GPCs and Barlows- real magnification on binoviewer

[Marian M]

Hi friends!

 

I did some testing today with the Baader GPC (1.7 and 2.6) and APM coma corrector ED 2.7x

Testing was pretty basic- just took some shots with the smart phone (from my hand, no other device), trying to keep a faraway antenna at the top of the image. Telescope- TS-Optics SD Apo 102mm f/7, MB bino and 25mm SLV. For each of the below antenna, from widest to the narrowest, the configuration is as follows:

• TS 102/714, Baader T2 prism, MB bino, SLV 25mm (shortest possible light pass)
• TS 102/714, Baader T2 prism, 1.25 clicklock, MB bino, SLV 25mm + GPC 1.7 (short adapter from T2 to 1.25, with GPC inserted)
• TS 102/714, Baader T2 prism, 1.25 clicklock, MB bino, SLV 25mm + GPC 2.7 (short adapter from T2 to 1.25, with GPC inserted)
• TS 102/714, Baader T2 prism, 1.25 clicklock, MB bino, SLV 25mm + GPC 1.7 (short adapter from T2 to 1.25, with GPC inserted) + APM barlow lens only screwed in 1.25 to T2 adapter
• TS 102/714, Baader T2 prism, 1.25 clicklock, MB bino, SLV 25mm + GPC 1.7 (short adapter from T2 to 1.25, with GPC inserted) + full APM barlow

Next I have opened the pictures from the laptop and measured the distance from the top of the antenna to a specific point, with a ruler. Last step, put in an excel and compute on par magnification/ mm. Some errors could occur because of the filed curvature, but the results are interesting

The spreadsheet attached as picture shows the measurements. In a nutshell, it seems that in above configuration:

• 1.7 GPC is adding in fact only 1.3 magnification
• 2.7 GPC is adding only 2.3 magnification
• APM barlow lens (only the nose), inserted on 1.25" adapter, with 1.7 GPC in MB, is giving 2.6 magnification (to get 2.6, I have considered in fact the real magnification given by 1.7 GPC, which as appointed above, seems being 1.3 only)
• APM full barlow with 1.7 GPC (working at 1.3) is giving 3.9 x

Was wondering if someone did these measurements and got some results....

 

O the other hand, Telescope- TS-Optics SD Apo 102mm f/7 can be used very nice without GPC. Use M68 to T2 adapter, screw the Baader prism in MB bino, then screw the whole bino with prism at once in the adapter (a little dangerous 😀), but will enter in focus, in fact around 6mm still free on focuser

 

Clear skies!

marian 

 

[Second Time Around]

Have you tried the APM 2.7x Barlow by itself without a GPC?  If so, what was the outcome?

Thanks.

[MalcolmM]

That's a great idea of measuring on a photo @Marian M. I'm forever wondering what magnification I'm getting with various combinations of eyepiece and GPC. I was surprised by the seeming lack of difference between a 1.25 and 1.7 GPC on the MB. If they were both showing less magnification as you found, that might explain it. I've done very rough experiments counting bricks on a distant chimney, but that was for field of view. I shall be using your method now to see what the magnifications are! Thanks for posting!

Malcolm 

[bosun21]

Where are you fitting the GPC’s? The clicklock is adding unnecessary length to the light path. You can either attach the MB to the T2 diagonal directly, or by a Baader quick changer utilizing the Zeiss microbayonet. 

[bosun21]

Also depending on how you mount the 1.7x GPC ie which direction the flat on it is orientated you may need to unscrew the GPC and reverse the cemented lens double.

[MalcolmM]

Hi @bosun21, if you are asking me ... I attach the GPC to the diagonal as per the manual. I'm pretty sure they are connected correctly. I need to do more experiments. It can be hard (I find) to judge relative magnification when the field of view is also changing. I was just surprised that the difference did not 'jump' out at me.

Malcolm

[bosun21]

14 minutes ago, MalcolmM said:

Hi @bosun21, if you are asking me ... I attach the GPC to the diagonal as per the manual. I'm pretty sure they are connected correctly. I need to do more experiments. It can be hard (I find) to judge relative magnification when the field of view is also changing. I was just surprised that the difference did not 'jump' out at me.

