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Old 04-04-2022, 01:05 PM
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OneCosmos (Chris)
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Focus distance help required

Hi everyone,


I need some help and hoping someone here will know the answer.


I have a Takahashi fsq-85 and I have replaced the focuser with an Esatto motor focuser which has only 30mm of travel. I am coupling it with a QHY 268c. I do have the Tak flattener 1.01x which I can use, but that was really only purchased because I formerly had a full frame camera. The fsq-85 is supposed to be good for APS-C sensors without the extra flattener.


The point of this thread though is that both with and without the flattener I can't bring the system to focus because there is insufficient travel. I do have 19mm of M54 screw thread extension tubes between camera and Esatto and the attached shows Acrux with the focuser fully extended.



Obviously I need extensions but we're not talking about a little off here, as you can see Acrux is massively way off focusing. I can have an extension piece made, but I do need to know how long it should be.


Is anyone using this combo and can therefore let me know how you reach focus?
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Old 04-04-2022, 01:35 PM
AdamJL
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Hi Chris

I don't have that scope, but do have that focuser. Sounds like you could be missing the 25mm adapter at the back.

FWIW, my image train is as follows:

Esprit 120 > Esprit to Esatto Adapter > Esatto 65mm Extension Tube > Esatto 3" > Esatto M81 25mm Extension.

Then to this, I have my Field Flattener, QHY spacing, QHY Filter Wheel, and 268M. From the back of the Field Flattener to the sensor, it's 55mm (actually 56.5).

If you're not using a flattener, obviously that changes your backfocus measurements. I don't know what that is for your Tak, but you can buy extension tubes as mentioned above to get to where you need to go. I use 25mm, you can start there? Maybe get two if you need the extra spacing.

Happy to take a picture for you later.

Last edited by AdamJL; 04-04-2022 at 02:02 PM.
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Old 04-04-2022, 05:52 PM
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OneCosmos (Chris)
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“Esprit 120 > Esprit to Esatto Adapter > Esatto 65mm Extension Tube > Esatto 3" > Esatto M81 25mm Extension.”

Thanks. Hmm I have Fsq-85 to Esatto Adapter > M82-M54 adapter > 1.01x flattener > 11.5mm M54 screw thread spacers > QHY filter holder > camera

I don’t have the 65mm Extension tube between Tak and Esatto or the 25mm one you mention the on the other end of the Esatto. I need to check why these weren’t sold as mandatory.
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Old 04-04-2022, 10:46 PM
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OzEclipse (Joe Cali)
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Geometric calculation

Hi Chris,

If the image you posted is an uncropped APS C, then the defocussed star diameter is about 15.7mm depending upon the specifics of your APS C sensor and blurring at the edges. If the FSQ 85 is the model with the 450mm focal length, then without any reducer this is a simple geometric similar triangles calculation:-

Extension required is approximately
15.5mm x 450mm = ~82mm
........85mm

I would retake the image with the Estatto at mid-travel rather than at one end measure the out of focus image and repeat the calculation.

Cheers
Joe
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Old 05-04-2022, 01:47 AM
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OneCosmos (Chris)
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Quote:
Originally Posted by OzEclipse View Post
Hi Chris,

If the image you posted is an uncropped APS C, then the defocussed star diameter is about 15.7mm depending upon the specifics of your APS C sensor and blurring at the edges. If the FSQ 85 is the model with the 450mm focal length, then without any reducer this is a simple geometric similar triangles calculation:-

Extension required is approximately
15.5mm x 450mm = ~82mm
........85mm

I would retake the image with the Estatto at mid-travel rather than at one end measure the out of focus image and repeat the calculation.

Cheers
Joe
Hi Joe,

That’s quite cool and not far off in agreement with the previous reply that appeared to have a 65mm and 25mm extension.

The only slight problem is I tried with and without the flattener and now I can’t remember whether that screen snip of Acrux was with or without! I’ll have to re-do it.
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Old 05-04-2022, 01:59 AM
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OneCosmos (Chris)
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Quote:
Originally Posted by AdamJL View Post
Hi Chris

I don't have that scope, but do have that focuser. Sounds like you could be missing the 25mm adapter at the back.

FWIW, my image train is as follows:

Esprit 120 > Esprit to Esatto Adapter > Esatto 65mm Extension Tube > Esatto 3" > Esatto M81 25mm Extension.

Then to this, I have my Field Flattener, QHY spacing, QHY Filter Wheel, and 268M. From the back of the Field Flattener to the sensor, it's 55mm (actually 56.5).

If you're not using a flattener, obviously that changes your backfocus measurements. I don't know what that is for your Tak, but you can buy extension tubes as mentioned above to get to where you need to go. I use 25mm, you can start there? Maybe get two if you need the extra spacing.

Happy to take a picture for you later.
I ordered the 65mm extension tube but it remains to be seen whether I also need the M81 25mm between flattener and Esatto.

If I choose to not use the 1.01x flattener (the fsq-85 has an internal
Flattener up to APS-C so they say), the 65mm ordered may be sufficient in combination with the 19mm of M54 spacers I already have between Esatto and camera. Based on Joe’s reply it looks like the 65mm M81 will do the trick for a configuration without flattener.
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Old 05-04-2022, 07:30 AM
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OzEclipse (Joe Cali)
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1.01 Flattener

Chris,

The 1.01x flattener, unlike a 0.8x focal reducer, will only very marginally change the position of the focal plane. However it will correct, flatten and importantly, act as extra extension that might save you needing to buy another 2 grams of $200 empty tube.

Joe
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Old 05-04-2022, 09:15 AM
AdamJL
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Quote:
Originally Posted by OzEclipse View Post
Hi Chris,

If the image you posted is an uncropped APS C, then the defocussed star diameter is about 15.7mm depending upon the specifics of your APS C sensor and blurring at the edges. If the FSQ 85 is the model with the 450mm focal length, then without any reducer this is a simple geometric similar triangles calculation:-

Extension required is approximately
15.5mm x 450mm = ~82mm
........85mm

I would retake the image with the Estatto at mid-travel rather than at one end measure the out of focus image and repeat the calculation.

Cheers
Joe
Awesome explanation and diagram!

Quote:
Originally Posted by OzEclipse View Post
Chris,

The 1.01x flattener, unlike a 0.8x focal reducer, will only very marginally change the position of the focal plane. However it will correct, flatten and importantly, act as extra extension that might save you needing to buy another 2 grams of $200 empty tube.

Joe
I think the 65mm is mandatory. As you noted, he needs circa 85mm. He isn't going to get that from the flattener alone. The flattener comes after the focuser anyway. He needs the extension before the focuser and the OTA to get the focuser to work as it doesn't have a lot of travel.
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Old 12-04-2022, 10:49 PM
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It's almost as if PrimaLuceLab read this thread. Here's a video they just posted on their YouTube channel

https://www.youtube.com/watch?v=lcR50-XPhH8
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Old 13-04-2022, 06:39 PM
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Chris, the spot diagrams show the stars are much tighter when using the flattener, even with APS-C sized sensor.

Given that you have a small pixel, high resolution camera, you only spoil the image by not using something you already have
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