Back-Focus....D'OHHHHHHHHHH !

[Hughsie]

I have spent the last hour debating whether to share my fortune/misfortune, stupidity/moment of inspiration or..... oh hell lets just call it what it is, I am an idiot.

This time last year I purchased a William Optics Z103 and the William Optics 0.8x reducer Flat 6AII. I was happily imaging with my WO Z61 for the early part of the year then, in the Summer, I decided to get into the Z103 and 'learn' how to use that. Armed with the Flat 6AII and the ZWO ASI1600mm Pro Cooled I made the following adjustments to the reducer as per the WO instructions below of 4.6mm and locked the reducer down.

So as you can see this boy is on a roll. I went as far as purchasing a digital caliper to really make sure that I had the 4.6mm distance dialed in then but please, be under no illusion, I was REALLY making sure I was at 4.6mm checking and re-checking the distance.

Then I had to get the back-focus right and according to the next diagram from WO, I needed a distance of 59mm. I was using a filter wheel with 2mm thick filter so I added a further 0.6mm in to the back-focus as well.  Easy right, I've got my digital calipers, instructions and an air of invincibility having spent weeks getting my 4.6mm distance sorted.

Game day approached, the BBC Weather app and Clear Outside were for once saying the same thing, Clear Skies. I have my new scope, new reducer perfectly set up and my new ASI1600mm camera. I AM READY.

First picture.....rugby ball stars in the corner...eh? Second picture, same outcome.

Purchased an adjustable spacer. Arrived a week later.

Game day 2. Rugby ball shaped stars. I adjust the adjustable spacer. No effect. In or out, I have oblong stars. Game day 3. I just carry on taking images, all posted here and cropping like crazy and in between sessions measuring and remeasuring the spacing.

That was all over the Summer.

On Monday this week I emailed William Optics explaining my dilemma. They responded the next day with a solution. This evening I implemented said solution and now I have lovely round stars in the corners of my images.

So the moral of this story is this.....

WHEN THE INSTRUCTIONS CALL FOR A 4.6MM SPACING ON THE REDUCER, REMEMBER YOU ARE NOT USING A DSLR CAMERA!

I am going to grab a cold towel for my head and a beer.

 

John

[sloz1664]

Hi John, I feel your pain & no you are not an idiot, back focal distance is the biggest bain in astroimaging. I had a WO Z71 and the Flat6 when they were first released and I had trouble obtaining the correct back focus as there were conflicting distances on various websites. I eventually contacted WO direct to obtain the correct distance. It is not an easy operation adjusting back focus, re-focusing, downloading the image and inspecting it hopefully to see a nice flat field. If not,to go through the whole operation again. I found it frustrating and I used the Baader VariLock adjustable extension tube in my imaging train. I could not contemplate having to go through the adjustments using delrin spacers and having to break into the imaging train. 😱

Steve