Skywatcher Heritage 100P vs. ST80 mini comparison

[rodhull]

Well, my 100P arrived yesterday so I had a chance to play with it last night. I'd been on the outlook for a replacement to my ST80 as a grab/go/travel scope since I've been less than impressed with certain aspects of the ST80...I often find it gives dim views, and its CA is very off-putting on the moon and planetary targets, so for less than £100 for more aperture, I thought the 100P was certainly worth a punt...

Conditions were far from ideal: a full moon had already risen behind my house which was at least blocking out its direct light, but it had obviously washed the sky out immensely already, some of the objects I chose to observe were not very high and thus the seeing conditions were poor, with a v. thin layer of wispy semi-transparent haze present for some of the time. I was also being periodically flashed by a neighbour's bright security lights spuriously going off and a garish streetlamp only about 15m away all of which meant that I could never get dark-adapted and I suspect was also causing a lot of stray light to enter the open tube of the 100P (the ST80's long dew shield is very helpful in this respect).

However, I couldn't resist trying them out together and had my ST80 set up on its tabletop EQ1 (without the counterweight and also slung over to 90 degrees so I can use it as an alt-az w/ slow-motion controls) and the little 100P on its Dob mount (which is excellent by the way - more on that later) both on a sturdy wooden garden table.

I checked collimation of the 100P first, which was spot on out of the box which I was relieved to find (my ST80 is also well factory collimated and I've never had to mess with it)...

I was using throughout my £50 Seben 8-24 zoom plus a Baader Q-Turret x2.25 barlow where necessary.

The 100P is a good-looking, convenient little instrument although corners have definitely been cut to get it sub £100 - the focuser is worse than my ST80's (and I don't think that's brilliant!) - there is a lot less travel, and finding the sweet spot at higher mags means you are turning it what feels like only a fraction of a mm to achieve correct focus which was a challenge at times.

The focuser also protrudes into the tube quite a bit (as is sometimes usual for Dobs) but it is extremely reflective which I can only assume will cause contrast issues down the line - when checking collimation, the effect of the focuser protruding into the light path was completely apparent - I had good concentric airy discs but on one side (where the focuser is) I saw a clear "dent" into the shape of the outermost rings - not sure what effect this will have on overall observations though?

Coma in the 100P was apparent to me for I'd say the best part of a third of the outer field of view - only the central portion of any given field shows well-focussed stars - this is a shame and the ST80 has it beat in this regard, but its to be expected for such a fast scope.

However, one of the stranger aspects of the 100P's design is its secondary mirror support vanes. They are very wide and cause quite pronounced diffraction spikes which can be seen on any bright object (Jupiter included). My 8" Skywatcher Dob has extremely thin vanes but the ones on the 100P are easily 3 times as wide...the ST80 focusses stars to a sharp, single point of light, but the 100P's views appeared to be not as pleasant on the brightest stars. I actually like the aesthetics of diffraction spikes sometimes, but when you are seeing them on a planet I wasn't feeling as happy!

Another puzzling aspect to the 100P, is the orientation in which the eyepiece and finder are mounted, which goes against Dob conventions - the eyepiece protudes at 90 degrees from the tube relative to the dovetail and the finder is where you'd usually expect the eyepiece to be - this often meant that I had to get right down on the ground to look through the finder, then hover over above and behind the tube to observe - as long as it was placed at the correct height this wasn't that much of a problem, but I really wonder why it was designed like that. However, all this is forgotten when actively using it since the mount is so nice to use! You can just pick it up and pop it down somewhere else whenever without any hassle...it never slips, and moves with ease.

The mount on the 100P really is an absolute delight - easily the best thing about it - it's sturdy and silky smooth, the scope is extremely well-balanced - the tension knob can be set for varying amounts and I never had any slippage even with the relatively heavy weight of the eyepiece/barlow combo. The ST80 struggles in this regard on its EQ1 and also when attached directly to a photo tripod - with the heavy zoom plus barlow in, and since it's a refractor with all this weight right at the back, slippage is often guaranteed unless you are very careful when mounted in this way.

So, onto optical performance: I have to say that the ST80 overall generally showed much better contrast on stellar targets (planetary and lunar are another story) - the sky definitely looked darker and stars stood out more prominently from the background haze. Now, I'm not sure whether this is due to the larger aperture of the 100P which was gathering more light thus showing more of the effects of light pollution and the full moonlit sky or whether it's simply better optics?

