How to focus D70 through newt???

Lee · #1 07-09-2005, 08:21 PM

I have tried to take some prime focus shots of the moon tonight with my D70 and 8"GSO Dob.... the dob (or is it Dob..

) has a 2" focuser...
I have a T-adaptor and tube from Andrews.
initially I thought the T-tube was too long, as the camera seemed to be heading toward focus as I racked in the focuser, but couldn't get close enough.... then I worked out the T-tube is two tubes in one, so I used the short one, which brings the camera in much closer.... still could not get it near focus....

Am I missing something really fundamental here???? The moon with Venus hovering alongside was such a nice sight too...

davidpretorius · #2 07-09-2005, 08:42 PM

if it the same as my toucam, then at prime focus without an eyepiece, the camera acts as a 6mm eyepiece, so you will see everything really big!

davidpretorius · #3 07-09-2005, 08:43 PM

can you take a picture of the problem with another camera and post it thru the attachments section.

Lee · #4 07-09-2005, 09:09 PM

I don't know what I'd actually take a pic of.....
I did just find this -

"Many newer Newtonians are designed with photography in mind and do not require modification for prime focus photography. To check, aim the telescope at the moon, remove the eyepiece, and hold a piece of paper just outside the eyepiece tube, moving it back and forth until a sharp image of the moon appears on it. If you can get the image at least 60 mm from the end of the eyepiece tube, you have enough back focus."
from http://www.covingtoninnovations.com/astro/updates.html

Does anyone know how GSO Dobs fare with in this regard???? Now the moon has set I can't really try it

iceman (Mike) · #5 08-09-2005, 06:00 AM

I'm pretty sure that newt's aren't capable of prime focus phtoography with a DSLR without moving the mirror forward in the tube.

Do some more searching, I'm pretty sure it's been discussed on this forum at least once before, and on other sites like CN.

Lee · #6 08-09-2005, 06:58 AM

Quote:

Originally Posted by iceman

I'm pretty sure that newt's aren't capable of prime focus phtoography with a DSLR without moving the mirror forward in the tube..

I did have a look around.... did get the impression that some Newts wouldn't do it... nothing definite though - did work out though that the bigger tube part of the T-assembly is for eyepiece projection..... hopefully that will work nicely....

gbeal · #7 08-09-2005, 05:47 PM

Gut feeling is you need to get the film plane/chip/camera closer to the primary. In my case this meant a slightly shorter tube, and an extension tube for visual use. Try the tracing paper trick as you know, and then remember that the D70 will need about 45 - 50mm of focus to account for the distance from the lens throat to the chip, my D110 was the same.
Gary

MiG · #8 09-09-2005, 12:03 AM

I can't get my 20D close enough to my GSO 10" dob, so you're not alone. And that's without any adaptors, just holding the body up to a fully racked in focuser.

RapidEye · #9 09-09-2005, 02:53 AM

My 10" GSO is the same way - doesn't have enough in-focus for the Digital Rebel.

I overcame it by using a bino-viewer's trick: put a shorty barlow in front of the T-Ring.

That pushes the focal plane out so that you can reach focus.

The obvious problem is that I've now turned my F5 scope into an F10 scope - but its better than trying to figure out a way to move the mirror up the tube a couple of inches!

G'Luck!

Lee · #10 09-09-2005, 08:06 AM

Quote:

Originally Posted by RapidEye

My 10" GSO is the same way - doesn't have enough in-focus for the Digital Rebel.

I overcame it by using a bino-viewer's trick: put a shorty barlow in front of the T-Ring.

That pushes the focal plane out so that you can reach focus.

The obvious problem is that I've now turned my F5 scope into an F10 scope - but its better than trying to figure out a way to move the mirror up the tube a couple of inches!

G'Luck!

