What to expect from a refractor with no corrector?

[rnobleeddy]

After using a Newtonian for the last year, I finally got around to having a go with a 60mm APO refractor I bought 2nd hand last year. It looks undamaged, but I'm having a lot of issues getting acceptable star shapes. I went back to basics and removed the reducer/flattener, and so wondering if anyone can suggest if these results are normal for a refractor with no corrector?

These are screen grabs of the data at full size. FL is 350mm and sensor is a 4/3 mono CMOS with a luminance filter. Short exposures (I think 10s) with the mount tracking.

I only captured 2 corners + the center, but the other corners show the same pattern, with the stars (for want of a better word) "flaring" out from the centre. The effect reduces as you go towards the middle and appears to be symmetric around the center.

I'm hoping that either looks normal or if not, perhaps it's obvious what the issue is? My mono camera + FW is heavy so there could be an issue with tilt, but I don't see how this would produce this pattern? Similarly, it's a doublet air spaced apo (photoline 60mm) so whilst color correction may not be perfect, I can't imagine this is related to that.

 

Top left

 

Approx central 

Bottom right

 

Bottom 

 

 

 

Edited September 8, 2021 by rnobleeddy

[ONIKKINEN]

Looks like field curvature, perfectly normal for an uncorrected refractor. Stars are nice in the middle and the curvature looks even. This is what the flattener should correct out if you put it back in.

[Louis D]

Yep, field curvature.  Put the flattener back in and adjust the spacing if needed to get the best star pinpoints in the corners once focused for the center.

[Ricky Graham]

I had exactly the same issue. Was always used to a Newtonian and didn’t know I needed a flattener with my triplet. But yep, all good with the flattener 

[rnobleeddy]

Thanks. 

I got pretty nasty results with the flattener initially. Part of this was my electronic focuser had some play in it, so the auto focus routine led to bad results. 

However, what also let me down was making the assumption that this would have the same tolerance as the Newtonian coma corrector I use, which appears to have a 1-2mm region which produce solid results. Based on what I experienced so far, the 0.79x flattener  I have will need to be adjusted much more accurately! 

 

 

 

[Louis D]

4 hours ago, rnobleeddy said:

However, what also let me down was making the assumption that this would have the same tolerance as the Newtonian coma corrector I use, which appears to have a 1-2mm region which produce solid results. Based on what I experienced so far, the 0.79x flattener  I have will need to be adjusted much more accurately! 

I've read of imagers trying get the spacing correct to the 1/2 millimeter.  You might also run into tilt issues where different corners are flattened at slightly different distances.

[iantaylor2uk]

Not sure what flattener you have used. I have got good results with the Hotech SCA 1x flattener on an 80mm f5.9 scope. Sensor needs to be 55mm from shoulder of flattener but doesn't seem too sensitive.

[Clarkey]

To get perfect results you need to be very accurate, particularly on larger sensors. Even on my ED80 and ASI1600 the spacing is critical within a fraction of a mm. Just go out and take a series of images whilst altering the spacing. If it varies from in different corners that is more likely to be sensor tilt.

[Louis D]

There are both variable length extension tubes and tilt adjusters to fine tune each.