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Reducer help / questions

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#1 Iowaboy

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Posted 16 February 2020 - 12:27 PM

I’m fighting back spacing issue / or a lousy flattener  issue 

I recently bought a Starfield .6 reducer from Canada. It’s made for refractors f7 and above - a 55mm backfocus is stated. I’m still getting comets toward the edge with a skywatcher 100 Ed pro (f9) 

And a Nikon D5300- all I had is a ruler and it looks like I have 54mm backfocus. 
the test image was bright and field reduced (f5.4)

 

question 1. Is 1mm really going to give me totally unsatisfactory coma ? 

 

question 2- this may be silly-  I have a Starizona SCT corrector lll that totally rocks on my 11” xlt-

now I know it’s for f10 and f11 SCTs - but what do you think would happen if I stuck it in the f9 refractor ???

 

thanks in advance -

 



#2 sg6

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Posted 16 February 2020 - 01:18 PM

Hard to say anything as the D5300 has an internal distance of 46.5mm - I believe it is a F mount ???

So you need an 8.5mm spacer between the reducer and the D5300 flange. You don't say if or what you have.

 

Next the details of the diagram seem to give the 55mm separation with the M42/M48 adaptor in place, if it is not there then the back focus is 65mm - again have you one in place, no mention of it.

 

Reading it Starfield have assumed a few things as they say a 10mm T-ring is required, but on your D5300 that would gve a back focus minimum of 56.5mm and you could never then reduce it to get 55mm. Seems Starfield have never heard of or considered a Nikon D5300.

 

So have you the M42/M48 adaptor on the reducer?

Have you an 8.5mm spacer or T_Ring attaching the D5300 to the reducer?

If no M42/M48 in use then what spacers etc have you ?



#3 wrnchhead

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Posted 16 February 2020 - 01:19 PM

I personally have only battled through one flattener's backspacing so my experience is limited to that. 1. Yes 1mm can be 200% of an acceptable amount. It's best to have an image to look at. I actually had my spacing right finally, but a barely loose cheap t-ring was causing my stars to blur in two corners. 

 

Do you have any "fairly short" exposures with stars for the experts to look at? They can usually tell which way it needs to go even. 



#4 rgsalinger

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Posted 16 February 2020 - 06:07 PM

When you buy a reducer you get a reduction in both focal length and imaging circle (where the stars are round). When you couple something that was NOT specifically designed for your telescope but is generic, poor results are likely. Finally, you're trying to use a camera with a large chip and you've reduced the imaging circle. I doubt sincerely that this is a back focus issue but of course you need to get it correct. I would call up the vendor and ask about the correct back focus and how to measure it. I would also call the company who makes your refractor and see if they have something specifically designed for it's optics. The reducer for my Televue cost a lot more, did not work very well (smaller image circle) and was specifically designed for my scope.. I think it's just not going to work very well and you may want to live with what you have or try a couple of others.

 

Now if you have a friend with exactly the same setup and it works perfectly then ignore everything that I said and get the numbers from him but that has to include using a DSLR with the same size chip and actually measuring the stars.

 

Rgrds-Ross 



#5 Iowaboy

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Posted 16 February 2020 - 06:07 PM

Hard to say anything as the D5300 has an internal distance of 46.5mm - I believe it is a F mount ???
So you need an 8.5mm spacer between the reducer and the D5300 flange. You don't say if or what you have.

Next the details of the diagram seem to give the 55mm separation with the M42/M48 adaptor in place, if it is not there then the back focus is 65mm - again have you one in place, no mention of it.

Reading it Starfield have assumed a few things as they say a 10mm T-ring is required, but on your D5300 that would gve a back focus minimum of 56.5mm and you could never then reduce it to get 55mm. Seems Starfield have never heard of or considered a Nikon D5300.

So have you the M42/M48 adaptor on the reducer?
Have you an 8.5mm spacer or T_Ring attaching the D5300 to the reducer?
If no M42/M48 in use then what spacers etc have you ?


