SW 0.85 Focal reducer, but still Field curvature?

[graemlourens]

Hey Guys.

I'm perplex.

Got an SW ED80 ->  SW FR/FF 0.85x -> SW DSLR-M48 Adapter ring -> Nikon D610 (Full-Frame)

Yesterday imaging M81 i was highly surprised that the stars at the edge are still bloated (just 'differently' than without FF/FR)

Honestly i checked 3 times if i really connected the FR and yes it was there

The distance from DSLR-Chip to FR should be correct as the Adapter ring puts it at the right distance or am i mistaken?

Here example WITHOUT FF/FR:

Here example WITH FF/FR:

Btw there was no star trails, i was guiding, and stars in the middle of those pictures are perfect.

What am i missing? Must be something straight forward, but i can't see it

Regards, Graem

[swag72]

I wonder if the reducer is actually able to cover a full frame chip? Scopes and reducer combo's have an imaging circle that they will illuminate without issue. If your chip falls outside of that size you will get corner issues. 

Just an idea - I have no idea of the size of your chip or the imaging circle of the SW reducer. Some investigation with your retailer may be in order.

[PeterCPC]

Looks like a guiding issue to me. I use an ED80 with FR and I don't get this.

Peter

[Owmuchonomy]

Could be a number of things going on here. Can you post a full frame to show the effect.

1) as Sarah says I doubt the flat field will cover a full frame DSLR chip. This is a similar experience I have with an Altair Astro flattener. It is claimed to fill a crop sensor but doesn't even do that.

2) chip to flattener distance could cause this but need to see the full image

3) field rotation could be going on due to poor polar alignment but again need to see the full image

Etc...

[graemlourens]

Hi guys.

Thx for your answers:

- Not a guiding issue, i made 40 Frames, 100% useable, PHD2 graphs look perfect

- Polar alignment dead on, aligning 40 Frames, not one had a rotational correction (over a imaging period of 1.5h)

I agree that i can't expect it to cover the full frame DSLR, but even close to the center, you can see slight 'blur' to the center of the image.

@Peter, have you got FX Chip when you image?

Attached JPEG of the raw (its pretty big)

Any help really appreciated

Regards, Graem

DSC_0051.jpg.zip

[PeterCPC]

I image with a Canon 1200d (modded) or Canon 550d (unmodded).

Peter

[graemlourens]

Hi Peter.

Thx, those are DX-Sensors so i guess thats why you don't see it. But I wonder if i could detect the same amount of curvature in one of your images.

Could you send me a JPG from a RAW of the ED80 with one of those cams? Would be great, then i could compare, maybe i'm just also expecting to much of the FR/FF.

Regards, Graem

[Owmuchonomy]

Hi guys.

Thx for your answers:

- Not a guiding issue, i made 40 Frames, 100% useable, PHD2 graphs look perfect

- Polar alignment dead on, aligning 40 Frames, not one had a rotational correction (over a imaging period of 1.5h)

I agree that i can't expect it to cover the full frame DSLR, but even close to the center, you can see slight 'blur' to the center of the image.

@Peter, have you got FX Chip when you image?

Attached JPEG of the raw (its pretty big)

Any help really appreciated

Regards, Graem

Very similar to the issue I have.  Your image shows a radial elongation to each corner which for me was concluded as poor flattener performance at the edges of my chip.  Altair sent me a duplicate flattener but that was the same. Altair also sent me spacers to try as you can get the same effect with inaccurate spacing.  However, they made no difference so I had to conclude that my chip was too big for my particular flattener.

[whizzbang]

have as look at this.......handy reference 

[Owmuchonomy]

have as look at this.......handy reference 

You beat me to it.  I was just searching for that graphic as it was posted in the thread when I had my problems.  Thanks.

[whizzbang]

You beat me to it.  I was just searching for that graphic as it was posted in the thread when I had my problems.  Thanks.

its been a real handy reference when setting up several flattners......

