SCT C8 actual focal length, Baader GPC magnification factor, visual back's light pass length

[YKSE]

There're many discussions on other forum about SCT's actual focal length depending on visual back setup and Baader GPCs' actually magnification factors. As it was a cloudy weekend, I took the chance to do some measurement to get some hard data to get better understanding of it.

1. The setup:

A yardstick on a table in the garden some 22meters away from C8, some combinations of my visual back setup are measured:

1.1 The stock 1.25" visual back

Despite its cheep shape, this is said to be the visual back (of total length of 100mm from the back plate of SCT) which gives the correct focal length (2032mm for C8), i.e. focal ratio f10. Every 1mm light path increase over 100mm give 3.1mm longer focal length. which I have assumed in this table for calculation

back_focus_length_160523.xlsx

The calculated focal length and focal ratio of each setup below is done with this excel form.

1.2 Baader 2" Zeiss prism + Baader SC to 2" receptacle + Baader Counter ring + (Baader 2" to 1.25" Clicklock reducer when needed)

Calculated focal length and focal ratio for this setup is 2032mm and f10.0

1.3 Baader 2" Zeiss prism + Baader SC Clicklocks + (Baader 2" to 1.25" Clicklock reducer when needed)

Calculated focal length and focal ratio for this setup is 2150mm and f10.63

1.4 Baader 2" mirror + Baader SC Clicklocks + (Baader 2" to 1.25" Clicklock reducer when needed)

Calculated focal length and focal ratio for this setup is 2224mm and f10.95

1.5 Baader T2 prism(none-zeiss) + SC Clicklock + Baader T2#15 + 2" to 1.25" Clicklock

Calculated focal length and focal ratio for this setup is 2128mm and f10.47

2. FL Measurement results and comments:

Here's the result file.

Millimeter is the measured unit with about +-0,5mm error, deviations in percentage are comaring to 1.25" stock focuser for 1.25" EPs, and to setup 1.2 for 2" EPs.

2.1 The calculated focal length seems to meet FOV change quite well, the 1.2 setup is practically exact, and 1.4 setup showed most deviation from calculated focal length (1.8%), the measured results are consistent across the board in accordance with the changed focal length.

2.2 Using a usually 2" mirror diagonal (none-SCT dedicated), as in 1.4, will have a system around f11.

2.3 An extenal dedicated 2" SCT focuser (none Feather touch microfocuser) should add another 40-50mm light path than the above 2" mirror diagonal, which should make C8 running in about f11.6 to f11.8.

2.4 I'll be using the updated focal length of my system when referring to power used.

2.5 Field curvature of the system (scope+EP) are quite readily seen when looking at the black and white yardstick, which I didn't know before.

3. GPC measurement setup:

With the same yardstick, EPs are in the maxbright binoviewer in turns with different GPCs.

Focal length of system is calculated in the same way, only two setups were tested:

First one is the following picture, only that Binoviewer is connected to diagonal via the 2" to 1.25" clicklock, which adds 33mm length, second one is shown in the picture:

4 GPC results and comments:

4.1 Results:

4.2 magnification factors are calculated by deviding the No GPC results by the ones with GPC, fairly consistent result, my 1.25x, 1.7x and 2.6x are more likely 1.16x, 1.36x and 2.43x.

4.3 focal length difference fit foreFOV difference very well (less than 0.5%).

4.4 The shorter setup (second one with counter ring) was my preferred setup, because of slightly sharper on-axis view.

Thanks for reading, hope the results are as useful for others as for me

Edit Info:

The GPC measurement results were renewed regarding the focal lengths/ratios, since GPCs shorten the back focus, therefore less increase in focal length.

[alan potts]

Nice work Yong, I always knew the F/L changed but never knew it was by that much, I guess mine will be more with it being 3048mm.

Alan

[Piero]

Very interesting! I didn't know about that increase in focal length. 

[YKSE]

Alan, 

yes, it feels good to have it quantified so that proper action(s) can be taken

Piero,

The increased focal length is a specialty with catadioptric scopes, i.e. SCT and Mak, where primary mirror needs to move forward (closer to secondary) when visual back is longer.

[Highburymark]

Another very interesting and useful piece of work Yong - I've referred back to your posts on barlowing the Leica zoom on countless occasions, and having just bought a C8 will do the same with this one.

