3.3 Reducer measurement advice

[KEJ]

A question regarding back focus(BF) when using a 3.3 Reducer and any comments or advice would be appreciated.

So last night I was intending on trying the 58mm approach as the BF distance, unfortunately it was way too cloudy, so that test is on hold.

In the meantime and regarding measuring, if you see the attached picture of my ASI533 and the distance between 1 - 2 = 17.5mm (sensor to top camera spacer).

Questions

When measuring or calculating distance and allowing for the 17.5mm (sensor to top of the camera spacer), should I measure to the Green, Yellow or Red lines/indicators I have added?

 

Example

Whilst this pictures probably show one of my earlier tests,  let’s say the gap between the camera spacer and the T-Adapter was approx 10mm at thats the spacers I am using.

Therefore, camera sensor to Green line could be 17.5 + 10 = 27.5mm

To the Yellow, could be 17.5 + 10 + 10 = 37.5

To the Red, could be 17.5 + 10 + 10 + 5 = 42.5

I am always slightly unsure where I should be measuring to exactly at the T-Adapter end ?

People say to the last glass lens of the Reducer(camera end) so I tend to guess and with the 3.3 you have to be more precise.

Thanks in advance

* where the spacer is closest to the T-Adapter, there is a tiny few mm collar, should I be allowing for that as well ?

 

** Or

Actually, should I be measuring inside the T-Adapter to where the tread ends inside the tube?

Sorry for all the questions and any comments much appreciated, I cut and pasted, the text has gone weird .......have to pop out will try to edit later

Edited April 3, 2023 by KEJ
formatting corrected for colour

[Cornelius Varley]

Done the reformatting for you. Tip, if you are copy/pasting from another source use "paste as plain text" to remove existing text formatting.

[Stuart1971]

First of all do you have the correct spacing or is it more than you think as these reducers in the f6.3 version have a 107mm backspace, secondly your red line is correct, it should be measured from the shoulder of the reducer assembly, as the bottom of the rear threads… 

you should be including any space in the backspacing measurement, that is from the sensor to the shoulder of the reducer, any space in between….

Edited April 3, 2023 by Stuart1971

[KEJ]

2 hours ago, Cornelius Varley said:

Done the reformatting for you. Tip, if you are copy/pasting from another source use "paste as plain text" to remove existing text formatting.

Back now and thank you, hopefully I'll remember.

[KEJ]

1 hour ago, Stuart1971 said:

First of all do you have the correct spacing or is it more than you think as these reducers in the f6.3 version have a 107mm backspace, secondly your red line is correct, it should be measured from the shoulder of the reducer assembly, as the bottom of the rear threads… 

you should be including any space in the backspacing measurement, that is from the sensor to the shoulder of the reducer, any space in between….

Thanks for the prompt reply.

I have a 6.3 reducer and I find 105mm BF is perfect(well with my dodgy eyes).

However, with the 3.3 Reducer I hear or read so many conflicting comments, so I have with the bad weather been trying to grab the odd picture and relying on the star patterns until I get a definitive answer. Currently the outer stars are all slightly radiating outward, which I believe suggests the gap was too small.

Hence why, I was just going to try 58mm approach later.

Question

Whilst you have answered my question(thank you) the glass or last lens of the Reducer I assumed sticks out slightly more, thats why I was possibly confused, but I will take your advice.  And just to be clear, on my picture on the Reducer,  I have the raised band/collar where it has Meade printed on it, to the left is the crack/join where the reducer meets the T-Adapter, that is where I believe the tread would be if they were separated/unscrewed, if that makes sense ?

Thank you

[KEJ]

2 hours ago, Stuart1971 said:

First of all do you have the correct spacing or is it more than you think as these reducers in the f6.3 version have a 107mm backspace, secondly your red line is correct, it should be measured from the shoulder of the reducer assembly, as the bottom of the rear threads… 

you should be including any space in the backspacing measurement, that is from the sensor to the shoulder of the reducer, any space in between….

Once I know where to measure from, which I do now........I can at least concentrate on the getting the spacing correct.

