Field flattener question.

[Ice (Ivan)]

Hello everyone, My question is as follows. Is the distance of a field flattener to the objective lens as crucial as the distance to the camera sensor to the flattener lens ? I have a 72 ed refractor, in the picture of the supplied flattener that I don't have, looks shorter than the flattener that I do have !! So setting the distance from the sensor is easy to get, but the distance from the objective is a fixed distance ? Any help will be appreciated Ice..

[The_bluester (Paul)]

Evostar 72? What flattener do you have? Apparently the "Correct" flattener is actually the one for the ED80. And they have a design issue. The ED72 has a female drawtube thread and the rest of them male, at the moment the only way to connect the two is a 2" screw in nose on the flattener but without modifying the compression fitting that screws in to the drawtube you wont get it to come to focus with the correct spacing.


I am in the process of getting a male 54 to male 56mm adapter made that will allow all threaded connection to eliminate the tilt that the compression fitting and nose results in. And it will increase my backfocus too. With the compression fitting modified in mine (Shortened by 4mm) I have about 1mm focuser travel left!


Apparently there is an official skywatcher adapter coming soon, but I am a bit concered they might be too thick from the photo I have seen, which means a choice between achieving focus and getting the spacing right. Time will tell. If they are thin enough I will probably buy one as they include a facility to rotate the camera.

[The_bluester (Paul)]

And I realised that I did not answer your original question.

The distance between the flattener and sensor is fixed, the distance between the objective and flattener is not, the flattener/spacer/camera become a single assembly that you move in and out to focus, just like you would have focused the camera before fitting a flattener.

[casstony]

There is an ideal/specified distance from the camera to the reducer. If you run out of inwards focus before achieving focus, the camera/reducer distance can be decreased by 2 or 3mm in an effort to achieve focus, but stars in the outer field will distort.

[Ice (Ivan)]

Thanks Paul, It makes sense now you put it that way

[Icearcher (Chris)]

Hi Ivan

I believe that the distance from the front objective to the FF/FR is your focus.

The distance from the FF/FR to the sensor is very important to get a very flat field. I have mine just running the normal spacing, 55mm I believe, and my field is pretty flat, maybe a tiny bit out but I haven't played to much yet as Im happy with the vast improvement over not having it. You can use delrin spacers to control the distance.

Paul, there is an adapter that screws into the 72ED drawer tube (after you take off the factory fitted compression ring) that allows the FF/FR to screw in, easy as pie. Not sure they are everywhere but I got mine in the package with the FF/FR.

With my setup, 72ed > FF/FR >DSLR, I still have about 3-4mm of inward focus but I haven't run into a situation where focus has been an issue.

[The_bluester (Paul)]

You could not use that one directly as it is M54 to M48 threads, but with an M48 to M56 thread ring you could, the flattener has an M56 thread.


The one I am getting made will be direct M54 to M56 and give a couple more mm of focus travel.

[Icearcher (Chris)]

Hi Paul

You are correct, I linked to the wrong one, Im pretty sure my adapter came with the 72ed FF/FR, thats what differentiates the package from the 80ed FF/FR even thought the actual FF/FR is exactly the same.

Little thing to note with the adapter, I believe the tread is M54x0.75, most other SW focusers are M54x1

[The_bluester (Paul)]

I can't recall the thread, I did measure it but the bloke making the adapter for me had just finished making one for someone else and had checked that it fitted properly, so I asked for "One just like that please"

No matter which way you slice it though, the decision to use a different drawtube thread in the ED72 focuser compared to the 80 and 100 which use the same drawtube OD is a monumental cock up on Skywatchers part. All they needed to do was simplify their stock lines and use the same drawtube and the ED72 would fit all the same adapters as it's bigger brother. The irony being that the two speed crawford is much nicer to use than the single speed that the ED80 gets!

[Ice (Ivan)]

hi guys and thanks for all your feedback, However My 72ed is a TS-Optics Photoline 72ED. I suppose I should have opened with that Info !! I think I may have found the problem, The flattener that I have doesn't appear to be compatible with my Scope. being it is only 400mm focal length. So I have ordered a Orion Flattener for short tube refractor's and fingers crossed it will be the answer to a nice wide flat field.


Cheers Ice..