Focal reducer spacing

[festoon]

Hello,

I'm hoping to resolve an issue that I just cannot work out

I am using a C5 OTA (f/10 native) with a Maxfield 0.33X focal reducer and a revolution imager R2 (976x582 pixels, 5um x 6.25um pixel size).

I've emailed the manufacturer of the reducer to get information on the correct reducer to sensor distance to obtain optimal results the following image was sent to me (ps the reducer is not the nexgen version but the original maxfield 0.33X). Referencing to the flange position of the reducer the spacing to the sensor should be 33.8mm

In my setup I'm achieving a set up with a f/3.2 with a spacing that is very different to what is described here. In my measurements I'm referencing to the flange back focus position (which is meant to be 33.8mm). My best guess would be my spacing is 40-45mm, likely around 43mm. At 33.8mm I'm still in the T-spacer!!

My solved image size is 41.9x31.4 arcmin. whereas the calculated FOV for the sensor, with the C5 OTA and no reducer is 13.2x10.2 arcmin (or as shown below 0.22x0.17 degrees). Therefore, with this set up I'm getting solved images at f/3.2.

So this is what I find very confusing. I've achieved almost the right reduction factor for the lens but the spacing I need to achieve this is very far out (maybe 10mm more than I would have expected).

Is anyone please able to think of what is going on? Why the discrepancies?

Thank you!!!

[fozzybear]

Are they not assuming you are using a Std C5 Sct adapter 2" I see you are using a Baader one? Just a thought I do not know just re-read ignore my silly reply Duh

[LightBucket]

The reason is you are using an SCT scope, where moving the primary mirror adjusts focus, this in turn alters the f ratio of the scope at any given position of the mirror, so the figures will be different on spacing depending on focus position, I have the same issue with my Meade SCT my reducer is about 10mm different to the specs, but I achieve correct reduction..

Edit, not alters f ratio, alters focal length...sorry 

[festoon]

Thank you @fozzybear. Its great to have some help on this!! Its a valid suggestion, and one I've suspected may be involved. Certainly the focal reducer is further away from the exit of the OTA than with a standard sct adaptor. 

Using the http://www.wilmslowastro.com/software/formulae.htm#FR_b webpage to see what effect this has. The first is for a spacing of 0mm between the focal reducer and the scope. and the second is for 50mm (a rough approximation based on my ruler measurements). To achieve a focal reduction to 3.3 with an additional 50mm spacing you would have to increase the spacing by approx 1mm based on this.

Since I don't have the published values for the focal length of the Optec maxfield I've substituted numbers which give the correct reduction to 3.3. So these are not necessarily correct. I assumed it all works fine for the C8 so this is my starting point

Then I've changed the OTA details to a C5. To achieve the same focal reduction you have to increase the spacing to the sensor by approx 1mm

And now if I add in a Scope-FR distance of 50mm to get the same reduction I need to increase the CCD-FR distance by approx 0.7mm.

So based on this I would expect that for the C5 with this extra distance due to the SCT adpator the maximum I would need to add is 2mm.

So yes you are right, based on this maybe some of the extra distance is due to the effect of the baader SCT adaptor. Also some of the effect may be due to that I'm using a C5 OTA.

But I still think I'm way out from where I expected

 

 

 

[LightBucket]

Anything you add to the back also alters focal length, so a different figure will appyle, same as I said above with moving the mirror..

[festoon]

Thank you for the reply @LightBucket 

It good to know you have also witnessed this effect. 

I'm just trying to understand the details of what you are saying. So the OTA is designed to be f/10 for a specific primary mirror position.

I will have achieved focus with the reducer for a primary mirror position that is different to this, hence as you say altering the focal ratio of the scope.

Forgive me if I'm wrong....is this not what the above calculations from Wilmslow Astro are taking into effect?

Thank you again  

[LightBucket]

5 minutes ago, festoon said:

Thank you for the reply @LightBucket 

It good to know you have also witnessed this effect. 

I'm just trying to understand the details of what you are saying. So the OTA is designed to be f/10 for a specific primary mirror position.

I will have achieved focus with the reducer for a primary mirror position that is different to this, hence as you say altering the focal ratio of the scope.

Forgive me if I'm wrong....is this not what the above calculations from Wilmslow Astro are taking into effect?

Thank you again  

Sorry, yes but that post was not there when I posted, they must have crossed..

Also the measurement is usually taken from the front of the reducer lens assembly, camera side..

 

[festoon]

cool  thanks @LightBucket

So based on this we can maybe explain 1.5mm discrepancy in spacing between the literature and what we are seeing in reality.

Can't work out where the rest is coming in!!!

The reason I ask this is Optec discuss the importance of the correct spacing to achieve the best results

To achieve these 'best results' should you aim to have the magnification correct, but the spacing wrong...or the spacing right, and the magnification wrong?

 

[LightBucket]

10 hours ago, festoon said:

cool  thanks @LightBucket

So based on this we can maybe explain 1.5mm discrepancy in spacing between the literature and what we are seeing in reality.

Can't work out where the rest is coming in!!!

The reason I ask this is Optec discuss the importance of the correct spacing to achieve the best results

To achieve these 'best results' should you aim to have the magnification correct, but the spacing wrong...or the spacing right, and the magnification wrong?

 

The spacing wrong every time, this will alter on these type of scopes and is not an exact science, go with the discrepancy and enjoy imaging..