[YKSE]

Had done another measurement to check the effect of having GPC afterand in front of the T2 diagonal. At roughly the same distance as before. Baader T2 to 2" par

Has both 48mm thread for 2' filter and T2 thread for further connection, which is handy for using Baader binoviewer's 1.25" nosepice (which has an internal thread for Baader GPC), as seen here:

Here's the setup and summary of measurement:

I'm not sure about the impact on SCT focal length of having GPC in front of diagonal, therefore I've assumed that it's the same as after.

Results seems to be fairly consistent as last time, what can be seen is that only 2.6x GPC gives larger jump in Mag 2.5x to 3.1x (about 24%) when placing GPC in front, while changes in 1.25X and 1.7x GPC are modest, only about 5% and 10% increase respectively.

[Adrian]

Nice work, Yong.

I got similar results with a Meade 10" SCT (see this thread), but I didn't go as far as measuring the path length with the different diagonals.

[YKSE]

Thanks Adrian.

I'll take my time on your measurement, Shoudl be very interesting

[YKSE]

Adrian,

Your measurement looks like quite consistent, similar to mine too, even though difference method Many good comment in that thread too.

I've uppdated my table for backfocus calculation with a few items, it can be used for finding good configuration for binoviewer, checking light path of some visual back combination, also getting an idea how much spherical aberration added in your original SCT.

You can insert new rows of items to get a more exact idea how your system works.

Light_path_length_cal.xlsx

[BGazing]

On 23/05/2016 at 16:47, YKSE said:

There're many discussions on other forum about SCT's actual focal length depending on visual back setup and Baader GPCs' actually magnification factors. As it was a cloudy weekend, I took the chance to do some measurement to get some hard data to get better understanding of it.

1. The setup:

A yardstick on a table in the garden some 22meters away from C8, some combinations of my visual back setup are measured:

1.1 The stock 1.25" visual back

Despite its cheep shape, this is said to be the visual back (of total length of 100mm from the back plate of SCT) which gives the correct focal length (2032mm for C8), i.e. focal ratio f10. Every 1mm light path increase over 100mm give 3.1mm longer focal length. which I have assumed in this table for calculation

back_focus_length_160523.xlsx

The calculated focal length and focal ratio of each setup below is done with this excel form.

1.2 Baader 2" Zeiss prism + Baader SC to 2" receptacle + Baader Counter ring + (Baader 2" to 1.25" Clicklock reducer when needed)

Calculated focal length and focal ratio for this setup is 2032mm and f10.0

1.3 Baader 2" Zeiss prism + Baader SC Clicklocks + (Baader 2" to 1.25" Clicklock reducer when needed)

Calculated focal length and focal ratio for this setup is 2150mm and f10.63

1.4 Baader 2" mirror + Baader SC Clicklocks + (Baader 2" to 1.25" Clicklock reducer when needed)

Calculated focal length and focal ratio for this setup is 2224mm and f10.95

1.5 Baader T2 prism(none-zeiss) + SC Clicklock + Baader T2#15 + 2" to 1.25" Clicklock

Calculated focal length and focal ratio for this setup is 2128mm and f10.47

2. FL Measurement results and comments:

Here's the result file.

Millimeter is the measured unit with about +-0,5mm error, deviations in percentage are comaring to 1.25" stock focuser for 1.25" EPs, and to setup 1.2 for 2" EPs.

2.1 The calculated focal length seems to meet FOV change quite well, the 1.2 setup is practically exact, and 1.4 setup showed most deviation from calculated focal length (1.8%), the measured results are consistent across the board in accordance with the changed focal length.

2.2 Using a usually 2" mirror diagonal (none-SCT dedicated), as in 1.4, will have a system around f11.

2.3 An extenal dedicated 2" SCT focuser (none Feather touch microfocuser) should add another 40-50mm light path than the above 2" mirror diagonal, which should make C8 running in about f11.6 to f11.8.

2.4 I'll be using the updated focal length of my system when referring to power used.

2.5 Field curvature of the system (scope+EP) are quite readily seen when looking at the black and white yardstick, which I didn't know before.

3. GPC measurement setup:

With the same yardstick, EPs are in the maxbright binoviewer in turns with different GPCs.

Focal length of system is calculated in the same way, only two setups were tested:

First one is the following picture, only that Binoviewer is connected to diagonal via the 2" to 1.25" clicklock, which adds 33mm length, second one is shown in the picture:

4 GPC results and comments:

4.1 Results:

4.2 magnification factors are calculated by deviding the No GPC results by the ones with GPC, fairly consistent result, my 1.25x, 1.7x and 2.6x are more likely 1.16x, 1.36x and 2.43x.

4.3 focal length difference fit foreFOV difference very well (less than 0.5%).

