A Bit Disappointed

[lavoisier]

I bought a Skywatcher 120 Evostar(second hand) a couple of weeks ago but have only just got it out to observe tonight. I was a bit disappointed when I turned it on Jupiter. I am not expecting Hubble type images but I couldn't get any image at more than x100 to focus. I did see some banding on Jupiter but I have bought a 4mm and a 6mm eyepiece for planetary work and I also have a x2 Barlow which should give me x200 with a 10mm eyepiece(my scope is f1000).

Can anybody suggest what may be wrong?

Thanks

Paul

[noakesy]

check the colimation ie mirrors are alingned, or not enough focal length

[lavoisier]

Can I ask what you mean by not enough Focal length? Are we talking extension tubes here?

Thanks

Paul

[Psychobilly]

Isn't the Evostar 120 a refractor?

Evostar - Skywatcher Evostar 120 (EQ3-2)

Billy...

[RikM]

Refractors normally should not need collimating unless they take a serious whack and I think you'd probably have to send it away to be worked on. But that may not be the problem. Certainly the seeing has been pretty awful for me the past few clear nights so I have been limited to 100-150x magnification before everything turns to mush. I don't know the Evostar refractors but I would imagine it to be similar to my Phenix 127mm f/9.4 and the resolving power on most nights is not what you'd call terrific.

Wait for a good night with steady air and don't push the magnification too much to get a better feel the capabilities of the scope.

Rik

Rik

[perrin6]

More than likely bad seeing. Also was the scope left to cool to the outside temperature? It wont be at it's best with high-powered viewing unless acclimatised. You could also try observing Saturn in the early morning as observing conditions are often at their best then. I test the seeing conditions regularly during an observing session at high power on close double stars I am familiar with to give me an idea how conditions are, if they are clear and steady I quickly skew over to a planet if there is one available !

[John]

Hi Paul,

The Evostar 120 is a 4.7" F/8.3 refractor I believe so I doubt very much if collimation is to blame. The focal length is 1000mm and it's potentially a fine planetary refractor.

I'm surprised that you cannot see any details at all on Jupiter. Even on nights of poor seeing you should be able to make out the slightly flattened disk, the main cloud belt and up to 4 moons. On some nights however that's all you will see, even at 200x - 240x which is about the practical limit for your scope. The additional details (some extra cloud bands plus the great red spot, if it happens to be on the right side of the planet) will gradually be able to be picked out with patience, and in moments of better seeing conditions. I've been finding 150x - 180x better with Jupiter in my 6" and 10" scopes - it's not been "taking power" well this time around.

Other reasons for detail being elusive could be the planet being low down in the sky so there is a lot of atmosphere getting in the way, the scope might need more time to cool - 30mins to and hour on cold nights, and heat rising from surrounding houses with all the central heating on !.

My suggestion would be to keep trying and keep to 150x. The more subtle details on planets don't jump right at you - they need to be teased out.

Hope that helps.

Edit: 3 posts while I was typing that - I need to type faster !

[noakesy]

soz didnt know it was a refractor

[lukebl]

In fairness, I doubt that there is anything wrong with the scope. Jupiter is getting low now, seeing is poor, and people often have unrealistic expectations of what they can Actually see.

It takes a lot of practice, and waiting for moments of good seeing, to tease out the detail on a small disk.

[Steve C]

It may have been the seeing quality, or should I say lack of it, that may have had a negative effect.

I've noticed, certainly in the area where I live, that although I've had several clear nights, the seeing quality has been questionable to say the least. It is more noticable at higher magnifications.

Where was Jupiter when you were observing it, was it getting close to the horizon as that in itself will lessen the quality of view due to the fact you're having to look through a lot of atmosphere.

Collimation may be a factor, however thats easy enough to check with a refractor.

[John]

I'd put collimation last on the list to check - F/8 refractors are reasonably tolerant of a little mis-collimation.

[lavoisier]

Wow, thanks to all have replied, awealth of information. The scope was set up in the afternoon about 2 hours before observing. It is kept outside in a garage so it shouldn't need much to acclimatise.

I could see a belt on a small disk but only using a 10mm eyepiece, any higher magnification turned to mush.

Jupiter was fairly high in the sky(as high as it can be at the moment) as it was just dark at 6-6.30pm.

I'm not sure about collimation, I have a collimation eyepiece with a mirror at 45 degrees but I'm not sure how it works. I know how to adjust the lens cell I just don't know how to use the eyepiece(any ideas?)

Thanks all again

Paul

[John]

This web page describes the technique I use for refractor collimation:

Refractor Collimation

But I'd like to reiterate that I doubt the collimation is causing the lack of detail - seeing conditions are more likely to blame. You won't see anywhere near the level of details that the photos taking though similar scopes show - but I exect you know that !.

