Diagonal question

[DevonSkies]

Hi,

I recently purchased a Skywatcher ST102 f/5 refractor to use as my grab-n-go scope. I was putting it through its paces last night and I have a couple of queries regarding star diagonals. (I'm quite new to all this!)

I currently have an Altair Astro 2" Quartz Dielectric diagonal. Altough it seems to perform well at low magnifications, I noticed something like astigmatism at high mags (above 100x) - basically, stars were appearing slightly elongated unless the focus was absolutely spot on.

I then tested the scope with the diagonal removed (using an extension tube instead) and saw that most of the "astigmatism" went away. More strikingly, without the diagonal I was able to clearly split the double star Rigel in Orion at 200x, whereas the view of Rigel through the diagonal was less crisp and had lower contrast, and hence the companion star was nowhere to be seen.

Now, I have a confession to make! Last week I managed to drop a Cheshire eyepiece into the diagonal (ouch!), and there is now a small scratch near the middle of the mirror (maybe 1.5mm in size). In fact, there are two such scratches very close together. Is it possible that this small mark is messing up the performance of the diagonal at high magnifications?

I am really quite surprised by how much the diagonal degrades the view at high magnifications. I didn't realise a simple mirror could have so much effect! Should I be looking for a more upmarket diagonal, or is it likely that I've damaged my diagonal with my misadventures? I'm willing to replace it, but don't want to waste money if it's likely to make no difference.

Thanks,

Ed

[John]

I don't think the small marks will be causing the image issues but if the mirror has moved a little within the body of the diagonal due to the accident then that could be causing the problem.

I owned a clone of the Altair 2" Quartz diagonal and it seemed to perform pretty well with my ED120 refractor.

Does the diagonal sit squarely in the focuser ? - sometimes they can be skewed slightly by the set screws being tightened.

You could always stick the laser collimator in the diagonal, shine it through the scope and see if it exits the objective lens more or less in the centre ?

[DevonSkies]

I don't think the small marks will be causing the image issues but if the mirror has moved a little within the body of the diagonal due to the accident then that could be causing the problem.

I owned a clone of the Altair 2" Quartz diagonal and it seemed to perform pretty well with my ED120 refractor.

Does the diagonal sit squarely in the focuser ? - sometimes they can be skewed slightly by the set screws being tightened.

You could always stick the laser collimator in the diagonal, shine it through the scope and see if it exits the objective lens more or less in the centre ?

Thanks. I have taken the diagonal apart to check, and the mirror is a thick chunk of glass (or quartz presumably?!) pressed flush against the body of the diagonal by a foam pad. There is nothing that can move, and I have checked the collimation of the diagonal itself using a laser and a mirror - the beam reflected back, bang on the centre spot on the laser collimator.

I haven't yet checked the beam alignment through the whole scope as you suggest, so I'll have a look at that tonight.

[DevonSkies]

Just a follow up on this.

FIrst, I tested a Revelation 1.25" diagonal in place of the original 2" diagonal, and found that the problem with the star shape went away. Pretty sure now that my diagonal was at fault, I bit the bullet and ordered a new Revelation 2" quartz diagonal.

Tonight, despite the bright moon, I had lovely still skies to test the new diagonal with, and bingo! - perfect Airy discs. My view of Castor now appeared as two nice bright spots at 250x magnification instead of two elongated "cats eyes". So somehow I had indeed damaged the original diagonal - either by scratching it or by perhaps minutely deforming the surface somehow so that high magnification views were disturbed.

Anyway now I'm happy, and I'm actually pretty impressed with the performance of the little ST102 coupled with a decent diagonal and eyepieces.

Ed

[John]

Good result !

[Dave In Vermont]

Very good job of troubleshooting the cause of the problem, Ed. Pity about the Altair 2" having been damaged, but you demonstrated excellent scientific-technique in deducing the causative culprit.

You'll go far in this field!

Dave

[happy-kat]

You are acheiving way higher magnifications then I do in my f5 130mmp heritage.

I may have to get my ST80 back from being stored at my Mum's.