4.4 The shorter setup (second one with counter ring) was my preferred setup, because of slightly sharper on-axis view.

Thanks for reading, hope the results are as useful for others as for me

Edit Info:

The GPC measurement results were renewed regarding the focal lengths/ratios, since GPCs shorten the back focus, therefore less increase in focal length.

I see you have tested the Baader Zoom in this configuration, but it seems it can be done also in this way, I would expect the lightpath to be much shorter in that case, correct? Any chance you might measure it, too?

[YKSE]

Yes, I could.

But it needs some extra work. The clicklock clamp on mirror diagonal is tightened there with 6 small screws, which makes it not so practical in field use. The 2" prism diagonal, on the other hand, is a simply screw on/off, therefore easier for measuring. But it'll be still longer than using clamp ring alternative, since mirror diagonal has longer light pass than prism diagonal of the same size.

[Highburymark]

Yong, 

Having experienced these impressive diagonals/prisms, how do they compare for planetary observing with the C8? Presuming that the Zeiss delivers the best views with minimum scatter, is the extra cost justifiable, or are the less expensive options better value? Would appreciate your thoughts.

[BGazing]

1 hour ago, YKSE said:

Yes, I could.

But it needs some extra work. The clicklock clamp on mirror diagonal is tightened there with 6 small screws, which makes it not so practical in field use. The 2" prism diagonal, on the other hand, is a simply screw on/off, therefore easier for measuring. But it'll be still longer than using clamp ring alternative, since mirror diagonal has longer light pass than prism diagonal of the same size.

Thanks. I have found the data...

" If having too little back focus (focus range) of the telescope or for accessoreis tht have to be attached "captiveliy" to the telescope (public observatories), you can attach the Hyperion Zoom directly to the diagonal prism, instead using the 2" barrel, thus saving approximately 47 mm of optical path."

[YKSE]

Mark,

For me, the cost of Zeiss diagonal is easy justifiied It works in all my three (relative slow) scopes, and with any eyepieces for planetary work. The less scatter in prism than mirror diagonal is a noticeable enhancement. Prism diagonal and binoviewer is my primary planetary setup, I can warmly recommend it if you don't have difficulty in merging images, especially if it's your primary observation targets.

BGazing,

Yes, 47mm is the height of 2" clickclamp, you can find more light path data of diagonals and adapters (mostly Baader stuff since most available data) in the excel file in my post above.

[BGazing]

thank you!

apart from bino viewers, and pardon my newbieness, could you please let me know what would be the pros and cons for 1.2 1.3 and 1.4? i assume that the first one, since it screws itself directly on, cannot be used as a 2inch diagonal in anything else but the sct?

[Highburymark]

1 hour ago, YKSE said:

Mark,

For me, the cost of Zeiss diagonal is easy justifiied It works in all my three (relative slow) scopes, and with any eyepieces for planetary work. The less scatter in prism than mirror diagonal is a noticeable enhancement. Prism diagonal and binoviewer is my primary planetary setup, I can warmly recommend it if you don't have difficulty in merging images, especially if it's your primary observation targets.

 

Thanks Yong - food for thought 

[YKSE]

Some more tests about GPC's actual barlow factors when placing GPC in front of a diagonal, T2 prism, 2" prism and 2" mirror diagonals were used on 120ED. As we might expect, depending on how the diagonal is connected to Binoviewer, the barlow factor changes.

Placing GPC in diagonal's filter thread has the advantage of easier changing GPCs, also possibility of higher barlow effects when needed.

Picture showed one of the setups (GPC to filter thread via Baader part #29, 2458199 in front of T2 prism diagonal to Binoviewer via 1.25" clicklock clamp)

Focusing on something 20 meters away without GPC pushes focuser travel much more out than focusing on infinity. 2x 47mm extension was needed to accomplish the test.

An excellent flexibility of these Baader SC clicklock clamps, t.ex. taking the click lock and nose piece right off from the Baader 2" heschel wedge, you get a 47mm extension, or you can use nosepiece of a 2" aspheric eyepiece....

Refractor's focus length doesn't get affected as in SCT, only the focus travel. Here's the measurement (done in 2 seperate sessions)

1.25x GPC:

1.7x and 2.6x GPC:

As we can see, placing GPC in front of diagonal increase the barlow effect, taking the 1.25x GPC as an example, it has only 1.16x when placed behind diagonal (measured in earlier post in this thread), and it becoms 1.32x, 1.45x and 1.50 depending of diagonals.

With 2" diagonal, 2.6x GPC can go as high as 4.87x, enough to push a pair median focal length EP to very high mag when needed.