Field flatness of refractor

[david_taurus83]

Eager to get going with new mono ASI when the clouds break, I still have one last obstacle to overcome. The setting up of the adjustable flattener for my WO GT71. The flattener is the Flat6A II which has an adjusting ring allowing up to 11mm of extra back focus. With my Canon, I was never really happy with star shapes in the edges. It was a case of trial and error and visually inspecting the test subs. With the ASI1600 and it's narrower width sensor I'm hoping it will be easier to set up. I've bought the licence for CCD Inspector and I'm hoping to use this to produce 3D models showing the field curvature while I make adjustments and take test subs. At the moment, I've left the flattener at the spacing I thought was ok with the DSLR. I've connected the ZWO M48/M42 16.5mm spacer, a 10mm spacer, EFW and delrin rings 20.8mm and ASI1600 6.5mm spacing with an assumed extra 1mm for filters and glass in front of the sensor. This gives me a back focus of 54.8mm so pretty close.

 

My question is, what is an acceptable curvature for an APO? Has anyone checked their field curvature just to see what it's like? I don't want to be chasing a result that's not possible!

[Davey-T]

According to WO it will give a flat field over 97% of a full frame sensor.

Dave

 

 

 

 

[david_taurus83]

2 hours ago, Davey-T said:

According to WO it will give a flat field over 97% of a full frame sensor.

Dave

 

 

 

 

Should I be aiming for a less than 5% curvature model? There are manufacturer figures and then real world figures. I know CCDI isn't perfect but it should be more reliable than visually inspecting single subs.

[Davey-T]

Never used CCD inspector but as I recall you need a suitable sub to start with so those already captured may not be suitable.

Single subs and the stacked result will / should be lots better than a single one.

As you say mostly trial and error as usual and depends on your standards, if you're OCD about perfect stars in all four corners you can spend more time faffing about in the pursuit of perfection than actually imaging 

Dave

[david_taurus83]

I have spent a few nights faffing about with the DSLR! I find myself being very critical of my results when I should be reminding myself I'm just an amateur with entry level gear though the cost of it certainly doesn't feel entry level?

 

 

[Davey-T]

I found it quicker to fix the dodgy stars in P'Shop, nobody notices in the corners 

Dave

[david_taurus83]

Do I leave it at this? Only adjusted a few times and it's the best I've seen the tilt centred. As originally asked, I don't know what acceptable curvature there should be!

[david_taurus83]

Trying something different tonight. Cloudy outside so I've brought the stars indoors...

[david_taurus83]

Loaded a blank screen on Autocad and generated a hatched surface of white dots. Scope is level with the screen and I've lined it all up as best I can. Don't know how well it will work but it's got to be better than wasting hours of clear skies.

[ollypenrice]

11 hours ago, david_taurus83 said:

Loaded a blank screen on Autocad and generated a hatched surface of white dots. Scope is level with the screen and I've lined it all up as best I can. Don't know how well it will work but it's got to be better than wasting hours of clear skies.

People also image a sheet of squared paper...

Olly

[david_taurus83]

31 minutes ago, ollypenrice said:

People also image a sheet of squared paper...

Olly

You mean like graph paper? Adjust until the lines look straight?

[gorann]

Before I also try to find the correct distance to a flattener I am struggling with I would like to be sure that the flattener to chip distance is the same if I foucus on something close by (like a computer screen or sheet of paper) as if I focus on stars at night. Does anyone know?

[Davey-T]

Doubt if you can focus your scope on the laptop screen unless you put it at the bottom of the garden 

Dave

[david_taurus83]

2 minutes ago, Davey-T said:

Doubt if you can focus your scope on the laptop screen unless you put it at the bottom of the garden 

Dave

I needed 4.3m minimum! I could even pick up the dark unlit pixels on the screen! Had enough by 3am. Got it somewhere close, a lot better than it was last weekend. Either way, it's getting used in anger tonight!

[gorann]

19 minutes ago, Davey-T said:

Doubt if you can focus your scope on the laptop screen unless you put it at the bottom of the garden 

Dave

I can probably get about 6 m between the scope and screen, so it could be worth a try but the question is if it would be the same distance between chip and flattener when I focus at 6 m as when imaging the sky. I assume David may find out tonight if the sky is clear. I have a sneaky feeling that the flattener is optimized for infinity focus (in my case it is an Esprit 100 flattener).

[AngryDonkey]

 

7 minutes ago, gorann said:

but the question is if it would be the same distance between chip and flattener when I focus at 6 m as when imaging the sky

Where is @vlaiv when you need him!!!!! ?

[david_taurus83]

13 minutes ago, gorann said:

I can probably get about 6 m between the scope and screen, so it could be worth a try but the question is if it would be the same distance between chip and flattener when I focus at 6 m as when imaging the sky. I assume David may find out tonight if the sky is clear. I have a sneaky feeling that the flattener is optimized for infinity focus (in my case it is an Esprit 100 flattener).

