How do these figures compare?

[swag72]

I have an image taken with my Pentax 75SDHF / ATik 460 and AP 0.67x reducer. According to CCDCalc I should have the pixel scale and field of view as highlighted in the image attached. When I got the image with this setup solved by Astrometry it gave me a very different scale and field of view.

Which of these is right and can someone explain the difference?

[Photosbykev]

The field of view values are very close to each other if you convert arcminutes to degrees

Did you resize the image before putting it into astrometry?

[Jessun]

Astrometry suggest a FOV of over 7 degrees (EDIT looking at the arc sec/pixel value). I'd guess that is way off. CCD calc sometimes makes the chip size in mm a bit small, but only by a fraction.

On the other hand CCD calc will assume you have nailed the distance for the reducer. It is 0.67 at its sweet spot, but did you nail it in the imaging train?

/Jesper

[swag72]

The field of view values are very close to each other if you convert arcminutes to degrees

Did you resize the image before putting it into astrometry?

I put the original size, uncropped onto Flickr - I don't know if it uses the original loaded up size for this task.

Astrometry suggest a FOV of over 7 degrees (EDIT looking at the arc sec/pixel value). I'd guess that is way off. CCD calc sometimes makes the chip size in mm a bit small, but only by a fraction.

On the other hand CCD calc will assume you have nailed the distance for the reducer. It is 0.67 at its sweet spot, but did you nail it in the imaging train?

My issue is that I cannot use the correct spacing on the reducer, so Iknow I'm not getting the 0.67x reduction, but wondered fi I'd be able to use this to tell me what reduction factor I was getting.

[Jessun]

There's a couple of tables and formulas on the AP website you can use to guesstimate what reduction you get for a given distance.

I'd experiment to find a sweet spot for your particular setup - judgeing only by image quality, going to start doing that myself with my CCDT67.

Is it very important to know if you're at say f4.5 or f4.6 by the way?

Apart from the pixelscale value of 9.63 arcsec/pixel which I don't understand at all, the other values as Kev said are pretty close.

[swag72]

It's not that important to know my reduction factor or speed of the scope, but I would like to know!!!

My spacing is looking at being quite short indeed, so I reckon I'll perhaps be 0.8x by the end .............. if it works!!! It's still possible that it won't!