However, when I looked at open cluster M36, despite the view initially being more pleasant in the ST80 (due to the darker looking sky and more well separated stars) - at second glance and once well-focussed, the stars were actually definitely brighter in the 100P and I could make out ever so slightly more of the fainter, more central ones...

I took a brief look at M13 when it rose higher and was just about able to make out some pin-pricks of individual outer stars in both instruments but it was generally a fuzzball! I think the 100P had the slight edge but conditions were very poor admittedly for a true comparison.

The moon, Jupiter and even Mars looked much better in the 100P than the ST80 however. In these poor seeing conditions the ST80 produced a wobbling, flared image of Jupiter that was awash with chromatic abberation all over (but more about the edges of the disc). I'm used to this - and is one reason I was looking to the 100P as a grab/go/travel replacement. The 100P however focussed Jupiter to a much crisper disc with no abberations, and allowed in moments of better seeing to see more detail. Even at the modest highest x112.5 magnification I was working with, I could easily identify the north and south belts of Jupiter, and even got a hint of darker patches of colouration on the surface of Mars in the 100P. Same results for the moon - since it was completely full, only the extremities were showing any shadow and contrast but these looked great in the 100P - no purple halo around the outside there - the full disc was blinding, and once in correct focus it looked sharp.

I split Alpha Gem (Castor) easily with clear separation at only ~50-60x in both instruments - I couldn't get the ST80 to split it with a lower mag...

Overall, I'm quite impressed by the convenience of the 100P and its ease of use but on balance the ST80's more contrasty stellar views looked nicer but again I wonder if this is actually a side effect of the 100P's higher light-grasp - can anyone comment on this?

I really need to test both under much darker skies (which is difficult over the coming few days due to the lunar calendar) and I suspect that under properly dark skies, the 100P might well win out - it's lunar and planetary performance is certainly better than the ST80 without a doubt.

I'll try and get out again tonight before the moon rises and see if conditions are any better if it's clear. I'm still undecided as to whether the 100P is a keeper/replacement to my ST80 yet...

[baggywrinkle]

That's a really useful review as many folk coming on SGL are looking at these types of scopes as a starter and more experienced people looking at them as you have, as a grab & go.

Be interesting to see you darker sky notes.

[rodhull]

Having read a bit on the subject today, the increased contrast of the ST80 is to be expected due to its design: a lack of central obstruction and interior baffle help to cut out light scatter. I suspect a light shield or similar round the end of the 100P OTA could well darken the sky background and improve contrast - I'll see if I have anything lying around in the house that would serve well for this...does all of this sound plausible? Any ideas for a suitable material to use?

[trynda1701]

That's a really useful review as many folk coming on SGL are looking at these types of scopes as a starter and more experienced people looking at them as you have, as a grab & go.

Be interesting to see you darker sky notes.

Yes, I'd like to see what rodhull sees when the nearly Full Moon is further along the lunar cycle to see what darker skies lets him see.

Does anyone have any comment on whether the 100P would benefit from any form of coma correction? Having never had any telescope as short as f/4 or so (50mm refractor years ago, and my f/10 8 currently), would there be any potenial bebenefit? I understand that if you don't already have a corrector, it would be an additional expense, one possibly not worth making for such an inexpensive starter/'grab and go' scope.

Mark

[baggywrinkle]

Having read a bit on the subject today, the increased contrast of the ST80 is to be expected due to its design: a lack of central obstruction and interior baffle help to cut out light scatter. I suspect a light shield or similar round the end of the 100P OTA could well darken the sky background and improve contrast - I'll see if I have anything lying around in the house that would serve well for this...does all of this sound plausible? Any ideas for a suitable material to use?

You might just use some black card rolled around the end and see how that helps..

Some, including me, use a cut down camping mat as a dew shied/light shield.

[rodhull]

Well, I took both scopes out again just before 10 last night before the moon had risen - I tucked myself down the side of the house, away from any interfering ambient lights - results were much better.

The contrast levels between both scopes were roughly matched this time in terms of darkness of sky background. Auriga was higher in the sky than from the first attempt, and the 100P clearly showed brighter stars in M36. When looking at M37, the 100P I saw the sprinkling of stars immediately, whereas in the ST80, I definitely had to look for slightly longer to adapt to the dimmer view. Once adapted the results were similar, but there's no doubt that the 100P's view was brighter overall.

