Optimal placement of ERF

[JohnM]

Has anyone noticed if there is an optimal separation between the OG and the ERF filter element?
Building this stage 2 mod I had originally placed an 82mm 25A filter internally.
Focus was less than desired, but illumination was even across the field.
Then I made a simple cell and mounted it out in front of the 102mm OG.
The next series of tests all showed a bright area in the field I can't seem to get rid of.
Rotating the filter makes no difference, it remains in the same spot.
I took out the etalon and checked it to be sure nothing had come loose, changed, etc.
I can get some of the bright spot dimmed by tuning, but it's still readily apparent in the finished image.
Could air gap between the front surface of the OG and the filter be related? Maybe tilt?, I've tried from none to about 2mm.
In another hour or 2 I'll have the Sun in a position to start experimenting again.
Images I posted from the 25th really show this thing.

[JohnM]

What a difference.
I reduced the air gap from ~1.5" to ~.5"
It's not much of an image, the seeing with the Sun as low as it is is always terrible here and I'm still off on etalon tuning, but the bright area seems to have gone.
Might try closing the air gap down to about .25"
First image from just now, second from yesterday.

[JohnM]

This with an air gap of ~.25"
Improvement?

[Allcart]

Its very hard to judge when the pics are all different john. I think you need to take the same pic with the same setup 3 or 4 times but only adjusting the spacing. Then we can compare like for like images.

[JohnM]

True enough, these were from too widely spaced time periods.
I'm clouded out now as usual for the PM.
I'm going to do some comparisons tomorrow, got this spacing change down pat now
From what I was seeing at my end with processing and all I think I'm about as close as I'll get with that 25A.
Then I want to start reducing the FL to see how much sharper I can get the finished images.
These were all at 2000mm.

[peter drew]

Interesting question John. I have a 102mm with a front ERF fitted to the front of the dewshield some 6" from the OG and a couple of others with an internal ERF, all give excellent results without the effect you describe.

[JohnM]

I'm using a 25A for an ERF, maybe that's the problem?

[peter drew]

I also have a 80mm F10 mod with a 82mm photographic 25A filter(Hoya) front mounted several inces in front of the objective which works quite well. Maybe this was a lucky one.

[JohnM]

Really?, that's interesting.
I plan on doing some more comparisons tomorrow.

[marktownley]

Looking forward to the results John

[JohnM]

The mornings test shots.
2000mm/DMK21
#1 0.25" gap
#2 1.25" gap
#3 2.25" gap
Each with it's own flat, identical processing in AS2






Hmmm. OK

[marktownley]

Hmmmm, indeed

Not sure what to say apart from that bright streak isn't there

[JohnM]

Very strange behavior. If I drop down to f10, 1000mm, that bright area comes back unless I move the filter back in close again.
Rotating, changing tilt angle has no effect.
It all has to be related to that filter. Wave front error?
I'm not sure if I've actually seen any images posted here by people using a 25A.
Just have to get a DERF as soon as I can.

[peter drew]

John.

I wouldn't wish to give the impression that my 80mm F10 mod with a full aperture 25A ERF is as good as a "proper" ERF but it is certainly much better than the standard 40mm PST and is very usefull for a quick view.

[JohnM]

So you use it visual and don't try to image with it?

[peter drew]

Yes, that's it John, I leave the imaging to those with the time and the skill. :

[JohnM]

The human eye is much more forgiving than the camera.