Wide field scopes - Are they worth the extra costs?

[jm_rim]

I want to buy my first wide field scope (300-400mm) for astrophotography and found the new William Optics Zenithstar 61 APO on FLO. I know there are not many reviews yet, but what I've read is positive.

Before I considered - and still do it - telescopes like Skywatcher Esprit 80 ED PRO or William Optics Star 71 II but are they the 2.5x extra costs?

[LightBucket]

The thing is when buying a scope, is that the prices don’t  go up relative to image quality, by that I mean if the scope is 2.5x the price, it won’t mean that it is 2.5x better, it may only be 10-15% better and in some cases not that much, so it’s really up to you and your budget, I have also read good things about the new WO offering, and I used to have a WO ZS66 and it was superb...

You could also consider the highly regarded Skywatcher ED80, that is an excellent scope for the money and a very popular starter scope, in fact many established imagers stilll use this scope....

Hope that helps a little

[MARS1960]

1 hour ago, jm_rim said:

I want to buy my first wide field scope (300-400mm) for astrophotography and found the new William Optics Zenithstar 61 APO on FLO. I know there are not many reviews yet, but what I've read is positive.

Before I considered - and still do it - telescopes like Skywatcher Esprit 80 ED PRO or William Optics Star 71 II but are they the 2.5x extra costs?

What you are really comparing is doublets and triplets and even the quads like 71-II.

In my lay opinion as an amateur who likes to take images of DSO's when i can (which isn't many times a year) i am more than happy with my used equinox ED80 doublet, it is very well colour corrected and takes fabulous images, i really don't need to spend 3x as much on a triplet/quad, however i did spend £39 on top for used SW field flattener, so for £439 i have what i consider to be a cracking good widefield, fairly fast (F/6) scope.

[Droogie 2001]

I purchased a WO Star 71 (Version1) a while back which was and still is my only refractor. The 5 element APO design, requiring no field flatener means I have not had any issues with chromatic colour aberation and have pin sharp stars to the corners.
Not suggesting that cheaper models cannot do this but when I went to purchase a refractor I was willing to accept paying extra to not have to deal with these possible issues. 

Overall its very made and has been a pleasure to use.
 

[jm_rim]

3 hours ago, LightBucket said:

I used to have a WO ZS66 and it was superb...

Very nice to know, thanks. Is it the Skywatcher Evostar 80ED you’re referring to?

[jm_rim]

2 hours ago, MARS1960 said:

In my lay opinion as an amateur who likes to take images of DSO's when i can (which isn't many times a year) i am more than happy with my used equinox ED80 doublet, it is very well colour corrected and takes fabulous images, i really don't need to spend 3x as much on a triplet/quad, however i did spend £39 on top for used SW field flattener, so for £439 i have what i consider to be a cracking good widefield, fairly fast (F/6) scope

I’m thinking the same, but since it’s only few evenings during the year, it would be nice with well corrected and fast scope.  

From what I can find there aren’t many fast (preferably F5 or faster) wide field scopes. 

[ollypenrice]

It's worth thinking about what you are paying for in very expensive fast APOs. Then you can think about how many of these things you need. (And to what extent you need them.)

I) Quality mechanical construction. Fast scopes have steep light cones which are intolerant of sloppy focus, sensor tilt (arising from the focuser etc) and mis- collimation. Every fast apo needs this qualtiy construction.

2) Colour correction. The faster the lenses the harder it is to bring all colours to the same focus, most notably blue. If you are a natural colour imager using OSC or LRGB this is very important. If you are a narrowband imager it matters far less.

3) Size of fully corrected flat field. If you're using a full frame sensor this is hugely important and not many scopes can deliver. The corrected image circle must be at least as large as your chip's diagonal measurement. Some manufacturers, including Takahashi, are distinctly optimisitic in their image circle claims.

4) Quality control. The most important factor, here, is to buy from a dealer with a no quibble returns policy. You'll find one at the top of this page.

It is certainly true that, the higher up the food chain you go, the more a small increment in quality costs you.

Olly

 

[LightBucket]

1 hour ago, jm_rim said:

Very nice to know, thanks. Is it the Skywatcher Evostar 80ED you’re referring to?

Yes thats the one..