[graemlourens]

Thx guys.

Based on the image, i conclude that its 'too close'

But i'm very surprised as i have the original Adapter thread, of the original sw FR/FR to the original ED80 so i would guess spacing is sorted out.

Will measure the distance myself to the DSLR-Chip once again, and try to reproduce it on terrestrial objects (even if very difficult, but will try and put some streetlight at the edge of the chip)

Nobody else having ED80+FR/FF with DSLR and having same problem?

Regards, Graem

[ollypenrice]

 A Takahashi FSQ85ED won't cover a full frame chip. I very much doubt that the ED80 and flattener will do so, but how big a circle is claimed?

Olly

[laser_jock99]

Not many scopes are able to cover a full frame CCD- in some ways going full frame is a waste of time unless you have the scope to match.

[ollypenrice]

Not many scopes are able to cover a full frame CCD- in some ways going full frame is a waste of time unless you have the scope to match.

Indeed.

Olly

[graemlourens]

Hey guys.

Ok, i can accept that, in the end it was not some kind of multi-thousand dollar piece of equipment. (I just didn't know what to expect and wanted to rule out any usage error)

But i'll tonight still do a few tests (also shooting DX), as i can see elongated stars even far from the edge, but maybe i'm now beeing too picky or underestimating how much bigger FX is comparing to DX

It could also have smth to do with focus, maybe i wasn't spot on, will have to doublecheck and get back to you guys tomorrow.

Thx for your help!

[lensman57]

Hey Guys.

I'm perplex.

Got an SW ED80 ->  SW FR/FF 0.85x -> SW DSLR-M48 Adapter ring -> Nikon D610 (Full-Frame)

Yesterday imaging M81 i was highly surprised that the stars at the edge are still bloated (just 'differently' than without FF/FR)

Honestly i checked 3 times if i really connected the FR and yes it was there

The distance from DSLR-Chip to FR should be correct as the Adapter ring puts it at the right distance or am i mistaken?

Here example WITHOUT FF/FR:

without.png

Here example WITH FF/FR:

with.png

Btw there was no star trails, i was guiding, and stars in the middle of those pictures are perfect.

What am i missing? Must be something straight forward, but i can't see it

Regards, Graem

The best you can hope for is coverage for a cropped sensor not full frame, that is why you have funny looking stars.

A.G

[iwols]

hi did you ever sort this problem

[StargeezerTim]

I get the same on my dslr crop sensor! From the  pics it is too close to the sensor but adding spacers makes no diff. I am using a 72 ed-r scope with a AA  or a hotech flattener. The aa flattener gives blobby stars near the corners and the hotech gives a  coma like pattern seen above. The same cameras used with the 150pds and coma corrector are fine!

I'll look at the thread with interest. I've more or less given up hope of getting good corners. Both the scope and aa reducer have been replaced but the problems persist. Like the poster said, my gear is not super expensive but i would love to solve it! A by product us that my star processing have come on leaps and bounds. 

[pete_l]

3 hours ago, StargeezerTim said:

I get the same on my dslr crop sensor

from the Take nothing for granted dept.  I would suggest shooting a frame and uploading it to astrometry.net  for a plate solve. See if the computed frame size matches the expected frame size with the FR in place.
Then start the diagnosis depending on what is revealed.

[graemlourens]

20 hours ago, iwols said:

hi did you ever sort this problem

Hi Everbody. Funny to get reminded about my problems over 2 years go in this thread  i was just starting out....

Fact is, that as olly Sara said, the ED80 + Focal reducer is not in the league of covering a full frame chip, and when i then (very soon) moved on to CCD, that problem was gone, as my Atik 460EX chip is much smaller than a full frame sensor.
If you having this issue though with a chip size in the range around 16mm (diagonal), then its probably spacing that you'll have to experiment with. Even simulating an FX chip, the stars were still not round in the corners btw.

Kind regards, Graem