Looks like a 8mm t- ring. No other adapters used. Thanks

Edited by Iowaboy, 17 February 2020 - 08:02 AM.


#6 kathyastro

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Posted 16 February 2020 - 06:16 PM

I’m fighting back spacing issue / or a lousy flattener  issue 

I recently bought a Starfield .6 reducer from Canada. It’s made for refractors f7 and above - a 55mm backfocus is stated. I’m still getting comets toward the edge with a skywatcher 100 Ed pro (f9) 

And a Nikon D5300- all I had is a ruler and it looks like I have 54mm backfocus. 
the test image was bright and field reduced (f5.4)

 

question 1. Is 1mm really going to give me totally unsatisfactory coma ? 

 

question 2- this may be silly-  I have a Starizona SCT corrector lll that totally rocks on my 11” xlt-

now I know it’s for f10 and f11 SCTs - but what do you think would happen if I stuck it in the f9 refractor ???

 

thanks in advance -

I am not sure how you would measure the 55mm with a ruler, as all of the distance is internal to the camera.  55mm means that you install the T-ring onto the camera, and then attach the T-ring directly to the reducer.  A DSLR (any brand) with its T-ring installed, is, by design, exactly 55mm deep.  They design the reducer with a 55mm spacing requirement with this in mind.



#7 Iowaboy

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Posted 16 February 2020 - 06:17 PM

Thank you for your answer. I don’t have any special thread adapters, and my tape seems to think my t-ring is about 8mm -
The reducer is screwed into t-ring and slid into the refractor -
My tape indicated 54mm from back of reducer to the focal plane mark on the side of the camera
Taking your info of 46.5 - and adding my t ring thickness of what looked like 8- I should be **** close or at 55mm-
I’m not too experienced with spacing - with my Starizona I just told them the camera and they sold me perfect spacing adapter for it. And it’s edge quality or better now.

#8 Iowaboy

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Posted 16 February 2020 - 06:18 PM

I am not sure how you would measure the 55mm with a ruler, as all of the distance is internal to the camera. 55mm means that you install the T-ring onto the camera, and then attach the T-ring directly to the reducer. A DSLR (any brand) with its T-ring installed, is, by design, exactly 55mm deep. They design the reducer with a 55mm spacing requirement with this in mind.



#9 Iowaboy

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Posted 16 February 2020 - 06:22 PM

If this is the case the correction of this reducer is lousy. Because if screwing it into my t-ring is the answer - that’s how it is now and the stars at edge are practically rainbow comments

I was measuring to the little sensor / focal plane mark on exterior of all DSLRs

#10 Iowaboy

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Posted 16 February 2020 - 06:39 PM

Yes I should just probably get the proprietary reducer from SKy Watcher. It’s only .85 but I know it works. But then I’ll only be f7.6. I was wanting faster -but don’t we all....

#11 Iowaboy

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Posted 16 February 2020 - 07:00 PM

When you buy a reducer you get a reduction in both focal length and imaging circle (where the stars are round). When you couple something that was NOT specifically designed for your telescope but is generic, poor results are likely. Finally, you're trying to use a camera with a large chip and you've reduced the imaging circle. I doubt sincerely that this is a back focus issue but of course you need to get it correct. I would call up the vendor and ask about the correct back focus and how to measure it. I would also call the company who makes your refractor and see if they have something specifically designed for it's optics. The reducer for my Televue cost a lot more, did not work very well (smaller image circle) and was specifically designed for my scope.. I think it's just not going to work very well and you may want to live with what you have or try a couple of others.

Now if you have a friend with exactly the same setup and it works perfectly then ignore everything that I said and get the numbers from him but that has to include using a DSLR with the same size chip and actually measuring the stars.

Rgrds-Ross


Thanks for Your answer. This reducer has a tough act to follow as my only other experience is with sct reducers. The celestron standard .63 was better than this and my Starizona SCT corrector lll turns my 11” xlt into an edge or better - but as you say. It is specifically built for my SCT - I may have to just settle for f7.6 with the proprietary Skywatcher .85 which I do know works. Because I can’t use this as is unless I want to crop


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