Focal reducers and flatteners for skywatcher ed80

[SuperG]

I’m having a dilemma. First is there a good alternative to the skywatcher ed80 0.85x reducer? The SW one is expensive and at that price a wider field ed72 would be desirable.

I’ve tried the televue one but it is not a good match. Has anyone tried the Orion reducer? But then I need an adapter to use that one.

[tvandoore]

Not the same scope, but I found the Astrotech 0.8 FF/FR to work quite well on my Explore scientific ED80, and it didnt cost a fortune. Having said that, the ES ED80 is f6 while the SW ED80 is f7.5, and the Astrotech is designed for f6. It says that it will work up to f7, but f7.5 would probably be pushing it.

[Ukastronomer]

This is a VERY good scope

[astro744]

Someone more into imaging than I once told me that the WO P-Flat-2 is a nice match for the ED80. (Only available used now, I think). I'm not sure if the model III is as effective with the ED80.

[Ukastronomer]

Stupid question does the 120 fit the 80, I assume not for reasons beyond me

https://www.ebay.co.uk/itm/Sky-Watcher-0-85x-Focal-Reducer-Flattener-for-ED-PRO-120-and-Equinox-Telescope/254159955815?hash=item3b2d1d2767:g: pSAAAOSwOwNchpnq

[Ukastronomer]

This is dirt cheap does this not work

https://www.firstlightoptics.com/reducersflatteners/ovl-field-flattener.html

[NorthernLight]

I have got the Orion 0.8x reducer and have some trouble with it. It just goes into the draw tube and is held with the screws. What I noticed is a distortion towards the frame edges: as if the stars rotate around the center of the frame. They are elongated little arcs. It does not show so much on subs but once stacked the effect is quite obvious. I have tried different spacing ie not fully inserted into the draw tube but that did not solve the issue or my method was not fine enough. I wonder whether it needs to go in deeper? But that cannot be done since it’s hard up against the flange.

[The_bluester]

I had that effect quite spectacularly with my C925 and reducer when the spacing between the reducer and sensor was too great.

[NorthernLight]

Could you fix it? Maybe there is a lower profile T2 Adapter I wonder?

[The_bluester]

In my case I was able to cut the spacing down a lot between the corrector and the sensor. First up was exactly what you mentioned, I realised a diagonal I had could be scavenged to give me an SCT thread and a much shorter adapter length. Then I was able to cut the spacing between my guider and imaging cam a bit. At this point the guide cam is only just acceptably in focus with the focuser for that part racked in as far as possible.

[SuperG]

I've heard a couple of things about the spacer between the corrector and sensor; I'm told 55mm is ideal but I don't know if there is any real evidence to support the distance or is it just anecdotal?

[The_bluester]

I have actually just bought an ED72 and am waiting to see what the "Correct" flattener for it turns out to be. I have read that it is actually an ED80 flattener (And is even marked as such) with a male/male thread adapter to fit it to the female drawtube thread of the ED72 (The rest of them with that size drawtube are male threads)

The issue with them seems to be that in-focus travel is very limited. If it proves to actually be the ED80 flattener I am going to see if I can get an ED80 drawtube to screw the flattener directly on to it, saving a couple of millimeters more.

It seems on the ED72, 55mm is the smallest flattener to sensor distance you should look at, but then the in-focus travel is an issue again.

I can tell you that with no flattener, the curvature is very obvious. I did a rough first light without one last night.

[Merlin66]

Max, et al,
The Baader diagrams for reducer spacing assessment are useful guides...

[The_bluester]

Hi Ken, a version of that showed up on Facebook for me this morning. In the case above of me fiddling with spacing on my SCT I certainly had an example of the second image.

For anyone else who does not know, the first one with the stars radiating away from the center of the FOV indicates spacing between the reducer and camera that is too small, the second with the radial arcs is for it being too large.

[SuperG]

Ken and Paul

This is great information. I have definitely seen the second case before.

[The_bluester]

I had seen 55mm mentioned for this reducer on an ED80 and greater spacing than that for the ED72 that I have, but at 55mm it seems to be almost spot on. I have to get rid of a compression fitting that is introducing some tilt before I can really fine tune it.

That 55mm assumes I am measuring from the same place as Skywatcher, I was measuring from the base of the sensor side threads but they might measure from the top of the threads, they don't specify.

[Merlin66]

Measurements are usually taken from the rear shoulder. The threaded section screws into the spacer/ adaptor and hence as no impact on the spacing.

[The_bluester]

That was what I assumed, but I have seen once instance where the end of threads was specified.

[NorthernLight]

That’s really useful. Definitely the second image. So that means I have to get the sensor closer to the flattener.

Does somebody know how to do that? The T-ring/bayonet screws to the flattener, the camera to the bayonet. How does one make adjustments?

[Merlin66]

Max, on a couple of setups I had to use a special t thread to eos adaptor which is only 1mm thick and add sppacers.
I got mine from Bern at modern astronomy in the uk.

[NorthernLight]

Thanks Ken, a good Suggestion. I will folllow up on that lead.
Cheers