 

[geordie85]

I was one of the lucky ones to get a series 1 WO star 71 that was re-collimated by Es Reid, thread here, 

 

It truly is an amazing scope and due to being a fast, flat field astrograph, I never have to worry about spacers, field flatteners/ reducers.

It has a very nice wide field too.

[rl]

There is a triplet option between (costwise) the ZS61 and the WO71..

The TS 65mm apo quad with the internal flattener is on special offer at the moment from several vendors...might be worth a punt. They normally retail at £800 which I always thought was a bit steep, but £600 now looks a whole lot more reasonable. There is also a s/h one on ABS. It's a very good imaging scope; the whole thing gives the impression of being designed by someone who had actually done some imaging and seen the problems for themselves.

The image plane is flat over a 44mm diameter circle and being a triplet (+flattener) the colour correction is good. 

Some of the first ones had pinched optics issues in the cold but this is long since sorted. Having the flattener included internally allows the use of a flip mirror which I personally find very convenient, without having to worry over the exact spacing.  They construction is very heavy duty for the aperture...built like the proverbial Panzer brick privvy. A bit too heavy for a Star Adventurer (which is why I bought a ZS61). I run mine on a Vixen Sphinx with a separate 60mm guidescope.

The focuser is particularly nice and the camera rotater does not rely on the focuser knobs having enough clearance not to collide with the mount. 

It looks like TS have several offerings at this aperture and price range and they are rationalizing the range. Maybe the TS65 will be phased out in favour of the 70mm version which might explain the summer special offer. 

RL

[newbie alert]

It's worth noting that the f ratio referred to the field of view so a 80ed that has a focal length of 600mm has a  native f ratio of 7.5...to get a smaller f-ratio scope the same 80mm will have a shorter focal length and therefore a wider field of view..

The sum is focal length divided by the diameter of the optics give the focal ratio..so a 80ed is 600÷80=7.5

[Max N]

20 hours ago, rl said:

There is a triplet option between (costwise) the ZS61 and the WO71..

The TS 65mm apo quad with the internal flattener is on special offer at the moment from several vendors...might be worth a punt. They normally retail at £800 which I always thought was a bit steep, but £600 now looks a whole lot more reasonable. There is also a s/h one on ABS. It's a very good imaging scope; the whole thing gives the impression of being designed by someone who had actually done some imaging and seen the problems for themselves.

The image plane is flat over a 44mm diameter circle and being a triplet (+flattener) the colour correction is good. 

Some of the first ones had pinched optics issues in the cold but this is long since sorted. Having the flattener included internally allows the use of a flip mirror which I personally find very convenient, without having to worry over the exact spacing.  They construction is very heavy duty for the aperture...built like the proverbial Panzer brick privvy. A bit too heavy for a Star Adventurer (which is why I bought a ZS61). I run mine on a Vixen Sphinx with a separate 60mm guidescope.

The focuser is particularly nice and the camera rotater does not rely on the focuser knobs having enough clearance not to collide with the mount. 

It looks like TS have several offerings at this aperture and price range and they are rationalizing the range. Maybe the TS65 will be phased out in favour of the 70mm version which might explain the summer special offer. 

RL

Very interesting, thanks for the heads up. Do you know if the £600 price is still available anywhere? The best I can find is £659.

Edit - sorry, I've just now done the conversion from the Euro price, which gets me much closer to £600...

[London_David]

The WO Z61 got me interested in picking one up on impluse too - it's cute and small and portable! But I've only ever had Newtonians or SCT's with refractors I'm also not sure where the cost goes in a refractor either. But now I've gone down a deep hole of wanting to buy a small refractor...

To throw another one into the mix... the Takahashi FS60CB TSK06210 is £629 for the OTA.

It has great reviews and everyone seems to hold Takahashi scopes in very high regard. But is it that much better? I have no idea what I'd see through it that would be different from the Z61 (other than a quick look at Astronomy Tools for the view). Plus... if I buy something small I'll be using it for visual and with an ASI290 or APS-C size. Does the 2x price jump justify that?

The TS65 sounds very interesting too... but I can't see them anywhere.

Or are you better spending even more (reaching 3x on the WO Z61 price now) on a WO 70 at f4.9 rather than the f5.9 of the Takahashi? Reviews seem to give the Takahashi greater performance than you would expect from a 60 because of the optics but... I think the 3x price jump is too much. I just want something fun and small. 

