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Old 29-09-2013, 05:47 PM
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Spookyer (Brett)
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Please check my logic in using a field flattener reducer

Hi all, I have the WO .8 field flattener reducer for my WO FLT132 scope.
I am using a full frame Nikon DSLR with it. This scope at 925mm focal length and the full frame sensor gives me a large field of view.

Using the flattener certainly improves the field flatness and makes a noticeable difference to the outer half of the image. It does however make my field of view even larger.

My thinking is this, if I am targeting an object that is large and goes close to filling my frame then I should use my field flattener reducer. I get to fit a bit more in and I get round stars to near the edge of my image. A win win.

However if I am targeting an object that is small and takes up say less than half of the frame then I am better off not using the reducer as I don't want to make the object even smaller and by cropping the image as I would for framing anyway I am getting rid of the field warping around the outer areas of the frame. So in this case by not using the flattener/reducer I am maximising the size of my subject and not suffering from field curvature in the final image anyway.

I hope I explained this clearly. Is my thinking correct on this? As a bit of a newbie I just wanted to check with some wiser heads.

thanks
Brett
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Old 29-09-2013, 06:06 PM
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The flattener is also a reducer. If you image the central area of your corrected field without the reducer it won't look as good as a crop of the same fov with the corrector on. So the answer would be no. Keep the flattener on.
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Old 29-09-2013, 06:49 PM
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If by chance you have a second crop sensor body, (doesn't everyone??) that will give a smaller FOV without removing the reducer.
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Old 29-09-2013, 07:40 PM
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Quote:
Originally Posted by multiweb View Post
The flattener is also a reducer. If you image the central area of your corrected field without the reducer it won't look as good as a crop of the same fov with the corrector on. So the answer would be no. Keep the flattener on.
But that cripples the image scale and you get a blocky pic?
Anyway, anything that widenes the FOV is evil by definition.
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Old 30-09-2013, 08:34 AM
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Originally Posted by multiweb View Post
The flattener is also a reducer. If you image the central area of your corrected field without the reducer it won't look as good as a crop of the same fov with the corrector on. So the answer would be no. Keep the flattener on.

Is that optically possible or is that another WO marketing hype claim?

I know the Astrophysics 155TCC is a reducer and I think its a fairly flat field although I have not tested it in CCD Inspector. Usually they are 2 different accessories.

I suspect marketing BS especially from WO who had no qualms claiming fluorite lenses when they were ED glass (which has a lot of fluorite in it).

Generally you use a reducer to get a wider FOV and also a faster F ratio meaning faster exposures due to more light coming from the wider FOV.

With full frame it takes a pretty high end reducer to give round stars to the corners. Usually a flattener is needed and it tends to be one or the other. If your use of it shows round stars to the corners and CCD Inspector shows a flatter field I'll stand corrected.

A wider FOV is handy for extended objects and a higher F ratio is handy for getting a bright exposure quickly. So the answer is it depends on the imaging target.

Have a look at Marcus Davies' images. He uses a flattener for regular targets, a reducer for wider extended objects and an extender to capture some galaxies/smaller objects. That's with a Tak TOA150.

Objects like the Large and Small Magellanic Clouds, Orion, Vela Supernova Remnant, Pleiades, Rosette, Eta Carina all are going to look nicer generally speaking with a wider FOV and faster F ratio to get extra depth.

Objects like NGC253 (Sculptor Galaxy), M83, Centaurus A, The Running Man Neb, perhaps the Horsehead require a bit more focal length so the reducer would not be used. In that case you would need a dedicated flattener as your scope may not give round stars to the corners on a full frame sensor - in fact it almost certainly will not. Virtually no scope does without a flattener/corrector of some type.

Often a flattener slightly extends the focal length by a small margin as with Astrophysics dedicated flatteners.

Greg.
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Old 30-09-2013, 08:55 AM
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Quote:
Originally Posted by gregbradley View Post
Is that optically possible or is that another WO marketing hype claim?
I had a P-FLAT IV. It went from x0.60 to x0.80 with an adjustable lens so yes you do get a noticeably wider field. Celestron SCT correctors sold by Teleskop.de also are tagged as x0.8 so again you get a wider field.
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Old 30-09-2013, 12:59 PM
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Quote:
Originally Posted by gregbradley View Post
Is that optically possible or is that another WO marketing hype claim?

I know the Astrophysics 155TCC is a reducer and I think its a fairly flat field although I have not tested it in CCD Inspector. Usually they are 2 different accessories.

I suspect marketing BS especially from WO who had no qualms claiming fluorite lenses when they were ED glass (which has a lot of fluorite in it).
Tak, ASA, TeleVue, and other brands that I think you'd consider reputable also sell combined reducer/flatteners. Even AP's CCDT67 and 27TVPH both have a (slight) flattening effect.

WO also sells a non-reducing flattener for the FLT-132, the PFLAT-68.
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Old 30-09-2013, 07:44 PM
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Well when I use the WO flattener reducer it certainly widens the field and it visibly makes the stars towards the edge of the frame much rounder (not always perfect though) so I don't think it is just marketing hype. I don't own CCD inspector in order to get a quantifiable effect.

Brett
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Old 30-09-2013, 07:51 PM
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Perhaps get to know it better with the reducer in Brett. Bit more messing with flats and focus point when you change things around.

I would have thought your logic is sound, however for a 20% change in image scale it doesn't seem like worrying about too much. Nothing to stop an experiment at a later date though.

Last edited by RobF; 30-09-2013 at 09:29 PM.
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Old 30-09-2013, 07:53 PM
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Brett,

I can't remember if you're playing with PixInsight, but if you are there's a PSFEstimator script that allows you to map the FWHM and aspect ratio in your subs. That's effectively what CCDInspector does. PSFEstimator doesn't distill the info into a few magic numbers but all the same info is there...

Cheers,
Rick.
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