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Is this the correct back focus setup for ASI2600mc camera?


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Hi,  I am starting out trying to get my imaging train setup correctly.

I am hoping someone might be able to advise me if i have this all set up correctly or not.

The imaging train is to be connected to a Skywatcher esprit 150

The camera is a ASI2600mc

The Esprit comes with its own Field Corrector.

According to the manual, it tells me that the Field corrector back focus is 96mm ( measured from the outside edge of the casing or 100mm if measured from the actual lens inside the Corrector.

So this is what i have setup and was hoping someone could please correct me or tell me if it is correct.

Below is a diagram ( a tad hard to read ) and a photo of the setup.

It should read as:

Field corrector, Adaptor 43mm, Spacer, 14mm,Filter

draw with filter inside 21.5mm, Camera with 17.7mm to the sensor.

The specs on the camera say 55mm back focus,  but i just can't figure out how that figure plays in with all of this,  I thought that there would only be the one back focus figure, 96mm for the Field Corrector,  so were dose this 55mm of back focus from the camera come into play?

 

WIN_20211128_11_53_14_Pro.jpg

WIN_20211128_11_35_46_Pro.jpg

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The spacing for 55mm back focus mentioned in the camera manual is for connecting to the many field flatteners which are designed for 55mm spacing from the FF rear flange to the camera sensor. This is because a DSLR + camera mount to T2 (or m48 if necessary) adapter is 55mm from the adaper front to the camera sensor so will screw directly on the 55mm back spacing FF without any extra spacing.

As your FF has 96mm back spacing this 55mm figure doesn't apply, though I believe there is an extension spacer included with your field flattener which  would allow a DSLR and adapter to connect to, and so achieve the 96mm. This spacer would be 41mm.

Possibly your 43mm spacer mentioned is intended to fill this role. If any glass is included between the FF and the sensor then 1/3 the thickness of the glass should be added to the 96mm to give the actual spacing distance. With a 2mm thick filter and a 2mm thick camera protect window this means 1.3mm should be added on to the 96mm. The 43mm spacer is possibly to help take account of this.

Your diagram is essentially correct for 96mm, though add the 1.3mm or the actual figure depending on your glass as necessary to the 96mm.

My only concern is the14mm spacer/adapter to your filter drawer in your picture. Its diameter looks rather small. Is it M42 (or T2) thread? Your camera is M48 thread, as well as the FF rear. Putting M42 that far in front of the camera may well cause some vignetting. If the 14mm section is actually M48 then no problem.

Alan  

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Thanks for the reply and advise, appreciated.

Yes that 14mm spacer is a M48 thread,  kind of looks small in the photo compared to the rest of the fittings / adaptors added to it.

It basically just screws Straight onto the ZWO filter draw without any reducers and the 43mm adaptor is a M48 - M68 adaptor.

I can see what you mean by adding 1/3 of the thickness of any mirror between the FF and the sensor.

That is were the 14mm spacers comes in,  The filter draw is 21mm,  so i added .5m to allow for the filter that is inside it bringing the filter draw up to 21.5mm.

I wasn't too sure if i had thigs installed in the correct order or not,  but from what you are saying,  it sounds like i have it all set up correctly?

Or sound i be putting that spacer in front of the filter so that it goes camera, spacer, filter, adaptor, Field Flattener?

 

 

Edited by bluesilver
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It's common to put the filters closest to the camera but this is when using 1.25" filters where vignetting may be an issue. If they are 2" filters like it seems you're using, then their position in the optical train is not really an issue. Having them further from the camera may be beneficial if there is evidence of reflection artifacts in your images from the filter surfaces but this is unlikely.

1 hour ago, The Lazy Astronomer said:

I believe the extra spacer/shim should go between the filter and the camera.

In my mind (and I may well be wrong!), the extra fraction of backfocus is needed after the light path has been slightly altered by passing through the filter.

The change in backfocus distance  is not dependent on where the light path has been refracted through the filter as the filter surfaces are parallel, so the filter location and subsequent extra spacing distance required can be put anywhere between the FF and the camera. 🙂

Alan

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Thanks for the replies and advise,

Yes, sorry i kind of missed a few things out in the original question,  so apologies for this.

The filter is a 2 inch OPTOLONG L-PRO FILTER

The filter draw is a ZWO FILTER DRAWER M42

I did notice a funny reflection when i was trying things out a few days ago at about 1/2 moon

I was using a bahtinov mask on a bright star to work on focusing and while i got a good spike alignment, i did notice that i could see sort of a light ghosting of the entire mask behind the star

I hope that make sense.

