A very unfair fight; ST80 vs FC-76DCU

[Stu]

I mentioned this elsewhere and thought I would give it a go. It’s obviously a very unfair comparison but hopefully does illustrate something of the differences between an entry level, fast achro and a longer focal length, high end fluorite apo.

EDIT for the record the ST80 is currently £128 at FLO including rings, finder, 45 degree prism, a couple of basic eyepieces and a Barlow, while the FC-76DCU is £1285 for the barebones OTA, over ten times the price, hence the unfair comparison comment!

If I do this again I’ll pop them on the AZ75, but for tonight Mrs Stu wanted me in watching Better call Saul, so I didn’t have too long and mounted them side by side on the ScopeTech. This worked ok, but the alignment was quite a long way out so I had to recentre when switching between scopes. Not ideal.

I tried two pairs of eyepieces; 17.5mm and 12.5mm Morpheus’ which gave x32 in each scope. For high power I used a 3 to 6mm and 2 to 4mm Nagler zoom. The 6 and 4mm gave x100 in the ST and x95 in the FC, and double those figures at the short end of the zoom. It was a very handy way of keeping things equal on the eyepiece front. The ST80 was equipped with a Baader BBHS mirror whilst the FC had a Televue Everbrite, just trying to keep thing equal again.

My first views were through the Morpheus’ at the Moon. The ST80 first, and it looked quite sharp, I could see part of Schroter’s valley and some nice terminator detail. Some CA around the limb of course. Switching to the 76mm and things tightened up a little. At these powers things were not so dramatically different; no CA around the limb and a sharpening up of detail ie I could see the valley much more clearly, plus terminator detail looked better defined with better contrast between black sky and lit up areas. Mountains peaks were brighter for example.

Switching to high power and that’s when things really changed. I can only describe the difference as like observing with a veil over the front of the scope with the ST and then removing it with the FC. The valley was finely etched against the surface, and I could clearly see the little kink in it which was vague in the ST. The upper section was also clearly visible. Aristarchus was basically featureless in the ST, just a bright blob, but in the FC it showed the brightness variations in bands down the crater walls and I could see the base of the crater separately defined. On the terminator, I picked out one particular crater with a clean arc of bright light around the rim, the middle filled with black in the FC but awash with diffuse light in the ST. There were a number of smaller craters which were either not visible or very hard to see around Schroter’s valley which were clear in the FC.

I checked a few other targets to verify what I was seeing. Castor appeared as a pair of unequal, tight white bull’s eyes with a single diffraction ring, albeit a bit broken up by the seeing in the FC. In the ST they were larger and messier, yellowish green with other colours thrown in and the separation was harder to see, although still obviously there.

Rigel was a similar story on the primary, with the tiny secondary easy to separate from it in the FC but harder to pick out against the flaring star in the ST. Still visible though.

The star test in the ST didn’t actually look that bad in terms of similarity between in and out focus. The diffraction rings seemed equally well defined either side, although the colour was much more yellow and they were smeared by CA. The FC was much clearer, white and with little or no CA. I will repeat this with the FC as I did this very quickly.

Anyway, a picture paints a thousand words as they say. These images were taken with the standard phone app and the x1 camera on my 11 Pro. I took them with ‘Live’ enabled and realised that the phone is clever enough to pick the sharpest image from the sequence, a simple form of lucky imaging. The first pair of images were at x100/x95 respectively and are the best of 3 or 4 taken, no processing. The second pair are cropped from the main image and processed a little to bring out some more detail. The final pair shows the CA and detail differences on the terminator. The results speak for themselves and are a good representation of the differences I saw at the eyepiece.

Does this make the ST80 a bad scope? No of course not. A fast achro is always going to struggle at high powers due to CA and SA. They are great for low and mid power observing of wide field targets, whilst also still showing some reasonable detail at higher powers. The focuser is a basic single speed R&P. It showed a little image shift and was more difficult to nail the focus. These scopes are inexpensive and less of a stress and worry to take around with you or use for outreach, and at the end of the day any scope is better than no scope. You can still have a lot of fun with an ST.

The FC showed its optical capabilities clearly and was much more pleasant to look through, particularly at high powers; the aesthetics of the view spoke for themselves. I cheat and use Feathertouch focusers on my Taks so could not do a direct comparison with the standard focuser. The dual focus makes such a difference in terms of easily achieving accurate focus though.

So, a highly unfair comparison with an expected outcome, but both scopes have their place and I will keep hold of the ST80 for my daughter to use if she starts paying more than a passing interest.

