Tag Archives: refractor

ST80 vs ST102

I recently wanted to purchase an ST80 or an ST102. I was undecided which to purchase and a wanted advert for an ST80 turned up 2 people wanting to sell me an ST80 AND an ST102. In such a pickle I decided to buy both so I could compare and then sell the one I liked least. I thought I’d share my findings here for those interested.

The ST80 is an 80mm f/5 (400mm) OTA and the ST102 is 102mm f/4.9 (500mm)

Both examples are made by Synta and are in Skywatcher blue. You can see the difference in size isn’t massive, with the ST102 being slightly larger. The weight of both tubes is comparable although when weighed “in the hand” I could tell the ST102 was slightly heavier.

Removing the dew shield revealed the lens cells. The ST80 seems to be made from plastic which perhaps isn’t a bad thing as it makes it lighter but it gives me concerns about the durability of the ‘scope. The ST102 lens cell seems to be a combination of metal and plastic, which is perhaps SLIGHTLY better than the ST80. Neither seem to be collimatable.

Viewing down the barrel the ST80 is well baffled and whilst the lens is coated it isn’t fantastic. That being said these are nice and cheap so I can’t really complain.

The ST102 is equally well baffled, but again the coatings leave something to be desired. I think the ST102 appeared a little better than the ST80 but thats subjective with both examples being second hand (although in good condition).

The ST80 has a cast aluminum 1.25” rack and pinion focuser whilst the ST102 has a 2” cast rack and pinion focuser. (Note the focus wheels on the ST102 have been replaced at some point, but this is a small modification). Both have been regreased but experience tells me that if brought new would be full of “syntaglue”. Easily fixed but god knows what the engineers were thinking when they used this “grease”.

All the following shots were taken with my Nikon D70 DSLR at prime focus. I’ve not had the opportunity to test both ‘scopes at night yet due to the weather (snowing here!).

The test chimney was fairly close, about 300 meters. You can see the difference in focal length immediately with the ST102 giving more magnification from the outset. You can also see that the ST80 image appears to be washed out a lot more than the ST102. I’m not sure if this was a function of the weather or if it’s due to the change in aperture. The ST102 definitely seems clearer though and despite it’s fast f/4.9 focal ratio in these daytime shots I can’t see much colour in either shot. I didn’t get a chance to barlow up to take a closer shot with both scopes but perhaps I need to test in that configuration too.



Taking a 100% crop of both these images from the 6 mega pixel originals gives more information. Again the ST102 seems MUCH clearer than the ST80 and I’m beginning to think that the added resolution of the 4” compared to the 3” lens makes a big difference to the contrast too, at least for these daytime shots. I was expecting the ST102 to show more colour as it’s f/4.9 not f/5 but to my eyes I can’t see much difference. Perhaps some moon limb shots will tell more.

ST80 Crop

ST102 Crop

All in all at the moment the ST102 is winning due to the larger aperture, however the ST80 is still the more portable OTA. Night time tests to follow as the weather allows!

Originally posted 24 January 2007


Skywatcher ED100

Well I received my skywatcher ED100 today. I brought it second hand (3rd? 4th?) from astro buy/sell for £500. This seems expensive but it’s the full kit with metal case, eyepieces, finder, diagonal and rings. Initial inspection was good with build quality about average. No real niggles although the dew shield squeeked something terrible when I removed it to inspect the lens cell! The crayford focuser is tight and there was no slop that i could tell. The finder is a half decent 9×50 with a single set of 3 pins for adjustment. Im not sure i like the SW (synta) method of using a collar and just 3 pins to adjust the finder. I think the 6 pin method gives better adjustment but thats just personal preference. I think i will be changing the finder for a right angle finder after getting into some strange positions lying on the ground trying to aim at the moon tonight!

This is the skywatcher setup on my Vixen GP. Please excuse the bare room, we were in mid-decorate!
Tonight was one of the clearest nights i’ve seen all year. Apart from the odd bit of cloud an a bit of wind the conditions were excellent. I first took a look at the moon. The cresent was bright in the sky and fitted my 18mm celestron x-cel eyepiece for a wide view of the surface. I was stunned. The best views i’ve seen so far. Bearing in mind my only comparisons have been through binoculars, a little 80mm f5 achro and a C8 that i never got to use in good conditions. I closely examined the terminator and was amazed at the detail I could see. Craters clearly had central peaks and the mares had pock marks that I’ve never seen before. I took a photo using my Nikon D70 at prime focus.

D70 at prime focus of ED100, 100th/s exposure. Straight from the camera other than a crop and resize for the web.

I was quite impressed with the quality of the image, but I think I need to find a 2” Barlow to increase the magnification to get detail at prime focus. At this point the clouds started to become denser and I swapped to my neximage ccd camera to try and get some stacked images of the moon.

Neximage CCD at prime focus. 500 frames stacked with Registax. Wavelet tweaked.

These images don’t really do justice to the scope as my imaging skills are limited. But I was quite impressed with the quality. I moved over to Saturn to get a shot of the rings but as Saturn was quite high in the sky (in cancer) I found it difficult to align the scope. By sheer chance I ended up looking at M44, the beehive cluster. It looked fantastic, with a myriad of pinpoint stars set in black velvet. One of the sights that we all wait for and love to get. I was amazed, mostly because of the clarity of the night, and how well the scope was performing compared to my other experiences. The longish focal length of 900mm meant that M44 wasn’t framed very well and I had to “scroll” around to see it all. I may invest in a focal reducer at some point to get wide field shots.

The cloud was coming in thicker and I moved to look at Saturn. After lying on my back virtually to align the scope I know I defiantly need a right angle finder! Saturn in my 5mm x-cel eyepiece was very nice. Quite low magnification compared to the C8 but this is as I expected as the focal length of 900mm is less than half that of the C8’s 2032mm. Even so I could easily see the Cassini division. I went to get my Barlow which I had left indoors to see if I could get some banding but after returning (minus night vision) the clouds had rolled in and obscured my view. Looking up I saw that a large dark cloud had completely filled the sky whilst I had been inside. A few patches were clear and I decided to wait it out. Something of a mistake as big black clouds in the sky usually only mean one thing: RAIN! Packed up in something of a panic as the last thing I wanted was water all over my laptop, new scope, and DSLR! I’ve been processing what little images I’ve gotten and wrote this report whilst waiting for the rain to blow over. Seems to have done so, so I’m back out to try and get Saturn again. Wish me luck!

ED100 on Vixen GP with Nikon D70 at prime focus and 70mm cheapo guide scope

Originally posted 3rd April 2006