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Looking for a good reducer/flattener

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After a break of about 3 years I'm back in amateur AP.

I have a new apochromatic refractor now, but I'm having a very curved field with comet like stars at the edges of the field.
What I don't understand is why the curvature of the field is apparent not only at the edges, but up to quite close to the center as well. The aberration is worst at the edges though.

It's a 130mm aperture, 910mm focal distance triplet with a FPL-53 ED element and 4 inch rotatable Crayford focuser.
You can see all specs here:

As it's an f7, I find the FOV too narrow as I'd mainly like to do DSO imaging.
Can you recommend me a good reducer/flattener that will work well with this scope?
I'd like a reducing factor of around 0.7x - 0.8x.
I'm now imaging with a Canon DSLR, but I'm thinking of upgrading to a QHY-163M with filter wheel.
Can you also tell me how I can connect this reducer/flattener to my focusser?

Thanks in advance!


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Robtics suggest this


as flattener, which seems to be the same as the TS counterpart for a similarly specced triplet. TS recommends the following flattener:




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Thanks Michael,

I had seen this before, but I was looking for a flattener/reducer in 1 package.
Or would it be better to place a non-flattening reducer behind (or in front of?) the flattener?

I'd very glad to get more advice on this as I have no experience with these parts.

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Any advice on this?

I'd like to buy a really good product for my scope.

Any thoughts on the APM Riccardi reducer-flatteners? 

They are extremely expensive, but really worth the money?

I'm kind of lost.


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Reducers *are* flatteners as well, you don't need both.

I currently image with the TS Photoline version with the TS 2.5" flattener, and find the FoV with my 694 sensor to be fine.

I may add a reducer later, and will probably buy the APM small reducer. Yes it's pricey but I think worth it.

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Although manufacturers say that their reducer-flatteners are compatible with scopes of within a certain range of specifications you are taking a risk on an untried combination. There can be problems with internal reflections, for instance. I may be wrong but your scope might be the same as the Altair Astro 130 F7 for which both a pure flattener and a a reducer flattener are available. Scroll down this page to see the accessories. https://www.altairastro.com/altair-wave-series-130-f7-ed-triplet-apo.html

The 0.79x three inch reducer flattener might be of interest to you. Note that I can't be sure that this is essentially the same design as your scope.


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I tried a low end reducer... It didn't focus.

I tried the top end, but couldn't find the required spacers. Arrgh!:tongue:

Then I tried the ONE FR recommended for my CCD camera. And finally, I found a good 0.5X reduction in my CCD which gave me what I wanted all along....

Note that the right Focal Reducer that works with your equipment, also gives you the ability to choose. You can choose to use it...

Or, you can chose to not use it.

If you can find that, then I'd say you have found the right Focal Reducer.

PS: Welcome Back to the Quest.

Edited by SonnyE
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Thank you all for your helpful advice.
Olly you are right, that Altair telescope is optical the same. I think it's built in the same factory.

I decided to go for the TS Photoline 3" 4 elements FF/FR because it is also recommended for TS scopes that have same optics and it has a M68 thread to which I already have an adapter from my focuser.
I'll let you guys know how it performs.

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