I was using Astro photography tool to run the camera.

Could this possibly be a refection form the filter and therefore try to put the spacers in front of the filter?

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This is how to get the correct backfocus for the Esprit150ED..  the m48 thread on the zwo supplied 16.5mm adapter screws onto the extender tube screwed to the back of the flattener..  you may need to put a spacer in at 1/3 the thickness of the filter to get the correct back focus and perfect corner stars .. with the Esprit150ED you can screw your 2” filter into a thread on the telescope side of the extender .. this  may mitigate your reflections but obviously makes it more difficult to change 

C1243F1E-31F2-4513-BAFF-9F4A0972C23B.png.38c46b7e4e201cd5caa6d427d75bcc25.png

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20 minutes ago, Laurin Dave said:

This is how to get the correct backfocus for the Esprit150ED..  the m48 thread on the zwo supplied 16.5mm adapter screws onto the extender tube screwed to the back of the flattener..  you may need to put a spacer in at 1/3 the thickness of the filter to get the correct back focus and perfect corner stars .. with the Esprit150ED you can screw your 2” filter into a thread on the telescope side of the extender .. this  may mitigate your reflections but obviously makes it more difficult to change 

 

Thanks for the advise and the images,  didn't look at it that way,  i was just looking at my camera in particular.

Well worth a try though.

I was going to use the 16.5mm adaptor but after doing the calculations from the figures given by the Field Flattener,  i cam up with the 14mm spacer to get the 96mm back focus,  the 16.5mm worked out to be 2.5mm too long.

But the connection theory is the same,  so will try it out next time i get some clear skies here again.

Might be worth while just trying the 16.5mm to see what happens though and compare the two.

 

 

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To complicate things, FLO say on their web page for the Esprit 150 flattener that the distance should be 98 mm. I once asked them about this and they said that the Esprit manual was wrong stating 96 mm. Maybe it is not that critical but something to think about if you end up with eggy stars in the corners:

https://www.firstlightoptics.com/esprit-professional-refractors/skywatcher-field-flattener-for-esprit-150.html

By the way, why do you want a luminance filter in the train? Is it to be able to replace it with other filters, such as a dual band filter? If not, you do not need it to block IR since your camera has a built in IR filter.

Edited by gorann
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9 hours ago, bluesilver said:

I did notice a funny reflection when i was trying things out a few days ago at about 1/2 moon

I was using a bahtinov mask on a bright star to work on focusing and while i got a good spike alignment, i did notice that i could see sort of a light ghosting of the entire mask behind the star

This is most likely just an artifact from the bahtinov mask and won't appear on normal images.

I assumed the ASI2600 was a M48 thread but it's actually M42. Not that that makes much difference. As you're using a 2" filter you can still place the 14mm spacer between the filter and camera if you wish. At some distance from the camera though the M42 thread size will cause vignetttting on an APS-C sensor, so try it out and see what happens.

The 98mm back focus distance mentioned by gorann does back up why the first spacer is 43mm, as my first post queried this. 🙂

Alan

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19 hours ago, gorann said:

To complicate things, FLO say on their web page for the Esprit 150 flattener that the distance should be 98 mm. I once asked them about this and they said that the Esprit manual was wrong stating 96 mm. Maybe it is not that critical but something to think about if you end up with eggy stars in the corners:

https://www.firstlightoptics.com/esprit-professional-refractors/skywatcher-field-flattener-for-esprit-150.html

By the way, why do you want a luminance filter in the train? Is it to be able to replace it with other filters, such as a dual band filter? If not, you do not need it to block IR since your camera has a built in IR filter.

Thanks for the link, and good to know,  at least that makes more sense as is in line with the 16.5mm adaptors that ZWO send with the camera.

In regards to the filter,  From what i understand it helps increasing the contrast of emission nebulas.

I could b wrong,  but i think that is what it is designed for and yes you are correct in saying that the ASI2600mc has it's own in built UV filter.

The filter i don't think would do too much for galaxies though.

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14 hours ago, symmetal said:

This is most likely just an artifact from the bahtinov mask and won't appear on normal images.

I assumed the ASI2600 was a M48 thread but it's actually M42. Not that that makes much difference. As you're using a 2" filter you can still place the 14mm spacer between the filter and camera if you wish. At some distance from the camera though the M42 thread size will cause vignetttting on an APS-C sensor, so try it out and see what happens.

The 98mm back focus distance mentioned by gorann does back up why the first spacer is 43mm, as my first post queried this. 🙂

Alan

Good to know,  just wasn't too sure if i somehow messed up the order of things.

 

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