f6.3 reducer, it really pays to do the maths

[wayne anderson]

A thorough search of many forums on the net shows Meade f6.3 reducer spacings to be anywhere between 85mm to 105mm, i was using 105mm thinking that was OK as it was the most common spacing suggested, so which spacing is right for my setup.

I decided to use the bintel astronomy calculator to find the correct arcsec/pixel using a 12inch Meade LX200 with f6.3 reducer and a ZWO ASI070MC pro at bin2x2.

https://www.bintel.com.au/tools/astronomy-calculator

The result was 1.03 arcsec/pixel for
1920.24 f6.3 at bin2x2

Now all that was needed was to image at various f6.3 reducer Spacings to find which spacing produced 1.03 arcsec/pixel by uploading the image for plate solving.

https://nova.astrometry.net/upload

Results:
105mm = 1.18 arcsec/pixel
100mm = 1.14 arcsec/pixel
90mm = 1.07 arcsec/pixel
85mm = 1.03 arcsec/pixel

The 85mm spacing produced 1.03 arcsec/pixel, this should be the optimal setting, i will image using this spacing in the future and see if it improves my images.

[bojan]

Wouldn't the coma (or lack of) in corners be the better goal to achieve?
Scale is pretty much irrelevant IMO.

[Merlin66]

Wayne,
Good stuff!


I would measure the focal length of the reducer before going further.
The "standard" focal length of the x0.63 reducer (Meade and Celestron) is 240mm.
This should give the x0.63 at a spacing of 105mm. See attached spreadsheet.

(There was a batch of Meade reducers a few years ago which were produced at a shorter focal length...)

[wayne anderson]

Yes, all my photos had very noticeable coma in the corners at 105mm and 100mm spacing it was at 90mm that coma started to be reduced and much better again at 85mm.

With the Meade f6.3 reducer image scale and coma from a non-flat field seem to be related.

The Meade reducer is a f6.3 reducer/flattener, it is the flattening of the image at 90mm to 85mm that results in less coma.

[bojan]

This is the case with other reducers... there is a "sweet spot" where coma is minimal, or absent.
I have Celestron C11 and Meade Series 4000 0.63 reducer.. and I am still in search for that "sweet spot".. it is definitely closer than recommended 105mm.

[wayne anderson]

Thanks Ken, i have read that info, lots to consider.

For many years i was advised to keep using the 105mm spacing, various lines of reasoning and equations to back it up said stay with 105mm so i did and had to keep cropping all my images to remove the stars with coma at the edges.

It was not until just recently i started reducing the spacing and have been getting better results, i am still working on this.

[Merlin66]

Wayne/ Bojan,

The design of the reducer is supposed to be 1. a reduction in effective focal length while 2. providing a flatter field.


My checking of reducers has always (excuse the pun!) focused on the on-axis image - I use my SCT for spectroscopy only.
Baader issued a chart for checking reducer spacing, not for the reduction factor but the suppressing of aberrations.
This infers that if you're seeing what looks like coma at the edges, the the reducer spacing is too close.......

[Stonius]

Could you explain a bit more about those images? As in how they're used? Or does it just show you what the aberrations look like across the field (for educational rather than diagnostic purposes)?


Cheers
Markus

[Merlin66]

Markus,
Basically a simplistic way of analysing your image when using the reducer.
If the star images in the corners look like comets - flaring away from the centre then the spacing of the reducer to CCD is too close and should be increased. It doesn't give a quantifiable outcome, just a basis for some trial and error.
It doesn't replace some of the commercial programs available to measure and quantify field curvature. (e.g. CCDInspector etc.)

[Stonius]

Ah, thanks! :-)


M

[bojan]

Yep :-)

[The_bluester]

From memory I found a little residual coma in my C925 once I was at my happiest spacing wise with the combination of correction, reduction and vignetting. I was substantially under the 105mm from memory. I am about to set that scope up again so I will have to measure and record it properly this time.

There was a little residual coma (A vanilla C925 really was not intended to be an astrograph) but I actually found it handy. If the coma was pointed into the corners (As in, directly away from the center of the image) then the collimation was bang on. I was able to dial it in without resorting to CCD inspector or similar programs.

[Merlin66]

I incorrectly said the SCT coma was "minimal" - this is not true for the pre- ACF and Edge HD models.
The coma in a "standard" SCT is equivalent to that of a f5 Newtonian.
(Rutten & van Venrooij, "Telescope Optics", p 82)