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Old 26-09-2015, 11:19 PM
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alpal
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10" f4 Newt. focus problem.

I still can't get my 10" f4 Newt to focus with my camera setup here:

https://www.flickr.com/photos/247194...in/photostream

I tried it out again tonight - this is my 3rd attempt.

I have tried the mirror in the holder as is = zero extension.
I tried 40mm extensions - the focuser was fully out for closest focus - didn't work.
Tonight I tried again with 20mm extensions & it still didn't focus with the focuser fully in.

It defies the laws of physics because the focuser has a 29 mm travel.
It's back to the drawing board.
You can certainly buy a telescope but there is no guarantee that
it will focus with your camera train.

Any ideas on how to make an infinitely adjustable mechanism to move the mirror
to find the right focus point?

cheers
Allan
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  #2  
Old 27-09-2015, 12:18 AM
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Shiraz (Ray)
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hi allan

you currently have the moon as a good target, so take the camera/rcc1 out of the focuser and hold a bit of baking paper above the focuser to find out where the moon image is formed - that is where the camera focal plane has to go. I don't know what your scope design is, but you will probably have the RCC1 hanging into the ota a fair way and you may need to move the mirror forward a bit to move the focal plane out and get the RCC1 out of the input column of light.

whatever, it'll work fine when you get it set up
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Old 27-09-2015, 12:20 AM
glend (Glen)
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Is it an imaging newt or a normal visual one? Many visual newts will not support camera focus because there is not enough in-travel. If your eyepieces all work fine without extension then you have a newt configured for visual use.
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Old 27-09-2015, 01:15 AM
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Quote:
Originally Posted by Shiraz View Post
hi allan

you currently have the moon as a good target, so take the camera/rcc1 out of the focuser and hold a bit of baking paper above the focuser to find out where the moon image is formed - that is where the camera focal plane has to go. I don't know what your scope design is, but you will probably have the RCC1 hanging into the ota a fair way and you may need to move the mirror forward a bit to move the focal plane out and get the RCC1 out of the input column of light.

whatever, it'll work fine when you get it set up

Hi Ray,
I can get it to focus with an eyepiece - at least I did with a 40mm extension.
I have to get this to work with my imaging train to use my QHY9m camera
+ varilock spacer + OAG + filter wheel.

cheers'
Allan
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Old 27-09-2015, 01:19 AM
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Quote:
Originally Posted by glend View Post
Is it an imaging newt or a normal visual one? Many visual newts will not support camera focus because there is not enough in-travel. If your eyepieces all work fine without extension then you have a newt configured for visual use.

Hi Glend,
What I need is some way to move the mirror up by turning screws till I find the ideal position.
I am thinking of using a circular ring of plywood with screws in it
embedded into nuts - that are glued in place.
I just need something that I can adjust in length
& also collimate to find the right distance.

Unfortunately this problem is orders of magnitude harder than I thought.
I am working full time & I don't have a lot
of time to muck around any more.
Already my 3rd attempt has led to failure &
seems to defy the laws of physics -
it's been very frustrating for what seems like a simple problem.

cheers
Allan
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  #6  
Old 27-09-2015, 07:51 AM
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Quote:
I have tried the mirror in the holder as is = zero extension.
I tried 40mm extensions - the focuser was fully out for closest focus - didn't work.
Tonight I tried again with 20mm extensions & it still didn't focus with the focuser fully in.
What I've written above doesn't make sense.
5 months ago -with 40mm extensions & the focuser fully out I would have
only just had to move the RCC1 corrector further out in the focuser
to achieve focus & lock it in place with the screws in the focuser.
I obviously didn't take proper notes.
Whatever happened I decided that I needed to have shorter extensions for it to work -
In this case 20mm as opposed to 40mm.
now I don't know why -
lesson learnt - take proper notes!

cheers
Allan
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Old 27-09-2015, 08:03 AM
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Pictures of 40mm & 20mm extensions.
Attached Thumbnails
Click for full-size image (Mirror holder with 40 mm extensions.jpg)
161.9 KB42 views
Click for full-size image (20mm extensions.jpg)
182.4 KB25 views
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Old 27-09-2015, 08:16 AM
glend (Glen)
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Try it without the corrector, just ignore the coma to test focus. If you can achieve focus with just the camera then you'll know where the problem is.
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  #9  
Old 27-09-2015, 08:52 AM
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Quote:
Originally Posted by glend View Post
Try it without the corrector, just ignore the coma to test focus. If you can achieve focus with just the camera then you'll know where the problem is.
Hi Glend,
Yes the coma corrector is tricky.
It actually extends the focus point out to allow for more back focus with a Newt . to take the
filter wheel & OAG.
I must have had the focuser fully in with the 40mm extensions
so why did I go for a shorter extension of 20mm?
I think I am confused because the first pictures I took were with
another mirror & no extensions.
The new mirror now - a higher quality one -is a bit thicker.