Will have to try that.....
The T-adaptor assembly I bought from Andrews actually has a barrell for an eyepiece, so ep projection worked quite well, apart from CA from cheapo ep..... so am on the right track anyway! may have to give the barlow a go though....
It would be nice to be able to leave lenses out of it, and get no false colour

thanks everyone for your comments.....

slice of heaven · #11 09-09-2005, 09:16 AM

Quote:

Originally Posted by gbeal

Gut feeling is you need to get the film plane/chip/camera closer to the primary. In my case this meant a slightly shorter tube, and an extension tube for visual use.
Gary

The r&p focusers on both my Parks newts have a screw in extension tube between the eyepiece holder and the focuser barrel.
Is the above statement the reason for this?
I've never known why their there. The Parks are supposedly designed for imaging ,given their age I'd say for film though not ccd.
Can anyone shed some light?

Lee · #12 09-09-2005, 10:10 AM

Quote:

Originally Posted by slice of heaven

The r&p focusers on both my Parks newts have a screw in extension tube between the eyepiece holder and the focuser barrel.
Is the above statement the reason for this?
I've never known why their there. The Parks are supposedly designed for imaging ,given their age I'd say for film though not ccd.
Can anyone shed some light?

Shouldn't make a difference I would think - AFAIK the CCD and film lie at the same distance in from the front of the camera, so the focal plane required should be the same (this is obviously for SLR's).... there is the crop factor with most DSLRs, due to the CCD sensor being smaller than a 35mm negative....
Does this extension tube have lenses in it? If it did I would guess it acts like the barlow....

RapidEye · #13 09-09-2005, 10:17 AM

Quote:

Originally Posted by plasmodium

Shouldn't make a difference I would think - AFAIK the CCD and film lie at the same distance in from the front of the camera, so the focal plane required should be the same (this is obviously for SLR's).... there is the crop factor with most DSLRs, due to the CCD sensor being smaller than a 35mm negative....

Actually, not quite. The chip size is smaller, so the focus plain is different, at least for the low end DSLR's. It doesn't crop them, it magnifies the FOV.

My Rebel has the effect of a 1.8 magnifier due to the focal plain/chip size difference: an 18mm FL lens works like a 28mm lens, for example.

High end stuff ($5000 cameras) that isn't the case, nor is it for CCD astro stuff.

slice of heaven · #14 09-09-2005, 10:27 AM

No lenses Lee, just a blank tube with a male thread one end and a female thread the other.
So for imaging I'd remove the tube and fit the SLR to the focuser barrel?
With the ever increasing usage of cameras on scopes these days ,it seems like an inexpensive addition that could be incorporated in the standard design of newts at the factory.

Lee · #15 09-09-2005, 01:54 PM

Quote:

Originally Posted by RapidEye

Actually, not quite. The chip size is smaller, so the focus plain is different, at least for the low end DSLR's. It doesn't crop them, it magnifies the FOV.

My Rebel has the effect of a 1.8 magnifier due to the focal plain/chip size difference: an 18mm FL lens works like a 28mm lens, for example.

High end stuff ($5000 cameras) that isn't the case, nor is it for CCD astro stuff.

I was under the impression it was only the reduced chip size.... as the higher end canons have the full frame CCD and don't have a 'zoom' factor, the high end Nikons still use the smaller, APS sized CCD and do have the 1.5 zoom.
So the lens still focuses an image onto the chip focal plane, but because the chip is smaller it only catches part of the frame - when this is then printed, the zoom appears increased, as the picture is a crop from the centre of the frame.
This is why you can use normal 35mm lenses on your DSLR, but have a zoom factor.....
Just how I understood things....

How does it magnify the FOV.... you actually get less FOV at a given focal length with most DSLR's.... not only the low end ones either... unless you consider a Nikon D2x low end

RapidEye · #16 09-09-2005, 10:27 PM

Quote:

Originally Posted by plasmodium

I was under the impression it was only the reduced chip size....

You're probably right - it makes perfect sense.

I just remember reading a LOT about the Rebel (D300) before I bought it and I was pretty sure I saw some writeup about the focal plane being further back as well. Its been almost a year since I read that and I can't find it again.

I do know the "stock" 18-50 zoom they sell as part of the kit will ONLY work on the Rebel. My understanding was that it was because of the focal plane issue.

Anyhow - its still a sweet little camera. Once you go DSLR, you'll never want to go back to a standard Digicam with a built in lens!