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New eyepiece - can't quite get things in focus


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Hi,

Just after a bit of advice. I've just bought a new eyepiece (BST Starguider 8mm) to replace the 9mm that came with my 'scope.

On the whole, I'm very happy with it indeed. With my old eyepiece, M42 was only really visible with averted vision, but with the BST I can look straight at it and pick out detail - (that was as far as I got on my first night out with it last night before frostbite started to set in). That view alone justified the upgrade to me!

The only problem I have is that I couldn't quite seem to get stars to be as pin sharp as they were with my other EPs. I could start off under focussed (is that the right term?) and go through to over-focussed but at no point between the two were stars 100% sharp. I could do it fine with the 9mm and 25mm EPs that came with the scope. I'm not sure why that would be.

Now, I'm being picky - the stars are just veeeery slightly out of focus and I'm very happy with my new EP. I'm wondering whether this could be a sign that I need to collimate the 'scope. Maybe it's only really noticeable when you start to push the magnification to its limits with the new EP, but not with the 9mm & 25mm EPs?

I'd appreciate any thoughts!

Edited by Bongo
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Some nights are not as good as others for clarity.

It could be that there was some thin high cloud or the jet stream was particularly turbulent overhead which was making the seeing a bit mushy and that was the reason you were not getting pin sharp focus.

Give it another go on a clear crisp night and check what you get.

(Yeah like we get lots of those in the UK 🤪).

 

You are also going to higher magnification with the 8mm eyepiece so it will start to show up poorer seeing more than the lower powers.

That all said collimation is always worth checking as it can be the reason you are losing sharpness.

 

Seeing quality can be checked here (set for your own home location)

Astronomical seeing Edinburgh - meteoblue

 

and here for an easy jet stream visual

Jetstream Forecast - Jetstream Map Updated Four Times Daily - Netweather.tv

 

Telescopius is also useful, move the hour slider across to see how seeing varies during the night

Weather Forecast for Cupar (telescopius.com)

 

 

Edited by fifeskies
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I find with my 8mm BST that it is a very fine line to in focus and slightly out of focus, I find its worse on very bright targets (i.e. Sirius / Rigel) to find correct focus but that is probably more due to refractor CA issues. I also find its harder to focus when I don't screw the eye cup all the way up :D

If you can achieve focus on the same target with the 9mm vs 8mm that does seem a little odd to my mind as I wouldn't expect 1mm to make a huge difference when swapping between EP. 

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11 minutes ago, wibblefish said:

If you can achieve focus on the same target with the 9mm vs 8mm that does seem a little odd to my mind as I wouldn't expect 1mm to make a huge difference when swapping between EP. 

Yeah, that's what I thought.

Maybe I was just being a bit clumsy with the focusser, especially since you say it's pretty fine. 

I have to admit, I didn't spend long out there because it was so incredibly cold! I think I'll get the collimation checked, try on another night and spend a bit more time with it and see how it goes.

 Cheers!

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The 8mm BST Starguider is a good eyepiece and should be very sharp, especially in the central area of the field of view.

I have compared the 8mm Starguider with my 8mm Tele Vue Ethos in my 12 inch dobsonian at a star party a few years back and I was surprised just how good the much less expensive BST actually did.

With the screw up and down eye cup section, the positioning is usually up for non-glasses wearers and down for those who do wear glasses when observing.

In your scope the 8mm is giving you 81x magnification which is really just a "medium" magnification with the scope.

Might be worth checking the collimation ?

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1 minute ago, John said:

The 8mm BST Starguider is a good eyepiece and should be very sharp, especially in the central area of the field of view.

I have compared the 8mm Starguider with my 8mm Tele Vue Ethos in my 12 inch dobsonian at a star party a few years back and I was surprised just how good the much less expensive BST actually did.

With the screw up and down eye cup section, the positioning is usually up for non-glasses wearers and down for those who do wear glasses when observing.

In your scope the 8mm is giving you 81x magnification which is really just a "medium" magnification with the scope.

Might be worth checking the collimation ?