It would be so easy if I could just move the mirror up & down to any point I wanted.
Changing to new extensions is a major job.

cheers
Allan
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Old 27-09-2015, 09:20 AM
glend (Glen)
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Allan, as I said in my PM on CN, I did not realise you were talking about primary mirror extensions as opposed to normal focuser tube extensions. Personally I would not be fiddling around with the primary location until I had a good understanding of how the scope was performing visually with a range of EPs, then, and only then would I move to camera testing. This gives you a baseline to work from. As I said on CN, go back to basic setup of the primary and work from there. Forget all the accessories and try it with just the camera. Try a DSLR if you can as well. I don't know anything about that corrector that your trying to use but anything with lenses in it in the optical path is going to mess with possible focal point locations so get rid of it for testing would be my advice. Always work from a point where you know things are ok.
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Old 27-09-2015, 09:26 AM
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Quote:
Originally Posted by glend View Post
Allan, as I said in my PM on CN, I did not realise you were talking about primary mirror extensions as opposed to normal focuser tube extensions. Personally I would not be fiddling around with the primary location until I had a good understanding of how the scope was performing visually with a range of EPs, then, and only then would I move to camera testing. This gives you a baseline to work from. As I said on CN, go back to basic setup of the primary and work from there. Forget all the accessories and try it with just the camera. Try a DSLR if you can as well. I don't know anything about that corrector that your trying to use but anything with lenses in it in the optical path is going to mess with possible focal point locations so get rid of it for testing would be my advice. Always work from a point where you know things are ok.
Hi Glend,
I sent you a PM.
I've have tried all that 5 months ago.

cheers
Allan
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Old 27-09-2015, 09:37 AM
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as far as I can tell, the RCC1 has almost no effect on the location of the focal plane in my system - it just fixes coma and curvature and everything else stays the same. ie if you set it up without the RCC1 and then drop it in, the focal plane does not move much.

If you had 40mm spacers and the focuser wound right in, then you need a little bit more spacing. From rusty memory, I needed to cut 55mm off the OTA of my 250/f4 to get the mirror far enough up to get focus on my QHY8/MPCC - that would be consistent with you needing a bit more extension on your mirror spacing (about 20mm possibly). My SW CF 250f4 has the focal plane a long way out by design and I only needed to take about 15mm off that tube. Cutting the tube is the best solution from a stability point of view, and suggest that you consider doing that when you have worked out how much is needed.
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Old 27-09-2015, 09:39 AM
glend (Glen)
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Ok, well I am out of ideas then. Good luck with it.
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Old 27-09-2015, 09:40 AM
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Hi Allan,

When I used a mpcc I needed an extension tube. With the rcci on mine it sits quite far inwards in side my moonlight focuser. I hope this helps...

Cheers

Russ
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Old 27-09-2015, 09:49 AM
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Quote:
Originally Posted by Shiraz View Post
as far as I can tell, the RCC1 has almost no effect on the location of the focal plane in my system - it just fixes coma and curvature and everything else stays the same. ie if you set it up without the RCC1 and then drop it in, the focal plane does not move much.

If you had 40mm spacers and the focuser wound right in, then you need a little bit more spacing. From rusty memory, I needed to cut 55mm off the OTA of my 250/f4 to get the mirror far enough up to get focus on my QHY8/MPCC - that would be consistent with you needing a bit more extension on your mirror spacing (about 20mm possibly). My SW CF 250f4 has the focal plane a long way out by design and I only needed to take about 15mm off that tube. Cutting the tube is the best solution from a stability point of view, and suggest that you consider doing that when you have worked out how much is needed.
Hi Ray,
Thanks - I want to cut the tube but not before I find out
the correct focus point for the mirror.
The tube is carbon fiber so it's an expensive mistake if I cut it incorrectly.
I was actually going to use another - better - mirror holder.
This one was only supposed to be for testing.
Another mirror holder could require all of the carbon fiber tube.

cheers
Allan
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Old 27-09-2015, 09:49 AM
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Ok, well I am out of ideas then. Good luck with it.

Thanks Glen,
your input was appreciated.

cheers
Allan
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Old 27-09-2015, 09:51 AM
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Originally Posted by rustigsmed View Post
Hi Allan,

When I used a mpcc I needed an extension tube. With the rcci on mine it sits quite far inwards in side my moonlight focuser. I hope this helps...

Cheers

Russ
Hi Russ,
Yes - the RCC1 works well on my 8" f6 Newt.
I got that right first time when I moved the mirror up in the tube!

cheers
Allan
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Old 27-09-2015, 05:03 PM
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I am still thinking about this today.
Something is fundamentally wrong.
With the 20mm extensions for the MIRROR -
the large circle caused by the star was getting smaller as
the focuser was wound in.
With it fully in it was taking up about a 1/4 of the camera chip size.

This is telling me that the mirror is too far away.
the mirror needs longer extensions.

Yet when I had 40mm extensions I still couldn't get focus.
40mm extensions bring the mirror up another 20mm which
should be exactly what I needed.
As the focuser has a 29 mm travel I should have been able to get focus.
It makes me want to put the 40mm extensions back on again.

This is a diabolical problem.

Allan
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Old 27-09-2015, 05:17 PM
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Put the 40mm mirror extensions on. Start with focuser fully racked in, and start testing as you focus out. If it's improving as you rack out, you know the right answer is somewhere between 20 and 40mm
If with the 40mm mirror extensions you get all the way out but still not in focus, just use ext tubes on your focused for testing purposes. 29mm isn't much travel.
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Old 27-09-2015, 05:34 PM
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Shiraz (Ray)
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if the star was taking up 1/4 of the chip area at focuser-full-in, then it was roughly 10mm across. At f4, that means that the mirror needs to be about 40 mm closer to get focus. If you meant that the star was about 1/4 the linear size, then you need another 20+ mm.

try your 20 and 40mm spacers together. my GSO f4 needed about 55mm and there was not as much stuff in the back-focus zone.
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