics-ED-APO-80-mm-f-7-Refractor-with-2-5--R-P-focuser.html I was much more comfortable with the price and weight of this scope as I could utilise my existing mounting options.

 

Further research brought up the suggestion that for deep sky visual use an Achromatic would do the job just as well and thanks to weight and cost saving I could probably go to a 100mm scope. Two obvious contenders are the SW https://www.firstlightoptics.com/startravel/skywatcher-startravel-102t-ota.html and Bresser https://www.firstlightoptics.com/bresser-telescopes/bresser-messier-ar-102s-600-refractor-ota.html To my eyes the Bresser looks better finished and the SW has mixed reviews with regards to the focus mechanism.

So, to get to the point, would the 102 Achros serve me better than an 80mm ED, or are there other options I should be aware of?

 

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As far as I am aware , the short focal length achromats will show CA on bright targets .. generally the longer the focal length the less CA is present . However it actually depends on you how much CA bothers you . The extra aperture on the 102 will be beneficial but if you are mainly looking at wide field targets then magnification isn’t really an issue . You already have your Mak for planetary and the moon . The st102 was the second scope I bought and I concur that the focuser became a bit sloppy after a time . I think bresser gear is well built ( apart from the appalling finder but you can change that of course ) As you don’t want to go down the photography route I’m not sure the ED scope would be needed . But of course the choice is yours . Maybe others have a different opinion 

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If widefield vistas are your goal, then I found the Skywatcher ST120 achromatic a great 'budget' option. Add a 2" diagonal and a 2" ED30 eyepiece and the wide view is splendid. Great contrast, pinpoint stars and as DSO observation goes, it saw as deep as my recently sold 150/750 reflector.

I found myself with too many telescopes and the Startravel lost, due mostly to its weight. Fully loaded with accessories and an eyepiece it was too heavy for an AZGTI mount. Chromatic abberation is visible on brighter objects, the lunar limb for example, but it didn't bother me too much.

Would you not consider a 102mm f7 ED refractor? Do you need two telescopes? This can replace your 102mm Mak for planetary viewing, while also allowing you to enjoy DSOs and a much wider field of view. They're relatively light weight instruments at around 4.5-5kg so easy to mount. I couldn't be happier with my recently purchased 102mm refractor. My own 102mm Maksutov is being kept as a travel/camping scope, otherwise it would be sold as the refractor has made the Mak redundant, and also my 150/750 reflector which was sold. The Altair Astro Starwave Ascent 102mm F7 ED refractor is sold under at least one other brand, but I can't remember which. One of the refractor gurus here will know.

A field of view comparison between a 102mm Maksutov, Startravel 120, and 102mm F7 refractor. All with a 1.25" 25mm 60° eyepiece.

905245289_astronomy_tools_fov(2).png.6c2bf9ebb71cf1a9aa4e44904cc5ead0.png

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The better question is, can you tell the difference between an ED and an APO visually.  In my experience at low powers, you can't.  It's only at high planetary powers that you can see the better color correction of the APO.

Really long f-ratio achromats can be very good indeed, but mounting them is a pain.  If you enjoy wide field views, this means short f-ratios.  You'll want at least an ED to avoid excessive violet fringing.

The ED you're looking at is made by Sharpstar Optics.  They're probably the highest quality Chinese made refractors.  I have the TS-Optics 90mm FPL-53 triplet and love the optical and mechanical quality.

All Skywatcher products are made by Synta.  Optically, they're very good.  However, mechanically, they are very basic to hit an introductory price point.  Swapping focusers (to MoonLite generally) on the SW 120mm APO doublet is pretty common because of this.

Here's a good chart showing the differences in CA across various apertures and f-ratios of achromats:

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Notice the trend that as you increase aperture, you need longer f-ratios to maintain constant CA correction.  This is also true of ED and APO scopes of a constant lens design.  If you want good color correction at f/5 and 100mm, you will need an FPL-53 triplet or equivalent.  However, if you can live with f/8, an ED or APO doublet will suffice.

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@Louis DI have always felt this chart useful. However I never really understood the black etched lines. If you look at a 90mms frac at f/5 it shows the black lines.Go to f/6 it's filterable. Go to f/7 you have black lines again.

There are a number of 90mms Astro fracs at f5.5 useful for airline travel but how good are they

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4 minutes ago, Mark at Beaufort said:

However I never really understood the black etched lines. If you look at a 90mms frac at f/5 it shows the black lines.Go to f/6 it's filterable. Go to f/7 you have black lines again.

Notice the hashed lines go in opposite directions and signify different levels of correction.

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10 minutes ago, Louis D said:

Notice the hashed lines go in opposite directions and signify different levels of correction.