I thought as long as the focal ratio is the same it shouldn't matter? It's not far off what it should be, 9.3mm according to WO. I had it set at 8.75mm with my Canon and it's now at 8mm from last night. The variable being a new camera, filters and spacers.

[ollypenrice]

1 hour ago, david_taurus83 said:

You mean like graph paper? Adjust until the lines look straight?

Yes. It would need to be a good way off. I've never tried it, just read about people using this method.

Olly

[gorann]

39 minutes ago, AngryDonkey said:

 

Where is @vlaiv when you need him!!!!! ?

Yes, I also hoped @vlaiv would read this!

[vlaiv]

Don't know if I can be of any use here - I have no idea if flattener distance will be affected by range to target.

My guess (and I need to emphasize it is guess only) is that it will be affected in some way, at least by "speed" of light cone. Closer you focus your objective, slower light cone is (if you bring it to focus on "front" side of the lens you should get parallel rays exiting on back side of the scope - equivalent to F/infinity ratio - beams won't even converge).

Some field flatteners have different recommended distance that they best work on depending on scope / FL you use them with. Not sure if close focusing changes field curvature in any way (making field flatter than when focused at infinity) - might also be the case.

May be that you can do it that way, but I suspect that distance needs to be a bit longer than 6m - same as collimation on artificial star - I can't remember but there is relationship between minimum distance and scope focal length of F/ratio - can't remember which.

If you are really keen on getting this sort of thing done indoors, it can be done, but I'm afraid you will need two scopes - one acting as collimation device - you use small screen (phone might do?) as a source of light and place it at focal plane of collimating scope to create parallel beams exiting in front (turn both scopes towards each other - front to front). Of course this technique requires collimating scope to already have pretty flat field and be of good optical quality (think opticians and mirror flats ).

 

[vlaiv]

It looks like my guess was wrong, here is quote from Telescope optics website:

"As the two relations imply, Petzval radius is independent of object and image distance."

Here is link to actual page (goes in depth explaining field curvature):

https://www.telescope-optics.net/curvature.htm

From above I would conclude that it is perfectly ok to adjust field flattener indoors as described above provided that distance of FF to sensor only depends on actual curvature being corrected (and I suspect that is the case, since curvature varies with FL - this is why recommendation is given for different distance depending on scope / FL).

[gorann]

1 hour ago, vlaiv said:

It looks like my guess was wrong, here is quote from Telescope optics website:

"As the two relations imply, Petzval radius is independent of object and image distance."

Here is link to actual page (goes in depth explaining field curvature):

https://www.telescope-optics.net/curvature.htm

From above I would conclude that it is perfectly ok to adjust field flattener indoors as described above provided that distance of FF to sensor only depends on actual curvature being corrected (and I suspect that is the case, since curvature varies with FL - this is why recommendation is given for different distance depending on scope / FL).

Thanks a lot Vlaiv! Then I have a go at it tomorrow if I manage to focus at 6 m. Supposed to be clear here tomorrow night so it would be nice to have it working.

[david_taurus83]

Just need to make sure the target is same height as scope and as perpendicular as possible to the FOV. I will post up a few subs later if my endeavours last night have paid off ?

[1DegreeN]

On 10/11/2018 at 19:52, david_taurus83 said:

Eager to get going with new mono ASI when the clouds break, I still have one last obstacle to overcome. The setting up of the adjustable flattener for my WO GT71. The flattener is the Flat6A II which has an adjusting ring allowing up to 11mm of extra back focus. With my Canon, I was never really happy with star shapes in the edges. It was a case of trial and error and visually inspecting the test subs. With the ASI1600 and it's narrower width sensor I'm hoping it will be easier to set up. I've bought the licence for CCD Inspector and I'm hoping to use this to produce 3D models showing the field curvature while I make adjustments and take test subs. At the moment, I've left the flattener at the spacing I thought was ok with the DSLR. I've connected the ZWO M48/M42 16.5mm spacer, a 10mm spacer, EFW and delrin rings 20.8mm and ASI1600 6.5mm spacing with an assumed extra 1mm for filters and glass in front of the sensor. This gives me a back focus of 54.8mm so pretty close.

 

My question is, what is an acceptable curvature for an APO? Has anyone checked their field curvature just to see what it's like? I don't want to be chasing a result that's not possible!

Hi David

Are you sure that you have the backfocus correct? The Flat6AII's backfocus of 54.8mm is from the wide rear flange where a T-mount would be up against, but you are connecting via the internal M48 thread and by my measurement the flange for that threaded connection  sticks out by 5mm so the backfocus becomes 49.8mm. Also, you have to add (not subtract) 1/3rd of the filter width so the backfocus you want to achieve is then  around 50.8mm. The components you list come to 53.8mm so you could be 3mm over.

I've got the Flat6A II on my Zenithstar 103 and am very happy with the results. I haven't used CCD Inspector but visually the images are good with just very slight coma remaining at the corners of my APC-C size frame and PixInsight's FWHMEccentricity script confirms this.

cheers

John

[geordie85]

Where did you get the Flat6AII from as they're not in stock at FLO. Thinking of getting one for my flt98