Star 71 extension tube

[Demonperformer]

To get the spacing on my new star 71 correct, I need 12.28mm. OK, I'm not going to find that, but I have some delrin rings if I can get close (slightly under, not over).

My filter wheel requires this extension to have a female t2 thread. (Yes, this one I actually know fits.)

According to the attached diagram, the ota has a M48 male thread on the end, but my "M48/T2" extension ring slips inside the opening, which I don't understand at all (no surpises there!).

Has anyone got any ideas as to exactly what will do the job, that won't break the bank (the ota has already done that)?

Responses from people who actually use this scope particularly welcome .

Thanks

 

[Droogie 2001]

Not sure why your M48 / T2 would slip inside the focuser?

I purhased an Altair Astro M48 17-29mm Variable Spacer Set (3 part) which attach nicely to the M48 on the OTA.

https://www.altairastro.com/m48-17-29mm-variable-spacer-set-3-part.html

I then used the 5mm and 10 mm only to get the back focus I requried. All I would add is that I have an OAG so you may need to select the correct rings for your setup.
I now only need to extend my focuser 3mm to gain focus.

 

[Davey-T]

Why do you need an exact length spacer when there is no flattener / reducer to worry about ?

I use part of a Baader VariLock on mine but only because it's got the right threads.

Dave

[Demonperformer]

19 minutes ago, Droogie 2001 said:

Altair Astro M48 17-29mm Variable Spacer Set (3 part)

Unfortunately, this would provide a minimum distance of 5mm more than the 12.28mm I require. Thanks.

[Demonperformer]

Just now, Davey-T said:

Why do you need an exact length spacer when there is no flattener / reducer to worry about ?

Admittedly this was my first thought (and was one of the reasons I chose this scope).

But then the question becomes why do they produce a diagram that says I need a total of 66.78mm (of which 54.5mm is made up by the fw/oag + camera distance) from the end of the OTA to the image chip?

And it doesn't help with all the different combinations of connector threads and adverts do not always say whether they are talking about the thread on the adapter or of the thing to which they are connecting. [This last paragraph is not directed at your response, Dave, it's just an additional frustration I feel about the entire exercise!]

[Droogie 2001]

12 minutes ago, Demonperformer said:

Unfortunately, this would provide a minimum distance of 5mm more than the 12.28mm I require. Thanks.

So you are over by 12.28mm already and you need to reduce this as opposed to my issue where I needed to extend the distance?

Can Starlight Express offer any help? It doesn't seem right that you need to remove distance.
That send it took me a fair while to get my head around this too. The only comfort I can bring is once its sorted you can just move on.

[Demonperformer]

No. The diagram says that I need 66.78mm, of which the oag/fw + camera takes 54.5mm, leaving me with needing an additional extension tube of 12.28mm. The one you suggest has a minimum distance of 17mm which is 5mm more than the amount I require. If I used that I would then be 5mm over. Not sure why I would go to starlight express anyway?

But then, there now seems to be the question of whether I need it at all ...

[Demonperformer]

Right, lets see if I can approach this from another angle.

If the diagram is (for some reason) wrong and I do not need an exact distance (which is what I think @Davey-T is saying?) then I just need an adapter that will link the ota to the fw/oag.

Now, the scope has a M48 male thread and the fw/oag has a female t2 thread. So I need an adapter that will have a female m48 thread on one end and a male t2 thread on the other end.

This would seem to fit the bill. Worst case scenario, I need to find another 8.78mm extension (either t2/t2 or m48/m48) somhow.

Good plan?

Thanks.

[Droogie 2001]

They are 5mm and 10mm M48 spacers so 15mm not 17mm. Will still leave you off by 2.72mm.  I mentioned Star Light Express as you noted it in your original post, the filter wheel being part of the imaging train. 

Sorry can't help anymore. 

 

 

[sloz1664]

Never bothered with spacing on my Star71. Just added the OAG, Filter Wheel & Atik 383L+ and perfect flat field. Using the Atik 460 I just swap cameras, again no spacing issues.

Steve

 

[Davey-T]

As long as you can reach focus ie: have enough inwards travel to achieve focus it should work.

As I said I use part of a Baader VariLock but only because it has the right threads, bit expensive to buy just to use part of it though.

Dave

 

[Demonperformer]

7 minutes ago, Droogie 2001 said:

They are 5mm and 10mm M48 spacers so 15mm not 17mm. Will still leave you off by 2.72mm.

Light dawns ... I see that it has spacers now, I was just going by the name "17-29mm adapter".

 

The concensus seems to be coming down on the fact that I do NOT need a specific distance, so I think I will go with the adapter I have cited above and hopefully this will get it all operational.

Thanks.

[Droogie 2001]

No problem. Hope you get it sorted.

[Datalord]

On 3/28/2018 at 10:51, Davey-T said:

As long as you can reach focus ie: have enough inwards travel to achieve focus it should work.

As I said I use part of a Baader VariLock but only because it has the right threads, bit expensive to buy just to use part of it though.

Dave

 

Davey, how did you get that motor attached? I tried opening that side of the focuser, but I simply couldn't get the thermometer thingy off. I couldn't find any screws and I was afraid of ruining the focuser if I had to use force.

[Davey-T]

3 hours ago, Datalord said:

Davey, how did you get that motor attached? I tried opening that side of the focuser, but I simply couldn't get the thermometer thingy off. I couldn't find any screws and I was afraid of ruining the focuser if I had to use force.

There are 2 grub screws down the hole, easy to miss one, loosen one rotate a bit and loosen the other, they're about 90 degrees apart.

You need to remove the circular spacer as well to fit a motor focuser.

Dave

 

[Demonperformer]

Right, the adapter ring has arrived this afternoon and I have checked it out. It certainly links the fw/oag to the scope, although I was initially a little concerned that I could not get it with the oag bit on top - until I realised that I could release the locking bolt on the focusser and rotate it through 90 deg.

The only issue I might have is that the adapter is so thin that there is still a few mm of thread on the scope that it does not engage. I don't think this will be a problem, as the weight of the fw/oag should keep it held in place. If necessary I can fill in the gap with a load of delrin rings, but I won't go there unless it causes a problem.

So, now the scope is operational, there will be no chance of any clear skies for the next month ...

[Demonperformer]

While I have been waiting for this to arrive, I have been doing some checking on how this scope will perform with my cameras (NOW, I think about that!).

* With the 224, it will give me a fov of 48'*36' and a resolution of 2.22"/px
* With the 1600, it gives a fov of 174'*132' and a resolution of 2.25"/px

Which seems to be pretty much spot-on for the 2"-2.5"/px, which I have seen recommended, and the fields-of-view will be suitable for a wide range of objects.

The canon (not that I use that much nowadays) is 219'*146' @ 3.38"/px, so not quite as good.

Of course, how the results actually turn-out will only be seen once I have tested it in anger.

Thanks.