Ad Astra Observatory

Hi Jimbo –

Glad to hear from you again! It sounds like you are now tinkering into
deeper waters. 🙂

Here are the backfocus distances:
SBIG ST-4 1.16mm
Meade DSI 7.43
Canon 300D 44.00

Nikon 46.50

So, the closest built-in algorithm would be for the Canon. But keep in mind
the error percentage there: 2.5mm/46.50
That will produce some interesting donuts.:)

If you kept the original spec on the Nikon above, your really best
alternative of all (unless you love to lug laptops around) is to simply get
an ‘Extra Stiletto Camera Mount’ for it. Extra mounts for any camera are

only $119 each and have been proven to beat the software every time. I know
you want to use the RoboFocus, but unless they have a way to enter a
customized number, you’ll likely be out of luck since the backfocus on the
Nikon is longer than the EOS. If it were the other way around, you could
possibly have a special adapter machined as you mentioned to add the
distance. The Nikon, btw, has the longest backfocus of all of the 35mm
cameras.

Let me know if this helps any, but unless the Robo folks can let a user

enter a customized number, you may be in for an uphill fight.

Thanks & Clear Skies!
Richard

Stellar Technologies International
408 E. Bowie Avenue Box 1423
Alamo, TX 78516
Tel.:1-800-232-9416

Intl: (956)787-2443

—–Original Message—–
From: Jimbo S. Harris [mailto:jimbo@jimbo.net]
Sent: Monday, February 04, 2008 8:17 AM
To: rshell@stellar-international.com
Subject: Focus depth: Canon vs. Nikon

Hi Richard,

I don’t know if you remember me, but I called you a few years back and

asked for some Ronchi gratings for an ATM project, which you happily
supplied, and which worked like a charm, by the way; thanks!

I have a new project, or the inklings of an idea anyway, and I need your
help again. This time, it’s just information I need, I think.

I’d like to know if the focal plane distances of any of the following
cameras are equal to (or can be made equal to) the focal plane distance of
a Nikon D70:

SBIG ST-4
Meade DSI
Canon 300D

Here’s what’s up.

I have a Nikon D70 DSLR, which I use the Stiletto IV to focus every session
and I love my Stiletto and I’m never, never giving it up so don’t ask. (: I

did the “filterectomy” modification on it myself, which threw off the
Stiletto and autofocus until I added in a chunk of same-thickness clear
glass (which I got from Lifepixel, in case you’re curious). This has worked
wonderfully for me.

But, software support for Nikon DSLR is seriously lacking in the astronomy
world. However, software for SBIG ST-4 (which I have, and also have the STI

Ronchi-focusing-nosepiece-doodad for, which I love and etc.), Meade DSI
(which I also have), and for Canon DSLR (which I don’t have, but read on)
is easier to find.
In particular, I’m looking for software to autofocus my telescope by moving
the Robofocuser and firing the shutter on my camera. For the Canon, for
instance, DSLRFocus is $45 and will do the trick. For the Nikon, only $300
MaxDSLR will do autofocus. The SBIG will autofocus for $100 with CCDsoft,
as will the Meade DSI (free plugin for CCDsoft).

So, what I’m thinking about is, use some other (digtial) camera, make it
parfocal to the D70 somehow, and use (whatever software is needed) to focus
the telescope with it, then remove the “focus camera”, pop in the D70, and
away we go.

So, I’m writing to ask you what the focal plane distances of the above 4
cameras are, and to see if you have any clues as to how I can achieve

precise parfocality (?) between one of the “other” cameras and the D70.
Clearly, you’ve done a lot of thinking about how to mechanically produce a
parfocal focal plane (and I am not looking for trade secrets — I am and
always shall be your customer. I love your stuff and keep it coming!). If
you can pass on some clues, though, I’d be most appreciative.

If you feel you can’t be too specific, a pointer in the right direction
would be most welcome; “the XXX has a shallower focal plane than the D70,
but it’s close enough that a parfocalizing ring might work there”, or “I’ve
measured such and so washers and they’re always XXX thick, if that helps”
or… I dunno.

Thinking it over, what I’d *really* like is a nosepiece for the ST-4 (for
instance) that held its chip in the same place as the D70’s. If STI made
one of these, I’d buy it.

I thank you in advance for your help, and thank you again for your great
products for the astronomy community!

Regards,

Jimbo S. Harris
Newark, CA

__________ NOD32 2847 (20080204) Information __________

This message was checked by NOD32 antivirus system.
part000.txt – is OK

http://www.eset.com