Another Field Flattener Spacing Question !

[Spaced Out]

Hi All

I bought my first ever refractor 3 weeks ago but, due to non-stop cloud cover, last night was the first time I have been able to point it at some stars.

First job was to try and space the FF/reducer correctly. To cut a long story short, after a couple of hours of faffing, this was the best I could manage. 

So, do these corner stars indicate under spacing or over spacing ? I’ve seen the diagrams posted on here for stars that are over and under spaced. To me, some of the stars (particularly on the left hand side) seem to be pointing outwards with small tails. I’d really value a more experienced opinion on this though.

The equipment is - Stellamira 90 ED, dedicated Stellamira FF/reducer for 90 ED and ZWO 2600MC.

Thanks

 

Single 10s frame

 

Top left

 

Bottom Left

 

Top right

 

Bottom right

Edited August 6, 2022 by Spaced Out

[newbie alert]

Looking at my phone it don't seem too far away, but the bigger the sensor the more crucial the spacing... I'd be inclined to add the thinnest baader shim , think the thinnest is the black one .3, red are .5 and gold 1.0.. delrin ones aren't that accurate, mine wasn't the same thickness all the way around and created tilt so I threw them away

 

If you're adding a filter you need to add 2/3rds of your filter thickness to your spacing

[Pompey Monkey]

Do you use an electronic focuser? I found this article about methodically evaluating backspace and tilt using NINA's "Hocus Focus" plugin that might be useful: https://www.astrobin.com/forum/c/astrophotography/equipment/fixing-the-dreaded-tilt-and-backspacing-error-in-optical-systems-with-objective-analysis/?page=1

It describes the potential pitfalls of analysing single frames for backfocus and tilt too. 

Btw, your stars don't look too bad anyway!

[michael8554]

TL too close

TR too far

BL too close

BR too close.

Very subtle, so correct the tilt and the spacing will probably be spot on.

I've seen a few posts that suggest the ZWO 2600MC is prone to tilt, hence its built-in tilt adjustment

Michael

[Spaced Out]

1 hour ago, newbie alert said:

Looking at my phone it don't seem too far away, but the bigger the sensor the more crucial the spacing... I'd be inclined to add the thinnest baader shim , think the thinnest is the black one .3, red are .5 and gold 1.0.. delrin ones aren't that accurate, mine wasn't the same thickness all the way around and created tilt so I threw them away

 

If you're adding a filter you need to add 2/3rds of your filter thickness to your spacing

Thanks. This is what I was struggling with tbh, it is subtle. TR looks best, but it's a bit squashed, but the others all looked a little more comet like. Confused.com ! I think what you say about tilt makes sense tho, probably bringing the TR corner back to match the others, then try spacing again if necessary.

It's a new camera for me, so I need to work out how to identify which corner to adjust first and then how to adjust it. Tbh I was dreading having to fiddle with those tilt adjusters !

[Spaced Out]

1 hour ago, newbie alert said:

Looking at my phone it don't seem too far away, but the bigger the sensor the more crucial the spacing... I'd be inclined to add the thinnest baader shim , think the thinnest is the black one .3, red are .5 and gold 1.0.. delrin ones aren't that accurate, mine wasn't the same thickness all the way around and created tilt so I threw them away

 

If you're adding a filter you need to add 2/3rds of your filter thickness to your spacing

Thanks for this. Yeah I think it's quite close, just struggling a little to work out what is going on as all 4 corners don't look the same, one seems a bit better and the others more comet like. A slight sensor tilt has just been suggested and thinking about it that seems to make sense.   

[Spaced Out]

22 minutes ago, Pompey Monkey said:

Do you use an electronic focuser? I found this article about methodically evaluating backspace and tilt using NINA's "Hocus Focus" plugin that might be useful: https://www.astrobin.com/forum/c/astrophotography/equipment/fixing-the-dreaded-tilt-and-backspacing-error-in-optical-systems-with-objective-analysis/?page=1

It describes the potential pitfalls of analysing single frames for backfocus and tilt too. 

Btw, your stars don't look too bad anyway!

Thanks for this. I am not using an autofocuser yet, but the ZWO EAF is on my shopping list for when funds allow !

Agreed the stars aren't bad, I'm just trying to get this new kit set up the best I can before I start to use it properly. 👍

[900SL]

That looks pretty good to me. Do some full stacks and I'm sure you'll be happy

[Spaced Out]

On 10/08/2022 at 15:50, 900SL said:

That looks pretty good to me. Do some full stacks and I'm sure you'll be happy

Thanks, yeah, you may well be right, it's not far off. I was just pixel peeping at first to try and get the best I can out of this new telescope.