I don't know -- I keep ratcheting up the price upsetting myself on scopes I don't really know about!

What do people think about the Takahashi, anyone used it?

[geordie85]

I've heard the Tak is very good for narrowband but the blue has bad bloating in broadband. 

I'm still tempted to get one and just use it for red, S11 and Ha then use my WO star 71 for Lum, green, blue and o111

[Sky-J]

Hi there, 

Re visual observing. I have an older WO scope, the WO Zenithstar 66 SD and I love it, its a really well-made piece of kit. I live in London so my bigger scope generally stays packed up and I invariably grab this one whenever the sky allows. Also as it packs into a small camera bag so comes with me everytime I leave town. If I ever upgrade, I think I'll stick to WO.

Good luck choosing,

Jeremy

[ollypenrice]

On 12/10/2017 at 15:04, geordie85 said:

I've heard the Tak is very good for narrowband but the blue has bad bloating in broadband. 

I'm still tempted to get one and just use it for red, S11 and Ha then use my WO star 71 for Lum, green, blue and o111

For imagng I don't think the FS60 is as good as it should be but it is very crisp visually.

Olly

[gorann]

Jesper,

you of course also have another option: to buy a telephoto lens. I just bought a Canon EF 300mm F/4 L USM in mint condition for about 500 pounds on ebay. This in an apo lens with and aperture of 75 mm and it is supposed to have a flat field that covers a full format DSLR chip. I just got it and weather has not permitted me to test i yet. I expect Nikon has a comparable lens for a similar price on e-bay. I was told to stay away from the latest versions with image stabilization since that introduces more lenses that are movable and they are just trouble and of no use for astrophotography. They are also more expensive. I see that you have a DSLR now, but if you are going for a chilled CCD/CMOS in the future there are adapters.

Edit: Martin Fransson has a Canon EF 300mm F/4 L USM and has taken some splendid images with it

Cheers

[London_David]

Maybe I’ll give the Takahashi a miss... 

I’m interested in eea and a occasional undescerning visual use, rather than glossy imaging. 

So I spoke to FLO about the performance differences between the Z61, the Z71, or something even fancier (Borg 72FL) and they said there probably wouldn’t be a noticeable in terms of performance unless I’m doing long exposure DSLR imaging — but...

They pointed out that the Z71 flattener is also a reducer which would help the speed taking the f/5.9 to f4.7 or so. So they suggested the Z71 over the Z61 to me.

But... It may be superficial but I like the look of the rack and pinion focuser with temperature gauge on the Z61 and the Gran Tourismo 71. Plus I’ve read on forums that some of the WO Crayford are not so reliable. They do seem to have r+p on all the new scopes. 

Has anyone used the GT71 compared to the Z71? Or had good / bad experiences with mechanical quality of the Crayford focus?

Or is the problem in this price / size range (60-70mm @ £400-1200) there are no clear answers only very small differences. 

[jm_rim]

On 6/10/2017 at 20:32, ollypenrice said:

3) Size of fully corrected flat field. If you're using a full frame sensor this is hugely important and not many scopes can deliver. The corrected image circle must be at least as large as your chip's diagonal measurement. Some manufacturers, including Takahashi, are distinctly optimisitic in their image circle claims.

There are not many wide-field scopes (as far as I can find) with a larger than aps-c size fully corrected flat field. Once in the future, I was planing to buy a CCD with a KAF-16200 (diagonal 35mm) sensor, but I'm not so sure anymore. Hopefully, ZWO makes their ASI071MC (diagonal 28mm) in mono, which a lot more scopes fit ...

21 hours ago, gorann said:

you of course also have another option: to buy a telephoto lens.

This i have thought of many times. I have looked at the, Nikon 400mm 3.5 ais, Zeiss Tele Tessar T 300mm f/4 and ... , but are they worth it, would a cheap ED or APO not be better corrected for astrophoto?

[gorann]

9 hours ago, jm_rim said:

There are not many wide-field scopes (as far as I can find) with a larger than aps-c size fully corrected flat field. Once in the future, I was planing to buy a CCD with a KAF-16200 (diagonal 35mm) sensor, but I'm not so sure anymore. Hopefully, ZWO makes their ASI071MC (diagonal 28mm) in mono, which a lot more scopes fit ...

