Question about size of objective lens

[dazzystar]

Hi All,

I understand that bigger is often better but how much better is my question. For example, I currently have an ST102 with a 102mm diameter objective lens. Would something like a William Optics Zenithstar 73 III or RedCat 51 outperform it for astrophotography use and if so why?

Cheers
Daz

[rl]

Bigger is generally better (portability and cost put aside for the moment). The history of telescopes since  Galileo in 1609 to James Webb is testament to this fact. But there are a few important caveats:

• The quality of the two scopes in question has to be comparable
• The sky always has the last word; if the seeing won't support the greater resolution of the bigger scope then you're in for a bit of a disappointment of detail. Objects are still brighter though.

The ST102 is a short-focus achromat made with cheapish glasses. It's great for low magnification wide field views but less so for detailed stuff. The lens aberrations (distortions) are too big for excellent results; the colour correction isn't great and this becomes more important for photography. You will see purple haloes around stars where the extreme red and blue light is not focussed quite tightly enough. It's more of a problem with photography because the camera is sensitive to a greater range of colour with low light compared to the eye. More expensive ED glasses get around this issue. There is also a potential issue with spherical aberration where the focus generally is not optimal- again, not an issue at low  visual magnification. 

With the small apertures of both scopes mentioned, you won't have too much of an issue with seeing compared to bigger scopes.

Either the WO73 or the Redcat will be better for photography. They are smaller but better made with more expensive glasses and designed with photography in mind. But they're also a lot more cash. The WO73 will be good for both visual and photography but will probably require a field flattener (more cash...). I believe the Redcat is basically photography only ..you don't need the extra flattener but the aperture is less (not good for visual). There may be a special diagonal  for it which will allow some eyepieces to be used, but check first

You might want to look at the mount first if seriously going down the photography route. It needs to be good enough to futureproof yourself against changes in telescope while possibly being portable. 

[Stu1smartcookie]

Hi , the ST102 is an Achromat single lens design that will display chromatic aberation when viewing bright objects . This would be the case for photography also , whereas the ZS73 is a doublet , which uses more expensive glass and offers a much cleaner image . Sure for viewing you will lose the aperture but for photography you will gain . 

To be honest i have viewed the planets with a zs73 and whilst they appear small , increased magnification gives sharp views . I have also owned an ST102 which is a fine widefield scope , its up to you how much chromatic aberation bothers you ( ie a false halo appearing around bright objects ) . 

Stu

[johninderby]

I’ve had an ST102 and while it was fine for low power wide field use trying to use higher magnification wasn’t nice due to CA and other aberations. 

One of the other scopes is far better suited for AP due to their excellent quality optics that will allow higher magnification and lack of CA.

BTW it shoild be The history of telescopes since Thomas Harriot in 1609 to James Webb. Thomas Harriot was the first person to use a telescope for astronomical observation not Galileo. 

Edited February 14, 2022 by johninderby

[Robindonne]

One of my first refractors was a 71 mm.  Bought an 80 mm and knew immediately the 71 wouldn’t get much use in the future.    Bought a 100 ed and the 80 was sold.  Bought a 120 ed and again knew for sure the 100 was going to be sold. All ed’s so comparable classes.   I cant share an experience when up-or downgrading to achro or triplet but for me the choice was clear.   The larger showed the most/best views visual.  
 

for Astrophotography i think a smaller apo is fine.  Like your Zenithstar for example

Edited February 14, 2022 by Robindonne

[Peter Drew]

Just to clarify, the ST102 and the rest of the ST refractors have achromatic doublet lenses.  It is their F5 focal ratio that limits the magnification performance as the higher end reveals the effect of chromatic aberration.  These are best suited to low power, wide angle viewing.  A review of posts asking for advice about telescopes shows that the majority want to know the maximum magnification possible for wide field models and the widest field possible for long focus ones more suitable for high power use.   The best advice is "horses for courses".     🙂   

[vlaiv]

Contrary to popular belief - even for astrophotography, bigger is better.

You will usually hear that for astrophotography, size does not matter and you can image everything with even modest 80mm scope.

While that is true - more aperture means faster imaging time as speed of system is best defined as "aperture at resolution". Larger scopes simply have more aperture and hence for given resolution / sampling rate - they will be faster. Another point is that larger aperture telescopes can image at higher resolution than small telescopes.

This is very pronounced in planetary imaging, but also holds true for long exposure deep sky imaging. In long exposure imaging aperture is not sole factor for resolution and things of course depend on mount used and sky conditions, but at smaller side of things, resolution does depend on aperture size (below 8" or so) significantly.