Thanks for that. I have never noticed that before. It does not help being badly colour blind.

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I know you are looking at a refractor, but honestly a 6” Newtonian will do all the things you want, it will give truly apochromatic views and will be much cheaper too.

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3 minutes ago, dweller25 said:

6” Newtonian will do all the things you want

Except provide diffraction free, pinpoint stars.  For wide field viewing, the difference is noticeable.

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I've ruled out the 102ED scopes due to weight as they won't fit my existing mount/tripod options, the extra aperture would be nice to have which is why I'm considering the Apchro option in that size. This is primarily to give me a grab and go option, to see if I like refractors and utilise kit I already have. A larger/nicer frac with corresponding mounting options could happen further down the line depending on how my astronomy interests develop.

I've also decided against the Sky-Watcher as the build and finish puts me off, I've since read on here a less than flattering user review of the Bresser so now I'm not so sure about going for an Apchro.

I saw this as being a complimentary scope to my Mak, wide filed views maybe better quality image with the ED glass that I can use for sky surfing and viewing the brighter DSO's whilst leaving the Mak for Lunar and planetary duties.

I guess it will come down to smaller aperture but better glass and hardware versus larger aperture with poorer optics and build quality.

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10 minutes ago, Mark at Beaufort said:

Thanks for that. I have never noticed that before. It does not help being badly colour blind.

No problem.  My future son-in-law is also color blind.

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6 minutes ago, dweller25 said:

I know you are looking at a refractor, but honestly a 6” Newtonian will do all the things you want, it will give truly apochromatic views and will be much cheaper too.

I knew that would come up 😁 

6" is a much larger scope and I don't want the collimation hassle. I may change my mind though after I've scratched the frac itch!

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14 minutes ago, Louis D said:

Except provide diffraction free, pinpoint stars.  For wide field viewing, the difference is noticeable.

Personally I quite the look of diffraction spikes on stars 🙂

Of course that will not be an issue with DSO’s.

Edited by dweller25
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4 minutes ago, dweller25 said:

Personally I quite the look of diffraction spikes on stars. Of course that will not be an issue with DSO’s.

Bright stars themselves are slightly bloated as well in Newts.  This is the difference I immediately noticed between the Dob and the fracs.  It makes splitting close, non-equal brightness doubles easier in the fracs.

Edited by Louis D
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19 minutes ago, Louis D said:

Bright stars themselves are slightly bloated as well in Newts.  This is the difference I immediately noticed between the Dob and the fracs.  It makes splitting close, non-equal brightness doubles easier in the fracs.

They don't have to be. I have built a number of Newts over the years which had pinpoint stars and track sharp diffraction spikes. Quality optics and precise collimation  is mandatory. 

Regards Andrew 

Edited by andrew s
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What are your skies like? If dark, and you’re not expecting to resolve more challenging DSOs, then an 80ED can be a great second scope which would complement your Mak nicely IMO. My first two ‘proper’ telescopes were a 105mm Mak and an Equinox 80ED and they gave good service - but 80mm is not enough aperture to go very deep unless you have dark skies, and even then there’s a limit. Open clusters will be stunning, and bright targets like M42 will also give fine views,  but galaxies and globulars could prove frustrating. I saw 50-60 Messier objects with my 80ED in darker locations, but - for example - I was never able to resolve M13 into separate stars until I got a larger scope. 

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5 hours ago, Aquavit said:

What I would hope to get from a refractor scope are wide field views with a nice contrasty image and maybe some colour in the nebulae

I think you would need a very large refractor to see colour - or possibly a good imagination, which is defintiely cheaper :)
People's tolerance of imperfections varies a great deal. Just like some people would never dream of drinking a wine that cost less than £100, or listening to music in MP3 format. So some will call CA "unacceptable" when others are quite relaxed about it.  Personally, I do not find it all that objectionable on the few objects where it is noticeable. But then, I have no problems with cheap wine, either!

I would say there is no hard and fast rule. Some achro's are better than others. Some Apo's are really not very good.

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17 hours ago, ScouseSpaceCadet said:

A field of view comparison between a 102mm Maksutov, Startravel 120, and 102mm F7 refractor. All with a 1.25" 25mm 60° eyepiece.

This FOV chart is really useful, thanks. Looks like the 102mm F7 ed doublet is almost as good as the ST120 in that respect. What about visual quality for DSO? Is the ST120 better or does the 102mm doublet hold its own? 

(I'm basically trying to decide between getting a cheaper refractor to get started – for widefield views and to ease my way into astronomy – or whether to just pull the trigger on a more expensive f7 ed doublet if it's going to be as good as the SST120 at deep space anyway)

 

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