How do I check my backfocus is tuned?

[Viktiste]

How do I check my backfocus is tuned in? I have measured it out and the best I can get with the spacers etc. at hand is about 2mm or so too far back according to the specs. The spec is 66mm (or 67mm with EFW) from flattener face to sensor for the Esprit ED80 if I understand it correctly.

How do I visually check it in the pictures taken? Can I check it during daytime? And how accurate does it need to be? This is for a F5 Esprit 80ED+flattener+ZWO OAG+ZWOEFW+ASI1600mm.

This is my setup:

 

There seems to be a flaw in that the  M48 threads on the M62-M48 crossover on the flattener are so long they hit the OAG prism pin before it it is screwed completely in. This crossover is 11mm. So therefore i had to put a 1mm spacer in the  connection between the crossover and the OAG to make a tight fit.

Theoretically according to the above picture: 11mm (M62-M48 xover) + 16.5mm (OAG) + 11mm +2mm +20mm (EFW) + 6.5mm (cam) = 67mm.

Measuring it out when screwed together (inclusive the extra 1mm spacer) from the face of the flattener to the face of the camera = about 62.5mm, then add the 6.5mm of the camera = 69mm.

 

 

Edited October 15, 2019 by Viktiste
Because English is not my first language...

[Thalestris24]

Distance from reducer to camera sensor is normally 55mm but can need some tweaking ~+/-1mm. A check with a digital caliper is useful.

Louise

[Viktiste]

I did measure it with a caliper, it says I am +2mm . I have no way to tweak it lower. Unless I grind down the 11mm female female spacer with sandpaper on a powertool. And I don't want to do that unless I can somehow confirm I need to.

[Thalestris24]

11 minutes ago, Viktiste said:

I did measure it with a caliper, it says I am +2mm . I have no way to tweak it lower. Unless I grind down the 11mm female female spacer with sandpaper on a powertool. And I don't want to do that unless I can somehow confirm I need to.

Not sure, but the above zwo scheme should give you 56mm which is ok because you subtract 1mm for the filter. Caliper measures are a guide. I would just test it to see what your subs are like. Fwiw I think using an oag with an 80mm frac is likely more trouble than it's worth. An ordinary guidescope should be fine but that's up to you.

Louise

[Physopto]

Don't forget the thickness of the filters will change the back focus.

It depends upon the filter thickness and refractive index of the glass used. Most filters have a refractive index about the same. Some manufacturers filters are 1mm thick and others can be 3mm thick. So the effective light path length is changed in a system depending whose filters you use.  As the refractive index of a filter can be around 1.45 the thickness of the filter affects the speed of the light and OPL is changed. So OPL for the filter is dn where d is the glass thickness and n the refractive index. A 3mm thick filter can change the optical path length by up to 1mm where as 1mm filter it is only about 0.3mm.

 

[Viktiste]

9 hours ago, Thalestris24 said:

Fwiw I think using an oag with an 80mm frac is likely more trouble than it's worth.

Perhaps, I have never used an OAG before, guess I will find out. 

My filters are the ZWO filters, they quote 'add 1mm'.

How do I actually see that the backfous is correct? Just that I can focus sharply?

[Thalestris24]

10 minutes ago, Viktiste said:

Perhaps, I have never used an OAG before, guess I will find out. 

My filters are the ZWO filters, they quote 'add 1mm'.

How do I actually see that the backfous is correct? Just that I can focus sharply?

It's not to do with focus - the flattener is to correct for a flat field, especially in the corners. If it's not flat you'll see distorted rather than round .stars.

Louise

[Viktiste]

Ah. Ok, thanks Louise. So waiting for a clear night now, might take a while.

Edited October 16, 2019 by Viktiste

[Pompey Monkey]

I have an Esprit ed80 too.

I'm afraid  that there is no way to get the oag and the fw behind the flattener without exceeing the back-focus distance of 55 mm.

As for tuning the spacing, some imagers luck out with the exact recommended 55 mm, but i didn't! Two years on and i stil haven't  got it perfect,  but it is "good enough" 😁

[Merlin66]

Can the OAG be fitted in front of the flattener?

 

[ollypenrice]

This thread includes diagrams posted by Demon Performer which show the effects of too much and too little chip distance. If you really can't get the OAG to work it's worth saying that you certainly don't need one with an 80mm refractor. A very budget guide scope or finder guider will do fine. It isn't necessary to buy guidescope alignment rings. You simply bolt the guidescope down roughly parallel with the main scope and get on with it.

 

[Viktiste]

Jepp, you were right about the OAG Louse/Olly. The OAG just has too small a FOV to be practical. So I fitted the finder scope from my 200P on the Esprit 80ED and use that as a guide scope. It was not possible to get camera focus with the finderscope that came with the Esprit. So I guess the OAG will live in a box, and might be used should I ever try imaging with the 200P again.

 

[Viktiste]

So what is the verdict on the backfocus here? Single frame of M31.