Above is of course related to proper imaging telescopes.  Using achromatic refractor adds quite a bit of twist to the whole story as chromatic aberration is kind of a blur and reduces resolution of telescope further.

[alacant]

1 hour ago, vlaiv said:

for astrophotography, bigger is better

+1

I can remember when this first hit me. We had a visitor with the then popular and oft recommended sw 80. I had an 200mm Newtonian. m33. We both had dslrs. 

The first thing you notice is each frame as it is downloaded. Whilst the 80 just about showed the core as a fuzzy patch, the 200 showed a galaxy.

Enlarging the image from the 80 to make it the same size as the 200 made it fuzzy and no amount of processing would get anywhere near. I got the impression that's you'd need a lot more frames from the smaller telescope to be able to get close.

On the other hand, it was much easier to image m31 with the 80 than with the 200. Other stuff may become important; an eq5, a sw 80 and a rpi go as checked in baggage on Ryanair. Airlifting an eq6 with a 200mm f5 would cost you a fortune. 

@dazzystar most will tell you that the redcat will give better images than your 102 because it focuses light better. Only you can decide what better means though. Maybe go along to an astro club and do a comparison. Words don't really do it justice.

Cheers

 

Edited February 14, 2022 by alacant

[vlaiv]

4 minutes ago, alacant said:

+1

I can remember when this first hit me. We had a visitor with the then popular and oft recommended sw 80. I had an 200mm Newtonian. m33. We both had dslrs. 

The first thing you notice is each frame as it is downloaded. Whilst the 80 just about showed the core as a fuzzy patch, the 200 showed a galaxy.

Enlarging the image from the 80 to make it the same size as the 200 made it fuzzy and no amount of processing would get anywhere near. I got the impression that's you'd need a lot more frames from the smaller telescope to be able to get close.

On the other hand, it was much easier to image m31 with the 80 than with the 200. Also, an eq5, a sw 80 and a rpi go as checked in baggage on Ryanair. Airlifting an eq6 with a 200mm f5 would cost you a fortune. 

Of course - smaller telescopes have their use and are much more portable.

You can also do wide field with large telescope. It is more involved - but 8" F/5 Newtonian is (almost) as fast at wide field as is 100mm F/5 refractor. Trick is to do mosaics and to bin data to get the same resolution as smaller scope. You then need to spend only a fraction of time on each panel of mosaic and it adds up to same time (say 2x2 panels - each panel for 1/4 of the imaging time).

[dazzystar]

19 minutes ago, alacant said:

+1

I can remember when this first hit me. We had a visitor with the then popular and oft recommended sw 80. I had an 200mm Newtonian. m33. We both had dslrs. 

The first thing you notice is each frame as it is downloaded. Whilst the 80 just about showed the core as a fuzzy patch, the 200 showed a galaxy.

Enlarging the image from the 80 to make it the same size as the 200 made it fuzzy and no amount of processing would get anywhere near. I got the impression that's you'd need a lot more frames from the smaller telescope to be able to get close.

On the other hand, it was much easier to image m31 with the 80 than with the 200. Other stuff may become important; an eq5, a sw 80 and a rpi go as checked in baggage on Ryanair. Airlifting an eq6 with a 200mm f5 would cost you a fortune. 

@dazzystar most will tell you that the redcat will give better images than your 102 because it focuses light better. Only you can decide what better means though. Maybe go along to an astro club and do a comparison. Words don't really do it justice.

Cheers

 

Thanks. Is there a UK wide list of astro clubs or do you know one near me in North London / Herts?

[alacant]

https://www.astronomyscope.com/astronomy-clubs-in-london/

Any good?

 

[dazzystar]

Perfect. Thank you!

[Andrew_B]

5 hours ago, vlaiv said:

Contrary to popular belief - even for astrophotography, bigger is better.

You will usually hear that for astrophotography, size does not matter and you can image everything with even modest 80mm scope.

While that is true - more aperture means faster imaging time as speed of system is best defined as "aperture at resolution". Larger scopes simply have more aperture and hence for given resolution / sampling rate - they will be faster. Another point is that larger aperture telescopes can image at higher resolution than small telescopes.

This is very pronounced in planetary imaging, but also holds true for long exposure deep sky imaging. In long exposure imaging aperture is not sole factor for resolution and things of course depend on mount used and sky conditions, but at smaller side of things, resolution does depend on aperture size (below 8" or so) significantly.

Above is of course related to proper imaging telescopes.  Using achromatic refractor adds quite a bit of twist to the whole story as chromatic aberration is kind of a blur and reduces resolution of telescope further.

Learning astrophotography is a lot easier and less expensive with a small scope though, and going big makes the mounting requirements even more demanding and expensive than they already are.