The Moon 8" vs 14" comparison and foggy corrector plate

[MarsG76]

Hello all,

I wonder if anyone can relate to what happened to my SCT.

On the night of 1st October I had the opportunity to not only observe the moon but also to compare the views of the moon between the 8" SCT and the 14" Dobsonian.

Initially at low power, 50X or so, the views were obviously brighter and somewhat crisper in the 14" scope compared to the 8". When I say slightly, I'm saying so little difference that unless I was looking at the moon through the both scopes one after the other, I would have thought that the views were the same.

Of course the similarities ended when I started to view the moon magnifying at the 200X and 400X mark in both scopes. Not only there was an obvious amount of fine detail visible through the 14" but the contrast was much lower through the 8".

At 400X the contrast was ridiculously low through the the 8". The view through the 14" showed a nice bright crisp image with lots of little craters dotted through Clavius, the wrinkled ridges of Copernicus with obvious ejects plains streaking out from it and the central mountains were distinct three visibly lit triangles.

Through the 8" the view was milky and almost featureless, the detail was there but it was soft. Soft not just in detail but it had a soft buttery look to it, with the blackness being a hazy grey glow, definitely not like I remember observing the moon at this power in the past, what happened to the contrast in my SCT???

Needless to say the views through the 14" more than ran rings around the 8 compared to what I saw through the SCT but I never had such bad views through it before during a night of good seeing, I know that the dob is meant to deliver higher contrast but this was ridiculous... I knew that's something is not right.

I had a look thought the SCT but without the eyepiece in the diagonal and what I saw was a very hazed over corrector plate... No it was not dew, the haze was on the inside!!! How da, what da??? I thought, there is always a rear cell on the SCT, if not the focal reducer than the skylight rear cell, so how did the moisture get in there?

During my last imaging session of M74, my subs were quite hazy, and I think that was caused by the same hazy corrector plate issue.

Looks like I'll be pulling off the corrector plate tomorrow to clean the inside of it to restore the high performance of my SCT I enjoyed in the past.

Googling the symptoms it looks like it is quite a common occurrence caused by out gassing of the lube or paint used in some SCTs.

Any one out there had a similar issue?

MG 

[michael.h.f.wilkinson]

No issues yet with my (nearly) 22 year old C8, but I have heard of it. On nights of good seeing, I have used my XW5 (normally reserved for use in my F/6 frac) in the C8 and been surprised at the crispness of the image. Fog on the corrector plate (outside or inside) does rapidly degrade the image. Be sure when removing the corrector to mark the orientaion carefully, so it goes back in the same orientation afterwards

[xtreemchaos]

great compo Mg, looks like the big Newt Wins by a mile but it should, I gave my c8 to my son a few years ago because it was in the loft and not getting used with it being too big to use in the obsyroom window thats why I mainly use fracs Nowa days , but I do miss it when Jupiter and Saturn are in range and the moon close ups where amazing , id love to try one of the 14 inch jobies, that might be a nice compo for your 14 inch Dob. very interesting read thanks . charl

[MarsG76]

20 hours ago, michael.h.f.wilkinson said:

No issues yet with my (nearly) 22 year old C8, but I have heard of it. On nights of good seeing, I have used my XW5 (normally reserved for use in my F/6 frac) in the C8 and been surprised at the crispness of the image. Fog on the corrector plate (outside or inside) does rapidly degrade the image. Be sure when removing the corrector to mark the orientaion carefully, so it goes back in the same orientation afterwards

The scope manufacturers must be using different lubes and paint since 2000 perhaps... but 22years old and no issues just goes to show that the phrase "they don't make them like they used to" is fact...

I'll definitely will be careful with the orientation of the corrector.

