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Old 16-10-2019, 08:33 PM
Capella_Ben (Ben)
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EdgeHD Focal Reducer question

Hi All,

I have just purchased an EdgeHD 9.25 and focal reducer and have been grappling with image quality issues when using the focal reducer.

Tonight, I did a test with and without the FR and I noticed that the focus point between the 2 configs is about 15-20 turns of the focus know different.

So, if I focus at F10 the add in the FR I have move the focus knob CCW about 15-20 turns to get focus.

Is this right? I would think that the focus point should be roughly the same.

I notice that with the FR I get heaps of curvature and elongated stars all over the place.
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Old 17-10-2019, 09:09 AM
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sil (Steve)
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focal reducer reduces the focal length so no it should NOT be roughly same focus point. EdgeHD is designed to gived flat to the the edge image. Hence the names of these products. EdgeHD doesn't require a focal reducer to remove distortion and you are removing distortion from an undistorted view hence creating distortion.
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Old 17-10-2019, 10:00 AM
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The_bluester (Paul)
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There is a focal reducer by Celestron that is specific to the edge HD scopes (Unlike the generic 0.63 reducer/corrector for their other SCT scopes)

What I have found with my SCT and reducer/corrector is that the spacing between the corrector and sensor is as critical as any other, get it wrong and in the case of the standard SCT at least you will have distortions and as the spacing increases, increasingly severe vignetting. I don't know if it applied to the Edge HD specific reducer but most corrector reducers, if the stars in the corners show comet tails pointed away from the center of the field then the spacing to the sensor is too small, and if it shows a fishbowl effect with the stars displaying arcs in the corners then it is too large.

And yes, add or remove the reducer to the train and your focus point will change significantly.
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Old 17-10-2019, 10:45 AM
Capella_Ben (Ben)
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Ok, cool. So I should expect the focus point to change. I can deal with that.

I thought it might have been the cause of the large amount of curvature when using the focal reducer (and it is the specific one for the edgehd 9.25).

I expected the focal reducer to give the same flatness as without it. F7 and f10 is supposed to have exactly the same back focus.
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Old 17-10-2019, 10:57 AM
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It takes a while to install the indexes but All Sky plate solver has a settings helper that involves plate solving an image online to get an arc seconds per pixel figure and using the same image in the helper, will work out your effective focal length. Depending on the camera and software used to take it you might have to tell it your pixel size, on my setup it pulls it out of the FITS file header.

When I set my reducer/corrector up as the internet suggested (Measuring from the end of the threads not the base) I found it was giving me about 0.56 reduction not 0.63. so it is useful to compare. If it came up with significantly under 1645 focal length for an image sub with the reducer in the train then the spacing is probably out.
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Old 18-10-2019, 10:34 AM
Capella_Ben (Ben)
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I have plate solved an image with the focal reducer and the measures focal length is 1649, so 4mm longer than spec.

I'll have a go at changing the back focus distance to see if I can get it to look flatter.

BUT, I should not have to do this. It is a VERY expensive reducer and it is advertised has having the same back focus.

Does anyone else need to change their back focus when changing from F10 to F7?

Thanks.
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Old 18-10-2019, 10:59 AM
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Back focus and change of focus are different things. I can't comment on the particular reducer you are using as I haven't got an edge scope or reducer, however back focus is referring to the distance between the reducer and camera sensor, not the setting of the focus knob. about 20 turns is probably similar to the change in focal plane with my reducer/corrector in or out of the train, but as mu scope does not have a built in corrector I can't compare any back focus difference meaningfully.

If it is supposed to have the same back focus as the bare scope, given an edge HD has a corrector built in to the baffle tube what it means there is if your corrector to camera spacing is right, you should be able to simply put the reducer in between the rear cell of the scope (Which has the corrector in it) and your imaging train, and if your field correction was good before, it should still be good after. The only other alternative would be if they mean that the back focus between the rear cell of the scope and the sensor stays the same with or without the reducer, but I really doubt that.


Another possibility (Again, without ever using an edge with or without reducer) is that your spacing was not correct to begin with and adding the reducer has made aberrations apparent that were off edges of the chip before.
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Old 18-10-2019, 11:11 AM
Capella_Ben (Ben)
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Thanks Paul.

Back focus is probably the wrong term to use. As you note, that will change between F7 and F10 as the FR is not parafocal. I found in an other forum that this is about 6.24mm of primary mirror movement which probably equates to my 20 turns of the focus knob.

MY problem (I think) is to do with a change a spacing between either the back of the scope or the back of the FR the sensor. This is supposed to stay the same says Celestron, but I found that the image is flat when in F10 and curved when in F7.

I'm thinking that the focal reducer is defective as it is not adhering to the specified spacing.
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Old 18-10-2019, 11:21 AM
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I havew often found that the "Correct" spacing is not really as iron clad as it is supposed to be. the 0.63 reducer is supposed to be 105mm spacing on my scope but I have found setting it at that produced abberations and pretty severe vignetting and a focal ratio more like F5 than F6.3 and I have reduced the spacing accordingly.

Rather than calling it defective straight away, I would be inclined to move the spacing around a bit and see what happens. The aberrations start in the corners and move inward as the spacing gets further from correct so if your previous spacing was not quite right you may not notice if the problems it caused were off the chip edges, and a reducer may bring them in to view.
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Old 18-10-2019, 11:41 AM
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Have you got a photo of your setup with and without the reducer in it?
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Old 18-10-2019, 11:47 AM
Capella_Ben (Ben)
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Quote:
Originally Posted by The_bluester View Post
I havew often found that the "Correct" spacing is not really as iron clad as it is supposed to be. the 0.63 reducer is supposed to be 105mm spacing on my scope but I have found setting it at that produced abberations and pretty severe vignetting and a focal ratio more like F5 than F6.3 and I have reduced the spacing accordingly.

Rather than calling it defective straight away, I would be inclined to move the spacing around a bit and see what happens. The aberrations start in the corners and move inward as the spacing gets further from correct so if your previous spacing was not quite right you may not notice if the problems it caused were off the chip edges, and a reducer may bring them in to view.
Yeah I'm going to do that soon. I might buy a variable T2 spacer first, just to make it easier.

I'm probably just a bit cranky that I have to go to so much effort. It is a VERY expensive FR!

Thanks for your help.
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Old 18-10-2019, 11:51 AM
Capella_Ben (Ben)
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Quote:
Originally Posted by The_bluester View Post
Have you got a photo of your setup with and without the reducer in it?
I do, but it is not overly exciting. I have paired down the imaging train to just an ASI071MC with spacers to give 55mm from the t-adapter. Wanted to remove any source of problems from other components (OAG, filters).

https://drive.google.com/open?id=1ri...hDYpiJTFTwKyHR

https://drive.google.com/open?id=1Sy...uREL3fqIW7h-tf
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Old 18-10-2019, 12:17 PM
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The sensor spacing is supposed to be 146mm according to Celestron, but that is to the end of the camera side threads of the reducer not the base of the threads, I have always though that was stupid, the measurement being given is to something that you cant directly measure when it is assembled!

Seems these work the opposite way to other reducer/correctors. Common wisdom is if you have comet tails pointing outwards then the spacing is too small and if you have a fishbowl effect with arcs around the edges, then it is too long, but this is taken from the Celestron blurb.

"Being outside of focus (too long) tends to produce elongated star images towards the edge"

I still suspect that your spacing was not quite right at F10 but the aberrations were not evident within the area of the chip. One other spacing impacting thing, I am not sure if the official camera backfocus measurement allows for the focus impact of the sensor chamber window.
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