[michael8554]

Hi KEJ

The Meade f/3.3 Reducer was designed for the tiny sensors of the 1990's.

So I'm not sure what the stars in your 1" Square camera are going to look like !

Michael

[KEJ]

3 hours ago, michael8554 said:

Hi KEJ

The Meade f/3.3 Reducer was designed for the tiny sensors of the 1990's.

So I'm not sure what the stars in your 1" Square camera are going to look like !

Michael

Thank you......I did read that previously. But I'm still trying with the 533 and that was going to be a try fallback with my planetary camera.

[Stuart1971]

4 hours ago, KEJ said:

Thanks for the prompt reply.

I have a 6.3 reducer and I find 105mm BF is perfect(well with my dodgy eyes).

However, with the 3.3 Reducer I hear or read so many conflicting comments, so I have with the bad weather been trying to grab the odd picture and relying on the star patterns until I get a definitive answer. Currently the outer stars are all slightly radiating outward, which I believe suggests the gap was too small.

Hence why, I was just going to try 58mm approach later.

Question

Whilst you have answered my question(thank you) the glass or last lens of the Reducer I assumed sticks out slightly more, thats why I was possibly confused, but I will take your advice.  And just to be clear, on my picture on the Reducer,  I have the raised band/collar where it has Meade printed on it, to the left is the crack/join where the reducer meets the T-Adapter, that is where I believe the tread would be if they were separated/unscrewed, if that makes sense ?

Thank you

Yes, the glass does stick out a bit more, but the shoulder at the base of the threads  is at the back of the first lens, which is where the measurement should be taken from, as this is a reducer and a flattener in one, with two sets of lenses, so the measurements is taken after the reducing lens but before the flattening lens…HTH

the red line is where you should measure from

 

Edited April 3, 2023 by Stuart1971

[KEJ]

6 minutes ago, Stuart1971 said:

Yes, the glass does stick out a bit more, but the shoulder at the base of the threads  is at the back of the first lens, which is where the measurement should be taken from, as this is a reducer and a flattener in one, with two sets of lenses, so the measurements is taken after the reducing lens but before the flattening lens…HTH

the red line is where you should measure from

 

Blimey, thats interesting and I would have never in a month of Sundays figured that out. - thank you.

So looking from above at the optical train, where the crack/join/seam is between the T-Adapter and Reducer?

Your red line on the right hand side is say on the threads of the Reducer?

[Stuart1971]

10 minutes ago, KEJ said:

Blimey, thats interesting and I would have never in a month of Sundays figured that out. - thank you.

So looking from above at the optical train, where the crack/join/seam is between the T-Adapter and Reducer?

 

yes, if that  screws directly onto the reducer then from the corner edge of the reducer, as every mm will make a difference….

[KEJ]

7 minutes ago, Stuart1971 said:

yes, if that  screws directly onto the reducer then from the corner edge of the reducer, as every mm will make a difference….

Thank you........

[KEJ]

So from last night a poor image

 

[Stuart1971]

55 minutes ago, KEJ said:

So from last night a poor image

 

Those star shapes show you need more spacing between the sensor and flattener….and you are not quite in focus….

 

Edited April 4, 2023 by Stuart1971

[KEJ]

Thank you, yes I agree on that.

I have been using picture as well previously.

 

[Louis D]

Kind of cool looking in a hyperspace jump or blackhole suction sort of way, though. 😁

[KEJ]

5 hours ago, Louis D said:

Kind of cool looking in a hyperspace jump or blackhole suction sort of way, though. 😁

Mine will be going down a black hole if I can't sort it out.

I'm parking the 3.3 issue for the moment, so back on the WO Z61 and maybe a crack at Venus/Moon with the Meade over the next few nights if its clear.

 

[ollypenrice]

I gave one away. If getting a large telescope with an F ratio of 3.3 were as simple as screwing one of these reducers onto an SCT, it is highly unlikely that anybody, anywhere, would use anything else! 

lly

[KEJ]

On 06/04/2023 at 08:02, ollypenrice said:

I gave one away. If getting a large telescope with an F ratio of 3.3 were as simple as screwing one of these reducers onto an SCT, it is highly unlikely that anybody, anywhere, would use anything else! 

lly

You could be right........Its just that I have one and I was determined to try and get it to work. Parked up for the moment......