This i have thought of many times. I have looked at the, Nikon 400mm 3.5 ais, Zeiss Tele Tessar T 300mm f/4 and ... , but are they worth it, would a cheap ED or APO not be better corrected for astrophoto?

At least a used Canon 300 f/4 in good condition is cheaper than any comparable Apo telescope with a 75 mm aperture. As I said I have not had a chance to try mine yet, but professional lenses like this, we are talking about an apo with ED elements and totally around 7 or 8 lenses to also correct for things like coma and curvature, are likely to be very well corrected. They are meant to be corrected for full size DSLR, but maybe that is to stretch it when it comes to astrophoto, or maybe not. I think you will find many examples on the net of high class astrophotos by these lenses. The Canon 300 f/4 without image stabilization is around 5000 pounds on e-bay but a new one (with image stabilization) is three times the price so they do charge for the nice optics.

Edit: Another thing to consider is the quality control. Cheap ED and APO scopes are made in China and then you often have to be lucky to get a good one, while I expect that the quality control of professional Canon and Nikon lenses (made in Japan) is very rigorous.

[London_David]

As I understand it (and someone may have mentioned this in this thread)... but the two things to be aware of with a regular photographic lens are diffraction spikes and the optimized f stop for the lens.

Since refracting telescopes are a fixed aperture, stars have no diffraction spikes: the aperture is perfectly round (whereas on a Newtonian you get the distinctive cross). If you have a classical photographic lens with a changeable aperture, there are moving blades to change the size of the hole and you get a spike for each blade. With some designs you could have 15 spikes coming from your stars. Maybe you like the look of tiny shiny spikes on stars, or maybe not, but it is something to be aware of and historical some have regarded them as "imaging errors". For me, I quite like them sometimes (and now you can even get filters on photoshop to add diffraction spikes to perfectly round crisp stars...).

Regular camera lenses are also optimized for shooting below wide open. So an f3.2 rated lens could have best imaging performance several f-stops lower.

Buying a great classical photo lens means you will be paying for features, qualities and mechanics you may not need in astro photography (changeable f-stop from mechanical shutter blades being one). Hence why it is possible for a well designed and made refractor to be cheaper than a traditional DSLR camera lens for the same performance. 

It doesn't make either type of tool necessarily better or worse: they're just different shapes of hammer.

[gorann]

3 hours ago, London_David said:

As I understand it (and someone may have mentioned this in this thread)... but the two things to be aware of with a regular photographic lens are diffraction spikes and the optimized f stop for the lens.

Since refracting telescopes are a fixed aperture, stars have no diffraction spikes: the aperture is perfectly round (whereas on a Newtonian you get the distinctive cross). If you have a classical photographic lens with a changeable aperture, there are moving blades to change the size of the hole and you get a spike for each blade. With some designs you could have 15 spikes coming from your stars. Maybe you like the look of tiny shiny spikes on stars, or maybe not, but it is something to be aware of and historical some have regarded them as "imaging errors". For me, I quite like them sometimes (and now you can even get filters on photoshop to add diffraction spikes to perfectly round crisp stars...).

Regular camera lenses are also optimized for shooting below wide open. So an f3.2 rated lens could have best imaging performance several f-stops lower.

Buying a great classical photo lens means you will be paying for features, qualities and mechanics you may not need in astro photography (changeable f-stop from mechanical shutter blades being one). Hence why it is possible for a well designed and made refractor to be cheaper than a traditional DSLR camera lens for the same performance. 

It doesn't make either type of tool necessarily better or worse: they're just different shapes of hammer.

If you need to step down a camera lens for astrophotography and want to avoid the iris-spikes, you should not use the internal iris but cheap circular rings that you put into the filter thread, like these:

https://www.amazon.co.uk/Belmalia-Step-Down-Rings-Filter-Adapters/dp/B00WQ0C7SU/ref=pd_sim_421_2?_encoding=UTF8&psc=1&refRID=Q3WW0YZVRAH8E0K09EGJ

However, the best telephoto lenses do not need to be stepped down, a prime example being the Samyang 135mm f/2 ( a 72.5 mm aperture f/2 apo lens that has virtually a flat field without coma for at least my APS-C), which is now so popular in astrophotography (I love mine). My expectation is that no stepping down is needed for the fixed focal length professional Canon and Nikon apochromatic lenses - but as I say I have still to find out about mine, although there are some great images out there on the net, like those by Martin Fransson.