Malcolm

No I was directing it to @Marian M. If you use the Zeiss microbayonet and quick changer then you need to fit the 1.7x GPC in the binoviewer itself, and reverse the lenses. If I had the choice again I would just use it the way you are. I never could work out what magnification I was getting.

[Marian M]

Hi!

Point by point

Steve, will do some more testing in the upcoming days and revert here

@bosun21 situation is as follows- I can screw the bino directly in T2 prism, T2 in T2-M68 ultrashort adapter and the Apo will be in focus, without any GPC. I have only 6 mm focuser travel, hence the quick changer will not enter, I remember it has around 1.2 cm light path. In order to setup like this, first I have to screw the T2 prism in bino, then the whole ensemble (bino+T2 at once)  to screw in the T2 to M68 adapter, which is a little crazy. Physically, there is no other place to screw, rotate, to fix first the T2 prism in the adapter and then the bino to T2 prism. I can screw a little the bino to T2 through bino ring but doesn't seem to be solid screwed, and when I want to unscrew, usually don't have place with the fingers or gets locked.

 

If the DSO is the target, usually I let the bino without any GPC but spend some time to setup the system.

 

If I want some Moon/ planets, I just add the clicklock. I am planning to buy more 1.25 to T2 adapters- for the moment I have only one. For me seems difficult to replace the GPC only in T2 prism, hence I found much easily just to replace the whole adapter, with the GPC screwed up, like in the below picture. Seems easier to replace like this and lower chances to touch the lens with the finger, mostly during the night, outside in the field.  

 

Very interesting the point with the GPC and convex part to the sky. Just checked mine (in the picture it is the 1.7x GPC and seems being positioned the other way around. So I should unscrew it from the metal body and put the lens the other way around? Seems not possible in mine to open it but also I did not have the courage to force it. For 2.7 I read that I should not do this. Thanks for this info! Would that affect the image, being placed the other way around?

 

Regards,

Marian 

[bosun21]

Yes it’s going to affect the image. To switch the lens assembly around you need to take the 1.7x GPC and unscrew the little lens locking ring that holds the actual lenses in place. Tip the GPC upside down onto a clean microfiber cloth letting the lenses drop out. Replace the lenses in the opposite way and replace the locking ring. The convex lens always points to the telescope/sky

PS it’s 2.6x and not 2.7x👍

Edited November 28, 2022 by bosun21
Typo

[Marian M]

Indeed 2.6 😀

 

Well, it seems that during the weekend some new experiments are to come 😀

What about the 2.6, how should it be placed?

 

thanks a lot!

[Marian M]

For 2.6, just checked it now, it seems that both surfaces are concave

[bosun21]

3 hours ago, Marian M said:

For 2.6, just checked it now, it seems that both surfaces are concave

The 2.6x can only be fitted in the one orientation and also the lenses should never be opened. The 1.7x lenses are cemented into a doublet and easy to reverse.

[Marian M]

Hi again!

 

Coming back with full test from beginning. I have put all pictures together, in one single file, and done all measurements from scratch.

Setup used is the same- TS 102/714 APO + Baader T2 prism + 1.25" clicklock + Maxbright 2 + SLV 25mm + below GPCs and either APM 2.7 barlow head or full barlow. Only the first picture has been done without clicklock, otherwise I would have not reached the focus. The GPCs were screwed in a 1.25" to T2 adapter, the adapter in bino head.

 

All measurements have been done on a larger screen (laptop), with a ruler. Pictures- done with a smartphone, from hand, directly through eyepiece. I acknowledge that pictures are not perfect, measurements the same and probably some field curvature leads to some wrong calculations. 

 

At least in this setup, the 1.7 GCP seems working more around 1.4 and the 2.6 GPC seems working more around 2.4, through my measurements.

APM barlow head seems giving a 2.9 magnification while the full barlow increase by 4. What is strange, but here I presume the field curvature is cutting some of the real size of the image/ ruler distance, is that the APM (head or full barlow) has a smaller magnification while using a more powerful GPC.

 

Without GPC, APM seems working 2.9/ 4 (head only/ full barlow), while with 1.7 GPC- 2.5/ 3.5 and with 2.6 GPC 2.1/ 3.5

 

Would by happy to receive any thoughts/ share experience on this.

 

Regards,

Marian