Field flattener help

[david_taurus83]

Hi everyone. Last night was a first light of sorts using the new WO GT71. It came with their latest adjustable 0.8 flattener, the Flat 6A 2. It's designed to serve most of their refractors and has adjustable spacing. I spent a good 2.5 hours last night tweaking, twisting, turning each way but I couldn't seem to find the sweet spot! I got it pretty close at one point but it still had ever so slightly eggy stars in one corner. Of course, after I tweaked I couldn't find that position again! It was all the more difficult and time consuming as after each adjustment the image, of course, goes out of focus, so I had to keep slewing back to Vega to focus and then back to NGC7000 for testing.

 

Is there a knack to setting up an adjustable flattener? Is there a quicker way or do I need to persevere through trial and error? Am I being too optimistic about the flattener capabilities considering I'm using a DSLR when the sensor is quite large in astrophotography terms?

 

Thanks

[Skipper Billy]

If the stars are eggy only in one corner its probably not a spacing issue but a tilt issue ie not orthagonal.

It can be a massive pain to sort out !

[david_taurus83]

That would be a big pain considering drawtube, flattener and DSLR are all threaded together! My camera has been modified but also has had the sensor re-shimmed. Could it be possible that the shimming is that much out? Is it possible the drawtube is slightly askew and can it be adjusted? It's at F4.7 with the flattener. It's my first refractor so still learning the ropes!

[newbie alert]

If you look on the flatener there are some adjustment screws..not sure if they could take out some tilt..

All modded cameras have been shimmed ..

[david_taurus83]

35 minutes ago, newbie alert said:

If you look on the flatener there are some adjustment screws..not sure if they could take out some tilt..

All modded cameras have been shimmed ..

Not noticed any screws but I'll have a look, thanks. It's made hard by the fact the image rotation moves when I adjust the spacing so it's difficult to pinpoint where the worst curvature is. Next time I think I'll try rotating the OTA back to a fixed position after each adjustment and keep an eye on the worst corner.

[newbie alert]

[david_taurus83]

The grub screws in the T ring?

[newbie alert]

Yeah..

Just a thought are you using it in combination with the  2 inch extension?

[david_taurus83]

2 minutes ago, newbie alert said:

Yeah..

Just a thought are you using it in combination with the  2 inch extension?

I thought those grub screws only held in the M* ring tight in the actual T ring. If loosened do they invoke tilt?

 

No, mine is different to yours. You appear to have the dedicated flattener to match your scope. That's discontinued now I think. The new one has an M63 thread one end to screw into the drawtube and an M48 thread the other for screwing on the camera. It also has  an adjustable ring to cover spacing for a range of scopes. Mine should work at 9.3mm.

[newbie alert]

Just to clarify,are you able to achieve focus but just have funny stars on one side?

[david_taurus83]

Yes, good focus in centre. Top and bottom also not bad but eggy stars in the corners. Like I said, I was just rotating the spacing ring last night and this was obviously rotating the image so was hard to pinpoint where the problem lay. Tonight I might try again but for every adjustment I will rotate the OTA the opposite way and keep the image in the same position. Should help narrowing down the problem.