[lavoisier]

Yes I did but thanks John. I shall have a quick look through my newly named 'Cheshire Eyepiece' just to be on the safe side. I think perhaps I was expecting to go to X200 + and get focused views of Jupiter. Oh well back to the real world.....

Paul

[BenM]

Test it on a more dependable and alternative target so you can be certain it's not an optical problem. Try a target at 1000 feet so the atmosphere won't be the issue.

FWIW, I'm working with a 5" scope now (an SCT) and I'm forming the opinion that 100x is about the limit. You can go to higher powers, but you just get a dimmer image, less field of view, and a ridiculously small exit pupil. I think the rule of thumb 50-60x per inch of aperature is overblown. While this 5" does focus and I can use nearly 50x per inch, it's not any better than 20x per inch. I cannot see additional detail on Jupiter for example, I cannot see the E star in the trapezium. At 100x, it's easier to focus, I have a generous exit pupil, I can probably see better contrast on the surface of Jupiter and although the trapezium is tighter, the view is just as detailed. This experience is over several different nights viewing where the seeing was mostly good but varied slightly. Last night was the worst because I started later and Jupiter was lower. I'm still working out my own rules of thumb, but I would say 10 to 30x per inch of aperature is where to work 95% of the time.

For 120mm unobstructed at the shorter focal length (1000mm) than the SCT I'm trying, I would suggest a high quality 6 or 8mm eyepiece like a Televue Radian (for planetary use). I wouldn't blame you for thinking 4 or 5mm would work. Perhaps it will without the atmosphere (at 1000 feet) or when the atmosphere is cooperating, but I would guess that 8 to 6mm will turn out to be your best bet (do try everything you can).

[lavoisier]

Thanks Ben, very helpful advice. I shall just play it as I go along.

Paul

[RikM]

My best combo in my refractor is 20mm plossl + 2x barlow (I haven't yet bought any really good eyepieces. 1200mm / 10mm = 120x.

Rik

[brantuk]

Don't think it's been mentioned yet but make sure you don't have dew on the optics. this will make any view poor to invisible. And start with low mags and build up to higher mags gradually till you reach what's best for the current seeing conditions.

[stafford_stargazer]

As kim says dew can make it terrible,i had this problem only last night,on the eyepieces,also what eyepeices you using?

My best views of joop last night were around 7.30,as that pretty much soon as i get in from work,lots of banding,but best with 15mm eyepieces in my mak 127 1500mm focal lengh,100x mag,not alot of mag but better than the 10mm standard skywatcher gave by far and less power.

Gaz

[Dave-Ryan]

I've been using a 10" LX200 for over 6 years and last night (which I did view with it) I founed was the worst seeing I had ever had under a moonless sky.

[LukeSkywatcher]

It could be down to the barlow.

[ollypenrice]

I know the scope quite well and rate it. It just might have had a bump but like the others I think the seeing (this is the degree of turbulence, not the transparency) is to blame.

One thing that people do with these scopes is 'settle' the lens elemnets because the lens mounting cells are pretty primitive, but effective. Stand the tube vertica on a hard surface, tiled, for instance, with the eyepiece end on the ground.

[ollypenrice]

Almost certainly the turbulent seeing but try this: sit on a chair on a hard surface and hold the scope vertically between your knees with the eyepiece end on the ground. Now listen to the radio and spend ten minutes tapping the tube with your fingers, rotating it occasionally. This was a well known trick in the heyday of these excellent budget scopes and is intended just to settle the lens elements which can get slightly misaligned. Tap, don't bash! I came a cross this on Ed Ting's scope reviews website. I travelled to Spain with my 120 and also to Majorca before moving permanently to a dark site. It gave excellent views. On the planets and moon it benefits from a minus violet filter or Baader Fringe Killer.

I still have the six inch version, though it rarely gets used these days, which is a bit of a shame because they are not at all bad and an incredible bargain.

Olly

[Mr Spock]

I have a Helios 120mm which is an earlier version of the Evostar. The Supplied 10mm eyepiece and barlow are rubbish. Also, x200 is the maximum for this scope due to chromatic abberation. With that combination you will get mush.

With a good 5mm eyepiece and a minus violet filter this scope is very capable.

[lavoisier]

Strangely enough the eyepieces have been mentioned before, but the best image I got was with the skywatcher 10mm that came with the scope. The Barlow is new too. I did however find I had the locking screw on the focuser tight which caused some problems, maybe that contributed. Glad to hear it's not a duff scope. Hope to get better images in the future.

Thanks all.

Paul