Skywatcher 6" Refractor focusing

[hippie]

Recently acquired this 6" refractor scope   Used it for the first time last night  With great views of Orion, M79, M41, Rigal, Sinus, Using my BST lenses 25mm 12mm was fine with good clear views,  But when i tried using the 8mm & 5mm BST Lenses both would not focus clearly ?,

Is the scope to fast for these lenses being F5? I have the same problem with my 10" dob F4.7, with both these lenses, 

Do I need to start saving for better EP's  .

[emadmoussa]

Does you refractor  have enough back travel?

[coulthamst]

Usability of higher power eyepieces wil depend (as ever) on sky conditions. It has been a very poor year in that respect here in Yorkshire,and although I don't know if Dorset has been the same, I would suspect that has been the problem

[John]

It should focus pretty sharply with those eyepieces - even the 5mm is only giving 150x. You will see loads of chromatic aberration on Rigel and Sirius though which may well affect the crispness of the images of those objects. How do the 8mm and 5mm BST's do on Jupiter and the Moon with this scope ?

[hippie]

It should focus pretty sharply with those eyepieces - even the 5mm is only giving 150x. You will see loads of chromatic aberration on Rigel and Sirius though which may well affect the crispness of the images of those objects. How do the 8mm and 5mm BST's do on Jupiter and the Moon with this scope ?

John I haven't tried it on the moon or Jupiter yet to be able to say,  But when using my 10" dob a few weeks ago i couldn't get any better results with the 8mm or 5mm looking at Jupiter from out in the forest with hardly no light pollution ?,

When looking at M71 & M49 last night  with the 12mm lens was fine as soon as i tried the 8mm lenes I couldn't get it to focus resulting in a blurred image The colour seemed true to the 12mm lenes it is just fine focusing problems I seem to be getting with these 2 lenses,    My 2" 20mm nagler televue is perfect also which is my favorite lenes in both scopes.

[John]

These eyepieces should be able to produce sharp images in both your refractor and dobsonian. The 5mm might be pushing things under poor viewing conditions with the dob but should still work well enough on the Moon.

The fact that both these eyepieces won't produce crisp focus in either scope makes me wonder if there is an issue with the eyepieces now. 

Do you have access to any other scopes to try them in ?

[hippie]

These eyepieces should be able to produce sharp images in both your refractor and dobsonian. The 5mm might be pushing things under poor viewing conditions with the dob but should still work well enough on the Moon.

The fact that both these eyepieces won't produce crisp focus in either scope makes me wonder if there is an issue with the eyepieces now. 

Do you have access to any other scopes to try them in ?

I don't have access to another scope at the moment John, So i will get in contact with alan  at Skies unlimited where i bought them from and see what he has to say !

I find it strange that I should be getting the same problem with this reflector scope I have just purchased, As i was thinking that it was a collimation problem with my Dob!

Which seems to prove it isn't that  now.   

[hippie]

I don't have access to another scope at the moment John, So i will get in contact with alan  at Skies unlimited where i bought them from and see what he has to say !

I find it strange that I should be getting the same problem with this reflector scope I have just purchased, As i was thinking that it was a collimation problem with my Dob!

Which seems to prove it isn't that  now.   

Hi John Update on my 8/5mm lenses  I emailed Alan at Sky's unlimited and he replied with :-

Hi Ian

Good to hear from you again.

I am sorry to hear that you are having problems with the 5mm and 8mm BST.  My thoughts are that as they are both fast telescopes that the problem may lie there.  I think the best thing is to send them back and refund you as for whatever reason, they don't seem suitable for your scope.  It is extremely rare that we ever have a problem with these eyepieces so I think it is probably a compatibility issue.

​If this turns out to be the case which was my suspicions in the first place I am left wondering what EP's to replace them with now?, 

I do like the televue Nagler lenses and will be considering replacing them with these EP's apart from the price of them?. 

[moriniboy]

Alan is an absolute star, what a Gent, you can't get better than that.

[hippie]

Alan is an absolute star, what a Gent, you can't get better than that.

My same thoughts exactly He is so very helpful always, I would recommend him to anyone looking for good quality lenses & advice! .

Those BST lenses are really good for the price Alan sells them at of £4 9.00 each!    

[ronin]

First I would have suspected conditions, the jetstream is rarely thought of but is highly relevant. Go to the US and all weather maps have the jetstream highlighted on them. And it has been over the UK quite a lot recently.

On the refractor I would suspect the scope at f/5 there will be CA, however being the condition it means that different colours are at different positions so the image created cannot have a sharply defined position. There isn't one.

Then with the diameter even the same colour is not at the same focal plane (SA), adding to a non-sharp prime image.

Basically although the scope is easy to use the image that the objective produces will leave a lot to be desired, and the eyepiece cannot correct this. Not sure therefore that anything will do what you want therefore. You may find that the BST's are as good as it gets. Had the scope been f/7 then I think what you see would be vastly improved.

The objective could also just not work well, I have 2 identical small f/5 refractors, one is better then the other when you compare them side by side.

On the reflector, maybe just too fast, hard to say. Do you know anyone to lend you say an 8mm TV plossl? They are said to be good to f/4 and should therefore give an indication. But at f/4.7 a coma corrector is certainly an item to be considered.

I think better eyepieces may be useful on the reflector, but I really doubt the refractor, I think too many aberrations on that to realistically get anywhere.

[F15Rules]

I have a Celestron 6" F8 refractor (so a focal length of 1200mm), and have no problem at all at 5mm which on my scope gives about x240 and is still very sharp (although with recent lousy atmospheric conditions I have struggled to get above x180 due to the seeing, and there is nothing anyone can do about that!  ).

I've not used an F5 6" but I could imagine it would be even more susceptible to the poor seeing conditions - ie the steadiness of the image is very poor, with a rippling or boiling effect with just the odd moment of clearer, calm and therefore sharper views.

My scope is collimateable (not all refractors are), but if yours is it's worth checking the collimation and if necessary adjusting it. Check the concentricity of the out of focus and in focus rings too...you should see a series of rings inside each other, and perfectly circular and in proportion to each other... 

If the inside central ring is noticeably off to one side as compared to the other rings, then your lens needs collimation.

Hope you can sort the issue out  ..

Dave

[Guest]

I have a Skywatcher 150mm F5 Star travel.I am visiting Swanage next week.Don't know how far you are from Swanage.I could bring it down and comapre it and my set of eyepieces.

[hippie]

I have a Skywatcher 150mm F5 Star travel.I am visiting Swanage next week.Don't know how far you are from Swanage.I could bring it down and comapre it and my set of eyepieces.

I am in Christchurch just across the bay from swanage, May be weather permitting we could meet up and compare EP's,

That would be a great help Thanks,  Do Keep in touch and hopefully we can meet up .