Explore Scientific 3" field flattener back-focus measuring point

[Bikeman55]

I posted this in another section but perhaps it better belongs here.

The title pretty much says it all...

Where do I go about measuring the 55mm of back-focus on the Explore Scientific 3" field flattener/reducer.

I have attached a couple pictures for reference so we can refer to the same locations.

I thought from the research that I have done I should measure from point A but after last night's first use it seems as though I got something wrong and I wanted to make sure I was measuring from the correct spot before I really start to scrutinize my spacing to sub-millimeter or perhaps my imaging train is not quite "plumb".

Any insight would be greatly appreciated.

Thanks.

[Waldemar]

 

removed because the link did not work. Sorry

 

[Fredmt]

Hello,

I've been looking for this answer a while already and found 2 different measurments ... C and A on your pictures.

I messaged ES support and they said to me that it's technically from the surface of  the glass which is A I think.

But still waiting for some other answer from users.

PS : I have an OSC ZWO ASI071 (Back focus 17.5mm) + 2 spacing rings provided with the camera (16.5mm and 21mm). So I get these 55mm but on the C mark ><

 

 

[Waldemar]

Maybe this is helpful to find the exact distance:

FF distance to sensor.docx

[Bikeman55]

Thank you all for the responses. 

Seems as though there are several different opinions on where the backfocus should be measured from. I did end up talking with Explore Scientific about it and they told me the same thing as Fredmt in that it is technically measured from the glass but they said point B from the pictures. I am currently waiting on a variable spacer to come in where I can do some star tests and figure it out once and for all then get a more permanent spacer afterwards. 

And thank you Waldemar for that diagram. That is fantastic and very helpful. I will definitely be referring to that the next time I'm out.

 

Cheers

 

 

 

[Fredmt]

Thank you @Waldemar for the doc.

Base on this, it seems that my backfocus is correct (55mm from point C on the image above). The image looks great (a few test frames on M51), eventhough there is a slight vignetting (which is not a problem actually with flats). I'm still waiting for 0.5,1 and 2mm spacer to see if I can get an even better image (some stars are not exactly round near the borders, but I need to zoom quite a lot to see that)

The second problem is 2'' filters. But I'm on my way to fix that (and this is not the subject of this post).

[JonathanA]

Hi,

All the above useful but I'm hoping to resurrect this thread as I have been having exactly the same problem and have spent hours trying to sort it.  I’ve tried various suggested distances between the focal plane of (Nikon DSLR) camera and the reducer haven’t achieved a flat field yet. I’m hoping someone has finally resolved this.

A couple of things to add to the discussion

The ES USA website (and some of the forums) are saying that 55mm is the back focus distance but there is no indication where this is measured from on the reducer. If it is from the lens surface, then would this be from the edge of the optical element or from the centre (it is obviously very curved)?

Puzzlingly, I know that the reducer used to come Nikon and Canon specific adaptors (arrowed in blue on the image below).  Studying images of these, it looks to me that using them would have given way more than the suggested 55mm of back focus.

To make this even more confusing on the ES Europe site they have the diagram below which seems to say something altogether different about the back focus required - ‘about 65mm’. (I’ve tried it – doesn’t work).

Has anyone got a definitive answer to where to measure from on the reducer? There are various suggestions in the forums but, looking at some of the posters’ setups and the absence of actual images taken through the reducer, I’m a bit doubtful of these especially as I've tried most of them!

Thanks.

[Waldemar]

The only way to get this sorted is by trial and error. Too many factors are  involved for a manufacturor to give you the exact distance.
 You need to be able to take steps of max 0,1 mm to find the ideal distance. An adjustable extension like this: https://www.firstlightoptics.com/adapters/baader-t2-extension-12-16mm.html or the Baader Varilock (2 lengths available) are the easiest way to do this.

[JonathanA]

21 hours ago, Waldemar said:

The only way to get this sorted is by trial and error. Too many factors are  involved for a manufacturor to give you the exact distance.
 You need to be able to take steps of max 0,1 mm to find the ideal distance. An adjustable extension like this: https://www.firstlightoptics.com/adapters/baader-t2-extension-12-16mm.html or the Baader Varilock (2 lengths available) are the easiest way to do this.

Hi Waldemar,

I think at the level of precision that you are talking about, tenths of a mm, you might be right about there being many variables. But the basics are pretty well defined - focal ratio being the key. Certainly other manufacturers and sellers seem able to provide detailed information. We have a situation here where on the USA version of the ES website they suggest 55mm back focus and on the European version they suggest 65mm! There's definitely something wrong there.

Fortunately, someone very helpful from ES USA did respond quickly to my enquiry - see below.

'I consulted with one of our optical engineers to confirm: The measurement starts at the rear flange (flat surface) of the corrector. NOT from the CCD M24x0.75 plate -- take that off, then start the measurements.

The M42x0.75 plate adds approximately 3 mm.

The backfocus is 55 mm. There is an additional 2 mm either side where the correct is acceptable.

You cannot judge T-rings by their total thickness – because that is not the amount they add. They have threads on one end that screw onto the M42x0.75 adapter – so those threads don’t count. The T-ring also have a bayonet (or other) mounting system that, when coupled with the camera, subtracts from the T-ring’s overall measurement. All those variables have to be accounted for in creating your system.'

It is puzzling that ES do not support some of their products with clearer information. No guidance at all comes with this product. A similar reducer supplied by Willams Optics comes with very detailed information about back focus and other parameters.

Thanks for suggestion of the variable extension tube - I had thought about that approach but unfortunately to get the correct backfocus with a Nikon DSLR I am going to have to go down the low profile T adaptor and very thin extension ring route.