Flattener spacing: Does it work?

[david_taurus83]

 

Most of us use this image as the go to reference when trying to sort out the spacing for for a nice flat field. Has anyone actually had any success in replicating it? For example, you have streaky stars as in the top image, you add a spacer and all of a sudden you are too far away and you get the opposite effect in the bottom image? So you space to somewhere in the middle to get nice round stars?

 

I have to say I have my doubts about it. Last night I spent 5 hours meticulously adding 0.25mm spacers to my Starwave 102EDR with TS Optics Photoline 3 inch 0.79 flattener. The recommended back space distance for this scope is 56mm. The minimum distance I can produce with the various bits and bobs I have here is 56.1mm. I started off with 56.6mm as I have 2 filters and the sensor window in the light path. I went all the way out to 62mm in 0.25mm steps and almost everytime I was still getting the elongation in the corners like in the top image. Sometimes I would get ok in 2 opposite corners but streaked like the bottom diagram in the other 2. It just seemed to generate any old random effect. It really is enough to drive you insane!!

 

Of course, at what point did I arrive at my best result? Yes, you've guessed it. At 56.1mm without any extra spacers at all.

 

Top left

 

 

Top right

 

 

Bottom right

 

 

Bottom left

 

 

Centre

 

 

 

 

This is a stack of 10 x 30s subs, unguided. This is the method I used to evaluate all the possible spacings, to try and achieve an average representation of the stars.

 

 

Clearly adding spacers is not beneficial. The only other way is to decrease the distance between flattener and sensor. But looking at the bottom corners it would appear I am already tilted too close.

 

Should I just leave it the hell alone?

[Davey-T]

YES 😂

Dave

[michael8554]

There was a thread a while back that suggested those white charts are wrongly captioned.

Should be:

Image 1 too Far

Inage 2 too Close

Which seems to match your results ?

Michael

[alan potts]

Not quite the same issue and I have never seen these patterns before. I have the same .79, 3 inch reducer which I was lucky to get S/H here. I require 55mm which is fine with the DSLR just bang it on the back. However the Zwo 071 if I ever use it again also needs 55mm but with a filter which means I am needing a 6mm sleeve to add to the 5mm filter. The wheels came off when I found out FLO don't stock these and I won't use other outlets, so looks like the DSLR will not be going anywhere soon as I have the reducer screwed to the scope. We all know what problems I caused last time i touched the scope.

Alan

[alan potts]

Actually Dave on the back of this post I have just had a really good look at a number of subs at 100-200% and the edges are not very clever at all. This is in a rig where the correct back focus is 55mm , camera straight into Canon bayonette fitting, then screwed to Reducer, not impressed at all. Can't adjust anything.

Alan

[sloz1664]

Absolutely despise getting back spacing correct for that ultimate flat field. I have an AA80 triplet and added a rather expensive focal reducer to the imaging train. Not only did I need to adjust the back spacing but I had a slight tilt as well. After adjusting, tweaking, refocusing, checking the image ang going through the same tedious routine, whilst being tired, cold and in a cramped condition, is not a pleasant experience. If the devil had appeared, whilst I was agonising over my scope and offered to miraculously sort the issue, my soul would have been forever his.

Steve

[Whirlwind]

On 03/09/2019 at 17:01, sloz1664 said:

Absolutely despise getting back spacing correct for that ultimate flat field. I have an AA80 triplet and added a rather expensive focal reducer to the imaging train. Not only did I need to adjust the back spacing but I had a slight tilt as well. After adjusting, tweaking, refocusing, checking the image ang going through the same tedious routine, whilst being tired, cold and in a cramped condition, is not a pleasant experience. If the devil had appeared, whilst I was agonising over my scope and offered to miraculously sort the issue, my soul would have been forever his.

Steve

If you have a relatively large amount of backfocus to play with a thin helical focuser between the corrector and the imaging set up can work wonders...

Edited September 4, 2019 by Whirlwind

[sloz1664]

Just now, Whirlwind said:

If you have a relatively large amount of backfocus to play with a helical focuser between the corrector and the imaging set up can work wonders...

I use a Baader Variable Locking T2 Extension in my imaging train. The tilt stems from the CCD sensor.

Steve

[Hughsie]

I have this joy to come when another clear night occurs. I recently purchased this https://www.astroshop.eu/extension-tubes/astoptics-extension-tube-variable-m48-extender-23-29mm/p,57790 to help make the adjustments to back focus. It's attached to the field flattener at one end and a couple of spacers at the other and wound and locked to give 55mm spacing. I hope, let me say that again, I hope that with this in the image train I can then calculate the exact spacing needed to put in permanently.

I hope.

John

[david_taurus83]

I've made another interesting observation on the eternal (infernal?!) quest for round stars! Lets take @ollypenrice's handy sketch below:

 

 

The answer looks simple. Adding glass between the corrector and the camera increases the light path/optical distance so we add a spacer to move the camera further away. I have never had much luck with this method. My attempts explained in the top post show this.

 

However, optical distance and physical distance can be 2 different measurements. Lets assume the required distance above for the corrector to perform optimally is 55mm. With no filter in the light path both the optical and physical measurements are 55mm so everything is perfect. Lets now add in an Astrodon filter at 3mm thick. Though the physical distance remains unchanged at 55mm, the optical distance has increased by approximately 0.33 the thickness of the filter, so 1mm, up to 56mm.

 

Any rationally thinking person will look at this and see the need to add 1mm to the spacing requirements so that the focal point matches the optical path. 

 

Thinking about it though, are reducers and flatteners not designed with an optical distance in mind? In the case above with the added Astrodon filter, the optical path is too long for the corrector to perform even thought the physical distance remains at 55mm. The focal point isn't determined by the distance from the corrector to the camera nor should it be by adding a 1mm spacer. The focal point is determined by the movement of the focuser on the OTA. So in the above scenario, we need to reduce the physical distance from 55mm down to 54mm in order to achieve an optical distance of 55mm as per the corrector specifications.

 

Case study:

 

At the weekend I switched back to my William Optics GT71 with the WO Flat6A II reducer. This reducer has an adjustable ring and gives from 0 to 11mm extra spacing depending on which scope its used on. It is designed with a Canon DSLR in mind so needs 55mm distance from the shoulder PLUS 9.3mm spacing on the adjuster to match the GT71, as quoted by WO. As I've already mentioned, I've not had much luck working with these quoted figures and adding spacing to take into account Baader filters (2mm) in the filter wheel and 2mm sensor cover glass.  I've always thought I needed to add ((2mm + 2mm)/3) 1.33mm to the physical distance.

 

Well this weekend I tried the opposite. I took 1.3mm away from the distance so worked with 55mm from shoulder plus 8mm on the reducer.

 

Stack of 14 x 900s OIII subs:

 

Top left

 

 

Top right

 

 

Bottom right

 

 

Bottom left

 

 

Centre

 

 

I have never gotten the corner stars this good before so it adds a lot of weight in my eyes to the theory explained above. Next time I give the 102 a whirl I'm going to try the same and reduce the physical distance and see how I get on. For the time being it would seem that I have, touch wood, found the sweet spot for my main imaging setup....

Hope this helps anyone else stuck down the rabbit hole as well.

 

David