Maxbright gpc question

[M40]

I have the baader maxbright binoviewer with 1.7x gpc and using this with a flip mirror I just cant get focus on my refractor. Just not enough back focus ( focuser inward travel). I will be trying a few mechanical arrangements first but just to help my exceptionally confused brain, if it comes down to changing the gpc, do I go up a gpc number or down? Currently the gpc sits in the binoviewer, would it help to move it closer to the telescope if possible? Many thanks for any help.

[Emperor!Takahashi!]

You'd go up to the 2.6x GPC. The Baader GPC is designed to only fit in one location (i.e., just beneath the binoviewer), so there isn't an option to move it.

You may also want to consider changing the diagonal since some have a shorter light path than others. For example, a T2 diagonal will conserve backfocus in comparison with a 2" diagonal. Which diagonal are you currently using?

Edited September 20 by Emperor!Takahashi!

[RobertI]

I would suggest getting a prism diagonal like this one. It has a considerably shorter light path and should allow you to reach focus. You will need to buy a nosepiece for it (one with a T2 fitting), and if you get a 2” nosepiece you can dispense with the 2” to 1.25” converter in the drawtube, saving a few more precious mm of travel. With this configuration I have 21mm of focus travel left on my refractor. 

Edited September 20 by RobertI

[omo]

38 minutes ago, Emperor!Takahashi! said:

You'd go up to the 2.6x GPC. The Baader GPC is designed to only fit in one location (i.e., just beneath the binoviewer), so there isn't an option to move it.

 

The GPC can go in the Baader 2" T2 nosepiece in front of the diagonal - thats what the plastic ring that comes with the GPC's is for.  Don't think this would work with the flip mirror, but does with the Baader T2 Prism or Mirror.

Other variations are possible, such as mounting it in a quick changer

[Emperor!Takahashi!]

25 minutes ago, omo said:

The GPC can go in the Baader 2" T2 nosepiece in front of the diagonal - thats what the plastic ring that comes with the GPC's is for.  Don't think this would work with the flip mirror, but does with the Baader T2 Prism or Mirror.

Other variations are possible, such as mounting it in a quick changer

I would recommend against other positions for reasons stated here: https://www.cloudynights.com/topic/820513-gpc-positioning-in-the-optical-train/

[omo]

35 minutes ago, Emperor!Takahashi! said:

I would recommend against other positions for reasons stated here: https://www.cloudynights.com/topic/820513-gpc-positioning-in-the-optical-train/

Read it, quite amusing, can't one GPC cope with the prisms in the diagonal and bino at the same time? Baader specifically state not to use 2 GPC's together, they seem to get there in the end.

I do take the point of closer to the bino is better. Considering your moving the GPC about 35mm between front and rear, image degradation is more likely darkening due to increased magnification.  I have used GPC's in both positions worked for me, but i have refined things since.

[M40]

I went for the 36mm zeiss prism diagonal so the prism shown by Robertl has a 3mm shorter optical length but I don't think that will be enough.

These are the two arrangements I am trying to make work

 

My thoughts were to ditch the clicklock in the first arrangement and use the quickchange as mentioned by omo. I have asked flo the question and I need to get back to them with more info, but as flo go home for the weekend which is terrible and shouldn't be allowed, you lot will hopefully help, which you already have in that the 2.6x gpc will be the one to go for if the quick change isnt enough. 

Does the 2.6x gpc still fit the 1.25" nosepiece or is it the 2"?

 

 

[RobertI]

Oh right, I understand now. I would have thought that connecting the BV directly to the flip mirror would be the best option, ditching the click lock adaptor as you suggest should reveal a male T2 thread that you can screw the BV directly onto? 

[RobertI]

1 hour ago, omo said:

thats what the plastic ring that comes with the GPC's is for

Ah, I was wondering what that was for!

[omo]

2 minutes ago, RobertI said:

Oh right, I understand now. I would have thought that connecting the BV directly to the flip mirror would be the best option, ditching the click lock adaptor as you suggest should reveal a male T2 thread that you can screw the BV directly onto? 

Seeing it helps, save 30mm, GPC would need plastic ring it pushes into the threads of the male fitting helps centre the GPC. No need for a quick changer - they take up 15mm.

The 1.7 may work like that. Easy enough to try

[Paz]

Another factor to consider is if you have any eyepieces lying around that require you to rack the focuser out a lot that might help get over the line, 32mm televue plossls being an example.

[PeterC65]

Definitely stick with the configuration with the binoviewer attached to the flip mirror directly as that will add considerably to the optical path length with the binoviewer attached to the diagonal.

There is a Baader Ultrashort Clamp that I use that has only 20mm of optical path length rather than the 38mm of the 1.25" Clicklock. One thing to bear in mind with these ultrashort clamps though is that the 1.25" nosepiece passes right through them and if anything is screwed into the nosepiece (even a filter) that can touch the diagonal mirror or prism.

 

[M40]

This is the latest iteration coupling the bino directly to the flip mirror using the flip mirror thread, thanks for the idea. The ring supplied with the gpc slips inside the flip mirror threaded part so unless there is an alternative flip mirror threaded part, it cant be used here. On the bino side of the gpc, there is a slightly raised ring which fits perfectly within the bino plastic ring and centers it, although not the most positive of fits.

 

Now the biggest frustration is waiting for the clouds to clear to see if it works.

[M40]

8 hours ago, Paz said:

Another factor to consider is if you have any eyepieces lying around that require you to rack the focuser out a lot that might help get over the line, 32mm televue plossls being an example.

I have a pair of 32mm plossls that I was playing with and couldn't get focus with the first arrangement. Out of interest, with just the diagonal in the optical path everything focused well.

[PeterC65]

1 hour ago, M40 said:

Now the biggest frustration is waiting for the clouds to clear to see if it works.

You should be able to check you can achieve focus by viewing a distant object in daylight. I had no end of trouble with my binoviewer initially (I was unable to merge the images) and did lots of tests in daylight.

 

[omo]

12 hours ago, M40 said:

Does the 2.6x gpc still fit the 1.25" nosepiece or is it the 2"?

 

 

All 3 GPC's are the same sizes. With a Baader 1.25 nosepiece the GPC will screw into it like it can on the T2 diagonal. In the T2 2inch nosepiece you need the plastic ring pushed / pressed into place.

Using your T2 diagonal is best with a 2" nosepiece as you avoid  the 2" > 1.25" adapter in the focusser which adds to the lightpath.

Edited September 21 by omo