 

[RobertI]

A nice demonstration of the different capabilities of the two scopes. 👍

[Alan White]

Nice comparison between scopes and the images help as well.
The outcome is of course as many would expect with a short Achro and a longer Apo compared.

I think the forum has plenty of room for scope comparisons, not to make any one scope look good,
but to show the differnces that do exist.

Nice one @Stu and I hope Mrs Stu enjoyed watching Better Call Saul with you as well.

 

[MalcolmM]

That's really interesting. If you get a chance it would be very interesting to compare say open clusters/DSO's and wide angle views (the ST's forte). 

I replaced a SW102 Star Travel with a 100DC. Very obvious difference on moon and planets at medium to high power. I also felt there was a big improvement with open clusters but this was not compared side by side so could have been combination of experience and sky conditions as much as anything else.

Malcolm 

[Stu]

2 hours ago, MalcolmM said:

That's really interesting. If you get a chance it would be very interesting to compare say open clusters/DSO's and wide angle views (the ST's forte). 

I replaced a SW102 Star Travel with a 100DC. Very obvious difference on moon and planets at medium to high power. I also felt there was a big improvement with open clusters but this was not compared side by side so could have been combination of experience and sky conditions as much as anything else.

Malcolm 

Thanks Malcolm. Yes, I’ll give that a go when I get a chance. It might have to be a couple of longer focal length plossls to keep the mags and Afovs similar.

[jetstream]

@Stu Great report!

What struck me was the impression of the ST80. For someone starting out or not wanting to spend much the lower power lunar views could give some nice detail. And no doubt way ahead of my ultra cheap first scope the Tasco I had as a kid. Widefield, filtered large nebula views in the ST 80 would be good as well I believe, such as the North American nebula.

I think the comparison is a valid one : it shows the ST80 is very capable in its intended role and it also highlights the difference in optics that more money can buy.

I wish I had access to this kind of info when I was young, I would have went and bought an ST80 immediately to try out and known what to expect.

Thanks, Gerry

[Merak]

It's nice to see comparisons, regardless of what the scopes cost.

I have the meade clone of the ST80 and it's good for what it is - a very light grab n go, starsurfing scope that can fit on a light photographic tripod.    The view of the Moon is quite pleasing at low to moderate powers too.   I've finished work at 2am before and it was clear so grabbed the scope for a quick 5 min look before bed, which turned in to 45!

Did find some coma at the last 10-15% when using a 24mm APM UFF but I can ignore that.

As for the Tak, I've never looked through one but I know they've highly regarded as a brand in the 30 or so years I've been astronomising.   Maybe if FLO get some stock of one of the smaller Taks I might get tempted....

Mark  

 

Edited April 11, 2023 by Merak
Typo

[Dazzyt66]

I flipping well love my ST80 - I use it A LOT more than my 150 Mak.

The ST80 is what it is, but as an entry level scope I don’t think it can be beaten IMO 🤔

[Stu]

11 minutes ago, Dazzyt66 said:

I flipping well love my ST80 - I use it A LOT more than my 150 Mak.

The ST80 is what it is, but as an entry level scope I don’t think it can be beaten IMO 🤔

Agreed Dazzy 👍

[Louis D]

If you have a Green #56 or Yellow-Green #11 filter on hand, try repeating the comparison on the moon with a filter in place on the ST80.  I've found a light green filter will greatly suppress the violet and red-orange flaring going on in the fast achromat on bright objects.  This then vastly improves sharpness by removing most of the veil of unfocused light splashed across the field of view.  Sure, you're left with a greenish view, but it's a much sharper view.

Try Venus next time.  It really sharpens up nicely with a green(ish) filter in the ST80.  It goes from a flaring blob of violet/orange-red light to a (mostly) sharp edged greenish disk in my ST80.

[Paz]

The ST80 is great value for money and is very light for the aperture so you can get away with using a cheaper or lighter mount. Various tricks can give it a boost for little additional cost like the semi apo filter and other filters,  and stopping down the aperture.

[Franklin]

7 hours ago, Paz said:

Various tricks can give it a boost

Neil English has written a whole book about this little telescope called "The Short Tube 80 Telescope. A users guide". A very good book and full of hints and tips on getting the most out of your little achro.

[Coco]

Better Call Saul - superb!

I wonder what the difference between FC-76DCU & ED80 Pro would be ?

[Stu]

3 hours ago, Coco said:

Better Call Saul - superb!

I wonder what the difference between FC-76DCU & ED80 Pro would be ?

I suspect those two would be much more evenly matched, the ED80 has an excellent reputation 👍