I will admit I didn't know that down was glasses wearers good to know! I usually don't wear mine at the telescope though I am tempted as I need them to find the RDF target and often swapping them back on they haze up and then are useless. I also worked out the other night that my right eye is much better at observing than my left when I was trying to find some fuzzies, I guess its slightly stronger. One day I will overcome my dislike of contact lenses and that will solve everything 😛 

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The 8mm BST was my first 'really ? £40 on an eyepiece ? ' purchase 🙂 and works fine in my little 150 heritage dob and 127 mak. , no problems with getting everything nice and sharp. The focus however I noticed from the start is at a very different point to that of the stock EPs, so refocusing always involved some major twiddling of my dobs agricultural/plumbing type focuser  (the mak focus requires the clothes peg mod for me to achieve sufficient precision !)

One thing though ... one of my BSTs, ( couldn't leave the 8mm all lonely now could I ? ) I think it was the 15mm , had a slightly unscrewed lower (silver) section when it arrived, presumably worked loose in transit . Easily tightened by hand, and I did it before using it in a 'scope so no idea if it would have adversely affected focus.

Heather

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20 minutes ago, John said:

The 8mm BST Starguider is a good eyepiece and should be very sharp, especially in the central area of the field of view.

I have compared the 8mm Starguider with my 8mm Tele Vue Ethos in my 12 inch dobsonian at a star party a few years back and I was surprised just how good the much less expensive BST actually did.

That is nice to know .

Now what shall I spent the £500 I saved by getting the BST on 🙂 ? *

Heather

* Not really the way my mind works, honest, just joking.

Sort of.

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36 minutes ago, Tiny Clanger said:

The 8mm BST was my first 'really ? £40 on an eyepiece ? ' purchase 🙂 and works fine in my little 150 heritage dob and 127 mak. , no problems with getting everything nice and sharp. The focus however I noticed from the start is at a very different point to that of the stock EPs, so refocusing always involved some major twiddling of my dobs agricultural/plumbing type focuser  (the mak focus requires the clothes peg mod for me to achieve sufficient precision !)

One thing though ... one of my BSTs, ( couldn't leave the 8mm all lonely now could I ? ) I think it was the 15mm , had a slightly unscrewed lower (silver) section when it arrived, presumably worked loose in transit . Easily tightened by hand, and I did it before using it in a 'scope so no idea if it would have adversely affected focus.

Heather

I have both the 8mm and 15mm BST and like Heather said, both very good EPs; no issues with focus unless viewing is bad then it is a different story? I had a topic a while ago, where I could not focus my 8mm and 15mm and I though it could be collimation (which was ok when I checked), so i think that night the high mag viewing was not good; my 25mm was fine. A few nights after that incident, clear skies and the 8mm/15mm were both in sharp focus. Worth giving the another go.

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On 12/02/2021 at 14:07, Bongo said:

have to admit, I didn't spend long out there because it was so incredibly cold!

Hi there, are you allowing sufficient cool down time for the scope, like you on these cold nights I don't like being out there too long.  Stick the scope out at least half an hour before viewing, at a good angle so it doesn't dew up. 

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On 12/02/2021 at 11:47, Bongo said:

Hi,

Just after a bit of advice. I've just bought a new eyepiece (BST Starguider 8mm) to replace the 9mm that came with my 'scope.

On the whole, I'm very happy with it indeed. With my old eyepiece, M42 was only really visible with averted vision, but with the BST I can look straight at it and pick out detail - (that was as far as I got on my first night out with it last night before frostbite started to set in). That view alone justified the upgrade to me!

The only problem I have is that I couldn't quite seem to get stars to be as pin sharp as they were with my other EPs. I could start off under focussed (is that the right term?) and go through to over-focussed but at no point between the two were stars 100% sharp. I could do it fine with the 9mm and 25mm EPs that came with the scope. I'm not sure why that would be.

Now, I'm being picky - the stars are just veeeery slightly out of focus and I'm very happy with my new EP. I'm wondering whether this could be a sign that I need to collimate the 'scope. Maybe it's only really noticeable when you start to push the magnification to its limits with the new EP, but not with the 9mm & 25mm EPs?

I'd appreciate any thoughts!


As others have said....

1. Ensure scope is fully cooled

2. Ensure scope is collimated

3. Enjoy 🙂

Edited by dweller25
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