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BF's: Skywatcher Esprit 150 with FTF3545 (TS Photoline 150)

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#1 widmerhans

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Posted 15 September 2020 - 03:26 PM

Dear All

 

I need help about the backfocuses for the combination Esprit 150 with Feathertouch Focuser 3545 with and without the TS-Flattener 2.5".

TS (Telescope-Service in Germany) isn't able to answer my questions; they don't know it or they won't give an answer (they pretend to have no time).

 

I calculated all parts by myself and they would only have to check the measurements marked with the stars in my two pdf-sheets:

 

https://www.dropbox....uslage.pdf?dl=0

 

https://www.dropbox....uslage.pdf?dl=0

 

* richtig? = * right?

* Bitte das richtige Mass angeben! = * Please give the right measurement!

 

Can anybody help me?

 

Best Regards

Hans



#2 KD58

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Posted 15 September 2020 - 04:23 PM

The back focus would be the distance from the flattener to your camera sensor which should be 87mm per TS specifications. The focuser should not come into the equation.

 

Khavar



#3 widmerhans

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Posted 15 September 2020 - 04:40 PM

Khavar

 

Thanks.

 

But please look at my pdf's. I know this measurements of 87 mm. You can see it on my sheet.

 

Hans



#4 Paul Garais

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Posted 15 September 2020 - 04:53 PM

As Khavar said: The focuser is not relevant. Even looking at your PDFs it is unclear what you are trying to achieve. Or I may be just too stupid ;).

I am a German and can fully read your sheet. Why even calculating the sum of the optical length of parts before and after the flattener?

Also you should be aware of the fact, that your sheet won't give you anything because there are manufacturing tolerances, which could be as high as 0,5 mm per part. If you really want to know, how far your sensor will be away from the flattener backside (which is the only relevant measure), you need to attach every part of your imaging train and measure with a caliper. Those theoretical sheet is just a very rough estimate.

Also your calculation regarding the filter is wrong. You need to subtract, not add the value, if you are calculating back focus.


—Paul
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#5 widmerhans

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Posted 15 September 2020 - 05:12 PM

As Khavar said: The focuser is not relevant. Even looking at your PDFs it is unclear what you are trying to achieve. Or I may be just too stupid wink.gif.

I am a German and can fully read your sheet. Why even calculating the sum of the optical length of parts before and after the flattener?

Also you should be aware of the fact, that your sheet won't give you anything because there are manufacturing tolerances, which could be as high as 0,5 mm per part. If you really want to know, how far your sensor will be away from the flattener backside (which is the only relevant measure), you need to attach every part of your imaging train and measure with a caliper. Those theoretical sheet is just a very rough estimate.

Also your calculation regarding the filter is wrong. You need to subtract, not add the value, if you are calculating back focus.


—Paul

Paul

 

Thanks. OK. Forget the filter.

 

For the correct focus you would need some way of the draw tube? Right?

How much way?

You can see that I ordered a PrecisePart for the adaptation on the FTF3545; this part has a length I assumed to be 68 mm because I assumed a total length of 125 mm to the flattener.

I simply want to know wether my assumptions were correct.

Do you think this is too much of a demand?

 

And there is an other pdf without a flattener also with PreciseParts (custom made).

 

Hans



#6 Paul Garais

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Posted 15 September 2020 - 11:33 PM

Paul

Thanks. OK. Forget the filter.

For the correct focus you would need some way of the draw tube? Right?
How much way?
You can see that I ordered a PrecisePart for the adaptation on the FTF3545; this part has a length I assumed to be 68 mm because I assumed a total length of 125 mm to the flattener.
I simply want to know wether my assumptions were correct.
Do you think this is too much of a demand?

And there is an other pdf without a flattener also with PreciseParts (custom made).

Hans

If you wan't to know, where the focus will be without other optical elements in the way, that is a value, that you get on the TS Website:
"Fokusabstand:185 mm ab der 2" Steckhülse"

So every other value that is missing, you can easily measure yourself. It is quick and easy done and you get accurate values for your sample.

#7 widmerhans

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Posted 16 September 2020 - 03:13 AM

If you wan't to know, where the focus will be without other optical elements in the way, that is a value, that you get on the TS Website:
"Fokusabstand:185 mm ab der 2" Steckhülse"

So every other value that is missing, you can easily measure yourself. It is quick and easy done and you get accurate values for your sample.

Paul

 

I know the value of 185 mm.

 

But here, too, the question is how much is the draw tube way that belongs to this 185 mm?

The 2" Steckhülse is a moving part and I don't wish to determine visually how much this way is by trial and error.

I want to know it beforehand.

 

This backfocus statement is incomplete because it is not measured from a fix point of the optic.

 

You could argue that the 185 mm are measured from the completely retracted draw tube but then no way would be left for the minus direction in order to focus. On my pdf you can see that I assume that the draw tube way that belongs to the correct focus is in the region of 65 mm (185 mm + 65 mm = 250 mm).

 

Hans

 

 

Dear All

 

I also would appreciate if somebody could answer who owns this OTA with the FTF3545 and could give me the draw tube ways.

 

Thanks a lot.

 

Hans




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