Jupiter was higher too, and the belts certainly stood out more on the ST80 in moments of good seeing, appearing darker and more distinct from the bright disc, but the lack of any chromatic abberation with the 100P again gave an overall more steady and more satisying view.

Later I observed M44, and although the views in both instruments were very similar since it is such a bright, open target anyway, the 100P again won out due to the very slight increase in brightness overall...

I think the first attempt the other night was definitely hampered by stray light and the 100P clearly suffers - I'll be in a very dark coastal location on the weekend, so I'l try to take both scopes with me and have another shoot-out...I'll look to create some kind of shroud I think - pretty sure I have some old thin camping mats lying about...

It's not all rosy though - I still think that the coma is definitely an issue on the 100P as is to lesser degree the very small critical sweet spot for focus at high mags which certainly makes achieving correct focus more taxing. Overall though, it's a satisfying grab/go scope since again, the mount is fundamental to its ease and pleasure of use.

I'll update this as i discover more!

[TrixieLoyalist]

Looking forward to seeing what you find out about this comparison I think I gave a friend the inkling for a telescope so seeing which of the cheapy beginner/travel scopes are better would be helpful.

[happy-kat]

For me another use for a travel scope is day time viewing and the mini newt does not provide that really.

[rodhull]

Absolutely. My ST80 will not be retired or sold for this purpose also (I actually have used it as a basic prime 400mm lens for my camera with a T-ring with better than expected results), but for visual astro purposes, the 100P is probably going to be taking over...

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[rodhull]

Well, I had one spectacularly clear moonless night (observing from about 10pm-midnight) on Saturday whilst camping in a remote coastal location (N. Norfolk) with almost zero light pollution - rated "night blue" on this page(only 2 orange streetlights about 200m away that I could avoid by crouching down behind our car).

The 100P was mounted on my Hama Traveller Compact Pro with the legs fully retracted. I was kneeling next to or crouching over to observe.

The upshot: the 100P is definitely a keeper!

I have never seen the kind of bright, clear views from my ST80 that I got through the 100P that night (I've been to other similarly dark sites with the ST80 once last year in July).

The double cluster in Perseus postively sparkled and didn't look too dissimilar to how it does in my 8" Dob in the back garden under a moderately light polluted sky.

The most visually dense of the Auriga clusters to my eyes (M37) positively exploded with pinpoint pricks of light - a totally different view to the one I got when I first tested the scope.

I found M81 and M82 very easily by pointing it in the general vicinity and as they were almost at the zenith, I got a beautiful view (not unlike the one sketched here. Pretty good for a 4" reflector - the inky darkness of the sky definitely helped, so did the lack of ambient light which was definitely affecting contrast in my earlier tests.

I actually didn't do a complete head-to-head with the ST80 since I was simply enjoying the views and using the mini-Dob mount so much (plus I didn't want the hassle of trying to look at things at the zenith on the ST80's tabletop mount which is tricky and I only have one tripod so would be unable to get a good direct comparison). The mini Dob mount worked great on the tripod - so much better than my ST80 does when directly attached. It's so well balanced (the refractor much less so) - there's zero droop!

I'd definitely not hesitate to recommend it to the beginner as a starter scope or to a more experienced observer as a travel scope. The coma is my biggest issue with it - I wonder if it can be corrected without spending the earth - all the coma correctors I've seen for sale are designed for 2" focusers and are pricey to boot.

I'll still be keeping my ST80 for its terrestrial use, it's guiding ability down the line if I ever get enough cash saved for an imaging rig, plus I like the built-in T-adapter for some lightweight imaging. I already also have a solar filter designed specifically for it so I'll be keeping it as a grab/go white light solar scope until I can get/make one for the 100P.

[NGC 1502]

Thanks for this review, very useful to many.

Regards, Ed.

[bomberbaz]

great final review rodhull, never thought about getting a mount for travel purposes. Think I will get one just for this, will be easier than taking a folding table around with you all the time 

Update: Just found the tripod you mentioned for only 25 quid from Tesco. usually buy from astro shops but at that price it would be silly not too.

[rodhull]

I won't pretend it's the most stable mount in the world if you had its head at a height, but with the legs well retracted it works great, and more importantly folds down super small...perfect for travelling with limited storage space.

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