Collimation Frustration

[ballaratdragons (Ken)]

As it is another cold rainy day I have spent several hours getting my 12" f5 dob just right.

I discovered through measurements and alignment that my 2ndry was out of wack.

I have set it at the recommended off-set - 1/10th of an inch off centre away from the focuser and 1/10th of an inch off centre towards the primary.

I have fully collimated it and the Cheshire reading is exactly centred and spot-on.

The clouds have parted so I took it outside to see how it would go on the daytime Moon.

I have posted a pic to show you what I now see with a 30mm 2" SuperView. It looks like this barlowed and un-barlowed. Then I tried a 25mm Plossl and got the same effect!

When I stepped up to a 15mm Plossl it went!!

I know it is the blurry reflection of the 2ndry and it is worst at perfect focus. I moved my eye around, away, closer, everything and it is visible at all times.

I have never had this problem before!!

What is causing this???

I am probably wrong here, but hasnt it got something to do with exit pupil and the fact that you are looking during the day when your retina is quite closed, or is it dilated, something like that anyways. Mind you I am not an authority on this or for that matter newtonians as well. Just my very amateur thoughts.

[ving (David)]

too much light... try a small EP and it will go. I dont know the technical reason for this but I am guessing too much light.

[ballaratdragons (Ken)]

That sounds like a very good answer from both of you but the camera shows it too! and a camera is different to our eyes.

And, I have never had it before on other daylight Moon obs.

[ving (David)]

yes it would... as this happens to me too... even more so with the camera

[astro_south (Andrew)]

Ken

It has nothing to do with collimation and everything to do with the larger exit pupil of longer focal length eyepieces. Perfectly natural - nothing to worry about.

[ballaratdragons (Ken)]

Thanks Andrew, but will it still be there during night viewing? If it is it will block out a lot of the sky.

[h0ughy (David)]

Originally Posted by Exfso

I am probably wrong here, but hasnt it got something to do with exit pupil and the fact that you are looking during the day when your retina is quite closed, or is it dilated, something like that anyways. Mind you I am not an authority on this or for that matter newtonians as well. Just my very amateur thoughts.

Sound good to me. I still think he left his pencil in the collimator . there are articles out these that describe what Ken is experiencing, just can't remember where those links are for the moment

[astro_south (Andrew)]

It will still be there - just not as noticeable (if at all). The dark sky background and the brightness of the moon will help disguise it. Rattle off a few moon shots with the digi tonight with the moon in various parts of the field of view and see if you can detect it as a slight dimming - making sure you use the same long focal length eyepiece that was the source for your daytime example

[ballaratdragons (Ken)]

I hope someone can find the links.

It's pretty scary when you spend hours correcting everything and then see a big black blob!

[ballaratdragons (Ken)]

I can't tonight Andrew. It's raining again and supposed to be the same tomorrow.

Wierd how I have never seen it before!!! Unless my scope was so far out of alignment it didn't happen.

[ballaratdragons (Ken)]

Hooray, there is a break in the clouds. Going to quickly get the dob out and check this problem. Oh, and see how my 2ndry alignment went.

Back soon with a quick report.

First night for the scope in 2 months!!!!!

[ballaratdragons (Ken)]

I got a bit of a view before the clouds closed up again.

No cool-down time!

AND the black blob is un-noticable unless I look hard for it! Thanks Andrew.

Below is Pic 1: The moon with the black blob gone. 2" 30mm SV
and Pic 2 & 3: Alpha Centauri triple (well 2 of them) through thin cloud! 9 & 6.5mm Plossls