[London_David]

As a rule of thumb most lenses are sharpest 2 stops down. It varies from lens to lens. A great place to explore about classical photographic lenses is Ken Rockwell’s website. His article on sharpness is very good on this: http://www.kenrockwell.com/tech/lens-sharpness.htm

More than anything, photographers pay high premiums to have lenses that give them more options in more situations. It’s more difficult to make a zoom lens that performs well at f5.6-f22 from 75mm-300mm than a fixed aperture prime f4.9 300mm. Hence why — for similar performance — zoom lenses are more expensive than primes, and fixed aperture refractors can be cheaper than adjustable aperture lenses.

That’s not to say classical lenses won’t perform for astro, they can and do. If the tool works, then great! The technology is only there to provide options and the ease of use.

On the diffraction spikes, personally I would avoid using step down rings as a solution. Step down rings are designed for changing the thread size on the outer thread of a lens so you can screw in filters or accessories of different thread sizes, they cant change the aperture, only the size of the light entrance opening of the lens.

Using rings to vignette the image may reduce the spikes but the aperture is set with the blades at the diaphragm inside the lens. If the intention is to optimise the optics and performance, blurring the light path is not a optimal solution. It may blur out the faintest stars for example. That may be fine for the intended final image, maybe a wide field nebula shot rich in colour — but for me I’m often trying to see deep faint objects not make things pretty so that’s not a solution. It just introduces noise in the whole image to soften the spikes in the highlights.

There are lots of great options at different costs but classical photo lenses are not necessarily better or cheaper. They can be, but it depends the intended use, and how it works with the camera being used.

 

 

 

[gorann]

11 hours ago, London_David said:

As a rule of thumb most lenses are sharpest 2 stops down. It varies from lens to lens. A great place to explore about classical photographic lenses is Ken Rockwell’s website. His article on sharpness is very good on this: http://www.kenrockwell.com/tech/lens-sharpness.htm

More than anything, photographers pay high premiums to have lenses that give them more options in more situations. It’s more difficult to make a zoom lens that performs well at f5.6-f22 from 75mm-300mm than a fixed aperture prime f4.9 300mm. Hence why — for similar performance — zoom lenses are more expensive than primes, and fixed aperture refractors can be cheaper than adjustable aperture lenses.

That’s not to say classical lenses won’t perform for astro, they can and do. If the tool works, then great! The technology is only there to provide options and the ease of use.

On the diffraction spikes, personally I would avoid using step down rings as a solution. Step down rings are designed for changing the thread size on the outer thread of a lens so you can screw in filters or accessories of different thread sizes, they cant change the aperture, only the size of the light entrance opening of the lens.

Using rings to vignette the image may reduce the spikes but the aperture is set with the blades at the diaphragm inside the lens. If the intention is to optimise the optics and performance, blurring the light path is not a optimal solution. It may blur out the faintest stars for example. That may be fine for the intended final image, maybe a wide field nebula shot rich in colour — but for me I’m often trying to see deep faint objects not make things pretty so that’s not a solution. It just introduces noise in the whole image to soften the spikes in the highlights.

There are lots of great options at different costs but classical photo lenses are not necessarily better or cheaper. They can be, but it depends the intended use, and how it works with the camera being used.

 

 

 

Step down filter rings (or homemade cardboard rings) placed in front of the lens do change the aperture and when used in this way they are referred to as aperture masks. They may not be in the perfect place but are often considered adequate and widely used in astrophotography. The best telephoto lenses (unlikely to include zoom lenses as David points out) can produce excellent results wide open (like the Samyang 135 f/2), or may be improved by bringing down the aperture 0.5 to 1 step (see http://www.astropix.com/html/i_astrop/lenses.html). Aperture masks do not necessarily cause vignetting, and may even reduce vignetting, see entry #9 in this thread: https://www.cloudynights.com/topic/518065-step-down-rings/