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Old 29-08-2012, 10:11 PM
04Stefan07 (Stefan)
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Newtonian DSLR Prime Focus

The other night when I tried to take a photo of the moon in prime focus with my DSLR I could not achieve focus. I was thinking that there may be something I am not doing correctly so after doing some research online and trying to find the problem I found a video online where they remove the 2" eyepiece section and they said you should be able to achieve prime focus.

Today I went out and attempted this. Positioned the telescope at some distant trees and did what the video said. Prime focus did not work.

Over the past few days I been looking up the problem on iceinspace, other forums, websites, videos, etc. Many others have had the same problem and it all comes down to the design of the telescope. Here are the only solutions to correct the issue that I have learnt.
- Purchase a low profile focuser (way too expensive and not worth it)
- Buy an adapter that will allow eyepiece projection
- Move the primary mirror further up the tube
- Replace the primary mirror screws and springs with longer ones which will place it higher in the tube

The only time I could achieve prime focus was today when I was playing around with the scope outside. I removed the 2" eyepiece section, then there was another bit of metal I removed. I then removed the EOS t-ring and placed the camera to the telescope and it was in focus! Problem was, there was no adapter or anything left.

When I purchased my scope about 10 months ago I did not plan to do astrophotography. I did some afocal stuff up but now that I bought a DSLR about a month ago I do want to do astro imaging with the DSLR (as well as photography on the side). I have the necessary adapters for the DSLR as well.

After reading the solutions I am now more confused than ever on what I should do and thought I should ask here.

The last thing I want to do is drill holes and play around with the mirror of the scope. I want to keep the DSLR (since i will make use out of it other than astro imaging) but I was hoping I could at least get prime focus. I do not want to spend heaps more money either.

I was thinking of buying a webcam, the Microsoft Lifecam HD-3000 $30, modding it and just do some basic planetary shots at prime focus.

I have had a look at the eyepiece projection method and seen some pictures which have come out very nicely. What are your thoughts on the eyepiece projection method? The cheapest one I can find is this one at just under $40 U.S..
http://www.ebay.com.au/itm/1-25-T-T2...ht_4560wt_1163

To do prime focus am I better off buying a cheap webcam and modding it (would the HD-3000 be able to be moddified?) or buying that adapter to do eyepiece projection?

Any help and advice would be appreciated!

Thanks
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  #2  
Old 29-08-2012, 10:15 PM
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Geoff45 (Geoff)
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You could move the focuser down the tube, rather than the mirror up the tube. You may also have to move the spider down a bit, depending on how much adjustment you have to play with.
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Old 29-08-2012, 10:27 PM
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Screwdriverone (Chris)
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Hi Stefan,

On my Skywatcher 200mm newt, the 1.25" eyepiece adaptor barrel unscrews, revealing a thread and this the screws onto the end of the EOS T ring, allowing me to achieve focus with my 1000D as the Tring and the remaining flange part of the 1.25" adaptor are very low profile.

Not sure if the Celestron has the same type 1.25" adaptor barrel arrangement, but worth a look.

Otherwise, the mirror MAY be able to be unscrewed from the bottom of the tube by loosening the retaining screws so the springs "push" it up the tube. Dont loosen them off till they pop out, but see if this helps too?

While my Skywatcher was collimated OK, the primary was screwed right down onto the bottom of the tube for transport and it took me a while to figure out why I couldnt get focus with the webcam in the focuser, until I thought about loosening the primary retaining screws a bit.

Give that a go as well.

Good luck.

Chris
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Old 29-08-2012, 10:58 PM
04Stefan07 (Stefan)
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I had a go at removing the 2" eyepiece adapter but the focuser isnt low profile like the skywatcher.

How can the focuser be moved down the tube?
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Old 30-08-2012, 08:28 AM
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Barlow (it will significantly extend the focal plane plane away from the tube) is better than eyepiece projection (lower distortions and better resolution, good for planets and Moon).. but neither is good for nebulae and extended objects (due to high effective F-number of the system).
However, you will have to get the low-profile focuser if you want to use prime focus directly (or with field flattener).
You can try to make it yourself (or with help of someone who has a lathe), it is really not a rocket science.

Last edited by bojan; 30-08-2012 at 08:42 AM.
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Old 30-08-2012, 10:50 AM
04Stefan07 (Stefan)
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I also forgot to mention that I have been using a barlow and is fantastic but would like some wide shots.

I wouldn't have a clue of making a focuser from scratch!

Quote:
Originally Posted by bojan View Post
Barlow (it will significantly extend the focal plane plane away from the tube) is better than eyepiece projection (lower distortions and better resolution, good for planets and Moon).. but neither is good for nebulae and extended objects (due to high effective F-number of the system).
However, you will have to get the low-profile focuser if you want to use prime focus directly (or with field flattener).
You can try to make it yourself (or with help of someone who has a lathe), it is really not a rocket science.
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Old 30-08-2012, 11:00 AM
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Quote:
Originally Posted by 04Stefan07 View Post
I wouldn't have a clue of making a focuser from scratch!
It is not too complicated, see here..
http://www.iceinspace.com.au/forum/s...hlight=focuser

Alternatively, you will have to buy it
Something like this:
http://www.telescope.com/Accessories.../51/p/8881.uts
http://www.meridiantelescopes.com/im...user/fochb.htm
http://members.shaw.ca/gargwolanski/FOCUSERS.HTM

Last edited by bojan; 30-08-2012 at 11:12 AM.
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Old 30-08-2012, 11:40 AM
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ZeroID (Brent)
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Quote:
Originally Posted by 04Stefan07 View Post
. Here are the only solutions to correct the issue that I have learnt.
- Purchase a low profile focuser (way too expensive and not worth it)
- Buy an adapter that will allow eyepiece projection
- Move the primary mirror further up the tube
- Replace the primary mirror screws and springs with longer ones which will place it higher in the tube


Thanks
Your fourth option is basically what I did. Added 20mm of spacers and longer screws to push the mirror up but my scope is DIY and lends itself well to modification. Not sure how easy it would be to do with yours.

It may make some EPS not have enough depth but that is easily fixed with screw in EP extenders. I have only one that really requires this.

Shifting the mirror up this way is not hard and is easily reversible so worth exploring. Better than drilling holes and spending heaps on things that may not work.
How much further do you think you need to reduce the focal length ? Are you close to focus when you run out of inward travel ? As I said, mine was only 20mm. Nice solid spacers reduces the chance of the longer screws wobbling and you can still use the provided springs. Just need longer screws. I turned my spacers up on the lathe so all equal thickness but you should be able to find something in the local hardware that could be adapted.
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Old 30-08-2012, 12:17 PM
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You have to be aware that moving the mirror up will increase vignetting, due to already too small secondary mirror (the case in most commercial newtonians) and this modification only makes that issue worse.
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Old 05-09-2012, 07:53 PM
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Quote:
- Purchase a low profile focuser (way too expensive and not worth it)
It is worth it.
The idea is get the camera sensor as close as possible to the secondary mirror.
This will allow better illumination of the sensor at it's edges.


You need to use a program like this:

http://www.dalekeller.net/ATM/newton...t/newtsoft.htm

to work out dimensions & be able to check for illumination & vignetting.
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Old 10-09-2012, 11:13 AM
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Quote:
Originally Posted by bojan View Post
You have to be aware that moving the mirror up will increase vignetting, due to already too small secondary mirror (the case in most commercial newtonians) and this modification only makes that issue worse.
A 20mm decrease is not going to make much difference to vignetting. That's all I needed.
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