[KEJ]

Not sure if anyone out there can give me a definitive answer on three hopefully simple questions.

 

Background

I have a Meade 8” SCT F10, ASI533 MC Pro camera and Hirsch 6.3 Reducer – see the attached.

 

Q1)  Back Focus Distance

Does anyone know the correct back focus(BF) for this setup as I have heard values from mentioned from 85mm, 88mm, 105mm and 115mm, when measuring from the camera sensor to the Reducer?
 

Q2) Measure to Where

When measuring, the camera end is simple as I know from the sensor “C” to the top of the camera spacer “D” is 17.5mm.
Should the measurement at the Reducer end be to point “A” or “B” on the T-Adapter ?

Example

Take the 105mm or 115mm BF figure I have used, now depending how you measure, to point “A” or “B” then that could be the approx. correct distance.

Q3) Meade 3.3 Reducer

In addition, I have a Meade 3.3 Reducer and once I know the correct point to take the measurement at the T-Adapter end is, does anyone know the BF distance for that potential setup as well ?

(With the 3.3 Reducer I could also possibly try it with one of my planetary cameras which have smaller sensors & more out of interest).

 

Hopefully thanks

[Stuart1971]

Ok, so with the 6.3 reducer, the back focus should be approx 107mm and the measurement should be taken from the base of the rear threads on the back of the reducer, as for the 3.3 the measurement should be taken from the same place, but the back focus distance is much shorter, there is a spacer set than comes with this reducer, which gives the exact correct back focus…

position A in your drawing is correct to measure to 👍🏻

 

 

Edited April 16, 2023 by Stuart1971

[michael8554]

Rule-of-thumb is to focus the sun on a flat surface with just the FR, to roughly measure the focal length of the Reducer.

Some figures that most seem to agree on :

210mm FL,  Back Focus is about 105mm.

105mm FL, Back Focus is about 85mm.

Adjust for best corner star shapes.

I posted the spec sheet for the f/3.3 on April 3rd.

Michael

 

 

Edited April 16, 2023 by michael8554

[KEJ]

3 hours ago, Stuart1971 said:

Ok, so with the 6.3 reducer, the back focus should be approx 107mm and the measurement should be taken from the base of the rear threads on the back of the reducer, as for the 3.3 the measurement should be taken from the same place, but the back focus distance is much shorter, there is a spacer set than comes with this reducer, which gives the exact correct back focus…

position A in your drawing is correct to measure to 👍🏻

 

 

Stuart hi

Thanks for the prompt reply and info.

6.3 Reducer

Is it safe to say the join, where the T-Adapter meets the Reducer or just step back into the T-Adapter collar a few mm ?

I will have to take it apart to check.

 

3.3 Reducer

I had a 15mm and 30mm tube/spacer that came with the adapter. I still assumed I would have to make allowances for the camera as I again assumed the older camera sensors could have been closer etc - fair point.

 

Thank you

 

 

 

[KEJ]

1 hour ago, michael8554 said:

Rule-of-thumb is to focus the sun on a flat surface with just the FR, to roughly measure the focal length of the Reducer.

Some figures that most seem to agree on :

210mm FL,  Back Focus is about 105mm.

105mm FL, Back Focus is about 85mm.

Adjust for best corner star shapes.

I posted the spec sheet for the f/3.3 on April 3rd.

Michael

 

 

Thank you also for the prompt reply and info.

 

I have never tried the Sun trick and whilst the 105mm figure is mentioned, I was exactly unsure up to exactly where as previoulsy I had always measured to B from the camera sensor.  

 

So I guess all my various tests were pretty much 10-12mm out all the time.

 

Thanks 

[Stuart1971]

3 hours ago, KEJ said:

Is it safe to say the join, where the T-Adapter meets the Reducer or just step back into the T-Adapter collar a few mm ?

Yes, try and work out where the actual shoulder of the reducer is and yes it will be a couple of MM inside the T adapter….👍🏻