[MarsG76]

20 hours ago, xtreemchaos said:

great compo Mg, looks like the big Newt Wins by a mile but it should, I gave my c8 to my son a few years ago because it was in the loft and not getting used with it being too big to use in the obsyroom window thats why I mainly use fracs Nowa days , but I do miss it when Jupiter and Saturn are in range and the moon close ups where amazing , id love to try one of the 14 inch jobies, that might be a nice compo for your 14 inch Dob. very interesting read thanks . charl

Thanks Charl,

SCTs do shine on planetary detail... I did compare the 8 & 14" on Jupiter and Saturn earlier this year... imaging wise the SCT was way better, but that might have been that the SCT has a electric focusser and I could hit the focus spot on... but during one night in particular, 24th February, the view of Jupiter in the 14" dob was amazing.

At 470X the planet was MASSIVE and there was so much intricate and fine detail within the many different shades in the cloud bands, White spots, breaks, waves and the GRS just stood out with some subtle shading within, namely there was a darker spot in the middle.

The moons were actual discs at that power... I couldn't stop looking.

Saturn detail was similar to the 8" during the best views that the 8" ever showed except that it was a lot brighter and that there were not only more moons visible around it but much brighter and easier to see. Saturn was huge too.

Ofcourse those views were during the nights of the best and most transparent seeing. During average or less than ideal nights the 8" is better and more comfortable to observe planets through... the larger mirror is more susceptible to atmospheric distortion than the 8.

[michael.h.f.wilkinson]

2 hours ago, MarsG76 said:

The scope manufacturers must be using different lubes and paint since 2000 perhaps... but 22years old and no issues just goes to show that the phrase "they don't make them like they used to" is fact...

I'll definitely will be careful with the orientation of the corrector.

Maybe part of the issue is that my scope has always been stored in a cool location, so whatever lubricant was used didn't evaporate, so couldn't cause condensation on the inside of the corrector plate 

[MarsG76]

5 hours ago, michael.h.f.wilkinson said:

Maybe part of the issue is that my scope has always been stored in a cool location, so whatever lubricant was used didn't evaporate, so couldn't cause condensation on the inside of the corrector plate 

Maybe... either way, the glass needs some invasive surgery...

[Paz]

Comparing my vx14 to my 127mm maksutov the vx14 shows much more detail.

At low power I stop down the vx14 to 137mm for super crisp views of the moon, and only use full aperture at higher powers if the conditions are favourable.

[MarsG76]

4 hours ago, Paz said:

Comparing my vx14 to my 127mm maksutov the vx14 shows much more detail.

At low power I stop down the vx14 to 137mm for super crisp views of the moon, and only use full aperture at higher powers if the conditions are favourable.

I understand that but my gripe is with the loss of contrast... definitely a level that is not normal. I'd say 30-50% loss. 

[Peter Drew]

I've had a 8se for a few years now, it is however based under better conditions than the UK but lunar views at 400x are still very contrasty and detailed. The optics on a SCT are very sensitive to collimation, a small amount out will have a large affect on performance. Easy to check to make sure.    

[MarsG76]

48 minutes ago, Peter Drew said:

I've had a 8se for a few years now, it is however based under better conditions than the UK but lunar views at 400x are still very contrasty and detailed. The optics on a SCT are very sensitive to collimation, a small amount out will have a large affect on performance. Easy to check to make sure.    

Definitely true, my SE had crisp contrasty views in the past, as I remember... collimation is crucial to detail but, I think, not contrast... I'll get to the bottom of it... just need a clear night to do one more test before pulling off the corrector.

[Peter Drew]

3 hours ago, MarsG76 said:

Definitely true, my SE had crisp contrasty views in the past, as I remember... collimation is crucial to detail but, I think, not contrast... I'll get to the bottom of it... just need a clear night to do one more test before pulling off the corrector.

Miscollimation tends to smear the image, smearing = reduction in contrast.  

[MarsG76]

On 10/10/2017 at 02:48, Peter Drew said:

Miscollimation tends to smear the image, smearing = reduction in contrast.  

I agree that to a point collimation problems will soften detail and lower contrast but the glow and hazyness that was visible in the EP definitely was not caused by miscollimation.

[MarsG76]

Update to the post, I bit the bullet and cleaned the corrector plate.. for the ones interested, description here: