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First Post, some questions from my first night!


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Hello all, I very recently began my journey into stargazing and after doing some research have some general thoughts and questions which I think all of you experts out there could help with.

I had originally had my mind set on a Dobsonian, preferably an 8", however I then noticed someone in my area selling a good condition Towa 339 which after reading glowing reviews, I purchased for $250. Overall, I would say it is in very good condition however a few of the eyepieces were / are VERY dirty. In particular, the 4mm eyepiece is extremely dirty so I followed numerous pieces of advice and used some q-tips and little bit of windex. Unfortunately on that particular eyepiece, I notice it isn't really doing the trick. I feel like it may be due to the fact that to actual space of the lens is sooo small that I can't really get great access to cleaning it. Does anyone have any tips?

Additionally, are there any other parts i.e. the actual objective lens at the end of the tube that need to be possibly cleaned? I have heard that this particular scope was known for having great optics, however I am not sure if this was aimed at the eyepiece or the actual internal optics.

Anyway, I set the telescope up last night and popped on the 4mm eyepiece (I know, that was stupid). Of course the moon appeared as a blurry blob no matter how much I focused. My question is....I know the concept of magnification involved here, but why would it be so hard to remotely get an object like the moon in any kind of focus with the 4mm. Keep in mind, I have not used the Barlow lens yet.

I then switched to a 12.5mm and had the same trouble at first...however after some fidgeting with the focus settings I suddenly brought it into focus and WOW...beautiful. I then moved over to Jupiter and was able to get a decent look at Jupiter which looked about the size of a pea maybe, but I could just make out the slightest hints of reddish bands. I could see the moons of Jupiter, but they appeared as bright stars....AWESOME. Once focused on Jupiter, I tried to switch back to the 4mm and again just got blurry nothingness! Finally I decided to go back to the 12mm and look at the moon again, but this time I couldn't get it to focus at all. So the question is....is there a sequence on focusing, i.e. turn the focusing knobs first then adjust the focusing tube?

Finally, can someone explain how I can do a better job keeping the scope from moving on its own even after I tighten the clamps. I am guessing that the balance weight would have something to do with it. I find that even after I set the scope on an object and tighten the clamps...it still drifts when I go hands off.

Thanks so much for your help.....I know some of this may sound silly.

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Hi & welcome to SGL.

Wobbly mount : If the tube drifts after tightening the clamps, try to look

for any play / loose joints, maybe tighten them, especially on the tripod.

It's hard to know why you had a good view of Jupiter with the 12mm at first

and then you didn't. Was the scope still pointing at Jupiter ?

Normally, best focus is achieved by going back and forth thru focus till you

get the sharpest view.

Cleaning of optical parts should be done with great care, and a lot of dirt has

to be present to degrade the view.

It's hard to be specific about sorting the issues without seeing the scope.

Don't know where you are, but if you can find a local astronomy club, most

are happy to help.

Good luck, Ed.

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Finally, can someone explain how I can do a better job keeping the scope from moving on its own even after I tighten the clamps. I am guessing that the balance weight would have something to do with it. I find that even after I set the scope on an object and tighten the clamps...it still drifts when I go hands off.

Thanks so much for your help.....I know some of this may sound silly.

Welcome to the forum, what mount are you using? Unless it is motorised and tracking the object there will be movement as the earth rotates, the smaller mm eyepiece the greater the movement.

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Well from the looks of it you have been enjoying yourself with this new purchase, though be it in a limited way. From what I remember, the Towa 339 was considered in its day to be the best of the Japanese refractors, so on that basis you've made a good choice. I am assuming that this scope has the familiar 80mm (3") objective lens and has a Focal Length of 1200 giving a Focal Ratio of F15.

Calc

1200/80 = F15

So what you may ask. Well the answer of F15 tells you that relative to the size of the objective lens the tube is very long and this will help facilitate a long and narrow light cone which when it arrives at the eyepiece, will generate very little chromatic aberration or colour fringing. This also helps produce a greater focusing point that can help in unsettled seeing, which in general is what most of us experience. There are shorter tubes of this type of scope but they will experience a halo of violet etc around the edges of bright objects such as stars, the moon and planets.

Calc

1200/4mm = x300 Magnification 1200/12.5mm = x96 Magnification

Jupiter was small when using the 12.5mm eyepiece as the magnification is low. To see more detail you would expect to ramp up the magnification by x150 at least. A barlow with the 12.5 effectively halves the focal length of that eyepiece giving you a 6mm eyepiece suppling x200 magnification - that's more like it!! But the barlow supllied might not be of great quality and magnification also applies to optical flaws that may be present. Normal seeing conditions allow x250 magnification with better seeing helping this figure to rise to perhaps x300. I personally would bother with that 4mm eyepiece especially as its now dirty and you've had a go with windex (is that a general cleaner for glass?) Eyepieces need special cleaning fluids such as Baader Wonder Fluid which is applied to the cloth not the glass and allowed to evaporate off once it is applied.

That's a start for now. You might think of purchasing an 8mm eyepiece (barlowed = 4mm) will probably allow you to place the whole moon within it (must have a 60 degree field of view) whilst providing you with the means to take a closer look at Jupiter when the seeing is really good.

Clear skies for now

James

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Thanks for all of the help so far.....your'e feedback generated some follow up questions though.

I have heard in numerous places to stay away from eyepieces starting with HM and SR. The TOWA, I thought was known for great optics, yet the eyepieces are all HM and SR...what gives? Also, this scope has .965 size, so the question is...are they that good that it's not worth it to upgrade to the average 30-50$ eyepiece on the market?

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On Cloudy Nights Forum, select the heading 'Articles' and then select 'How to...' section. Half way down is an article by Allister St. Claire (17th April 2003). The article itself isn't relevant as a whole only the introduction, in which he confirms that no one makes the .965 eyepieces as the 1.25" and 2" are the preferred sizes and goes on to argue that it's no great loss as the majority that came with their scopes are of poor quality.

So, not good news really and reinforces your early quoted disappointment. However, I have noticed in my digging around that Orion (U.S scope maker) actually supplies n adapter that will allow you to use 1.25 size eyepieces. The part No. is 7159 and it will cost you $20.95. I have no experience as to whether it works and if it does, how good it is but for now it represents a 'get of jail card' to help you continue observing with your scope.

I hope this is of some help to you.

Clear skies

James

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