Eye relief

[vlebo]

A friend of mine has kindly given me a Startravel 120 which has come with the 2 standard Skywatcher eyepieces ( 25 mm wide angle and a 10mm ) plus a Celestron X- Cel LX 8 mm. Looking at Jupiter last night the views in the 25mm wide angle were fine but using the X Cel 8mm I could not achieve focus. I have astigmatism in both eyes and normally use long relief eyepices. Is the fact that the X-cel has 16 mm eye relief the reason I could not achieve focus or is this normal with the Staretravel 120 ? 

Thanks in advance

Chris

[michael.h.f.wilkinson]

This does not sound like a problem of eye relief, but one of limited focus travel. Did the best focus occur with the focuser fully inwards or outwards? In the first case, you might need some extension tube, in the second case the mirror might need to he moved forward.

[vlebo]

The best focus was achieved with there still being focus travel either side of the best view. At best in the 8mm it was blurry but seemed ok albeit much smaller ( therefore probably looked in focus ) through the 25mm wide angle.

[ronin]

Doesn't sound right, and a shame as the 8mm would have given an excellent view of Jupiter.

The stupid answer is you were warm, and breathing  , and managed to fog the top lens of the eyepiece, therefore Jupiter was seen through a film of moisture. In simple terms that is about all I can think of.

I suppose stop breathing isn't a realistic option? :grin:

Best I can suggest is to try again and if it is repeated than take a look at the eyepiece for any fogging and check the front lens for the same.

I take it that you were wearing your glasses at the time.

I am short sighted and have astigmatism but usually find it easier to remove my glasses. The focus difference is minimal and it is easier to get my eye in a more comfortable location.

[michael.h.f.wilkinson]

The best focus was achieved with there still being focus travel either side of the best view. At best in the 8mm it was blurry but seemed ok albeit much smaller ( therefore probably looked in focus ) through the 25mm wide angle.

Maybe the scope had simply not cooled down sufficiently. If a large scope has been stored in a warm house it first needs to reach thermal equilibrium before it can really be used at high magnification, otherwise thermal currents in the tube ruin the image. Bad seeing could be a problem (lots of complaints about that lately), or perhaps a collimation problem (easily solved).

[bingevader]

Conditions? It's been very variable recently. I haven't had a good, clear and steady view of Jupiter for a few weeks.

One night I couldn't get a good image with any EP, another night low power was fine but high power useless and only this week, low power was poor and high power intermittent!

You might just need to try again and be patient!

Cheers

[vlebo]

Thanks  for the replies. The scope is kept in a very cold garage so cool down , I think , is not a problem. I will give it another go asap and check everything is ok and see what happens. I still feel it may be my eyes and the fact that the 8mm has not enough eye relief. That's just my gut feeling while looking through the eyepiece but I am sure you guys know more than I in these situations. Just makes me feel I need to get that large Dob on order

Thanks again

[John]

The X-Cel LX is a fine eyepiece and it's 16mm of eye relief is excellent compared with many eyepieces - the most you generally get is 20mm. At 75x it should have been well within the capabilities of the ST120 refractor despite it not being the optimum high power planetary scope. Even with moderate seeing conditions it should have given you nice views of Jupiter and better than either of the standard eyepieces that come with the scope. It does not have a particularly odd focus position either.

I wonder if one of the lenses in the eyepiece had become fogged for some reason ?. There is a lens in the X-Cel LX design that is right on the focal plane of the eyepiece so any dust / dirt / fogging is immediately obvious when you view a bright object through it. Another thought is that the eyepiece got cold in which case the heat from your eyeball is enough to fog the eye lens of the eyepiece almost immediately you place your eye too it.

[Pig]

Does the Xcel range have an 8mm ? I think not

Not that this would effect the issue

[John]

Does the Xcel range have an 8mm ? I think not

Not that this would effect the issue

Its a good point. The X-Cel LX does not have an 8mm but the older X-Cel series did. I've not used the earlier X-Cel series but the X-Cel LX's seem to have a better reputation. 

[Qualia]

I suppose stop breathing isn't a realistic option? :grin:

That's really not such a silly idea, or better said, thinking about this a few weeks back gave rise to some curious experiments. This first night with Jupiter, I figured that to avoid the misting steam up of the orthos on a cold night I'd simply cover my mouth with a type of thin neck scarf thing, but this didn't really work. So the next night I came out with a drinking straw and although an improvement, I found it quite annoying. Finally, it hit upon me to buy a child's cheap snorkle and use this. The improvement is more than significant for serious planetary viewing.

- - - -

The discontinued Celestron LX range included an 8mm with 20mm eye-relief. The Celestron LX XL range doesn't include an 8mm but rather a 7mm or 9mm with 16mm eye-relief.

[faulksy]

That's really not such a silly idea, or better said, thinking about this a few weeks back gave rise to some curious experiments. This first night with Jupiter, I figured that to avoid the misting steam up of the orthos on a cold night I'd simply cover my mouth with a type of thin neck scarf thing, but this didn't really work. So the next night I came out with a drinking straw and although an improvement, I found it quite annoying. Finally, it hit upon me to buy a child's cheap snorkle and use this. The improvement is more than significant for serious planetary viewing.

- - - -

The discontinued Celestron LX range included an 8mm with 20mm eye-relief. The Celestron LX XL range doesn't include an 8mm but rather a 7mm or 9mm with 16mm eye-relief.

thats a good idea, imagine the neighbours seeing that or anyone, that would make anyone laugh

[John]

thats a good idea, imagine the neighbours seeing that or anyone, that would make anyone laugh

Wearing a balaclava and a snorkel would confirm my neighbours opinion of my sanity I reckon   

[Naemeth]

Wearing a balaclava and a snorkel would confirm my neighbours opinion of my sanity I reckon   

Imagine going out to a dark site wearing a balaclava and snorkel and being pulled over by the police while wearing them!

Back to the OP, it seems either you were pushing the seeing or the heat from your eyeball was enough to dew the eyepiece up - however dew is very easy to spot when it's on the eyepiece - it should look visibly misty rather than just out of focus.

[Moonshane]

I doubt it's an eye relief issue as most eyepieces allow you to draw back your eye a little, to see less field of view if the eye relief is a little short for you. this does not affect focus.  some eyepieces do black out of course (kidney beaning) but you don't mention this. I suspect the fast scope might be the issue but as others say this seems to be a decent quality eyepiece based on online reviews only and you were at pretty low power so it's a mystery to me.

[vlebo]

It is the older LX range and i thought that only had 16mm of eye relief. If it has 20mm eye relief then it must be another factor coming into play. I will take more time next time I am out and try to find the problem. One thing which may make a difference is I recently had a pair of varifocals made for me and also had a pair of distance glasses made which are the ones I used that night. Next time I will try the varifocals for viewing to see if that makes any difference going on the premise that the varifocals are a higher quality lens compared to the distance glasses which were a freebie.

[Qualia]

I gave up worrying about what folk think when I realized how seldom they do

[Pig]

I didn't realise there was such a difference in eye relief between the 2 eyepieces, 4mm can make a big difference

I hope you get to the bottom of the problem.

[vlebo]

I didn't realise there was such a difference in eye relief between the 2 eyepieces, 4mm can make a big difference

I hope you get to the bottom of the problem.

Thanks  I will try and get out tonight and have another go