In-focus issues with Newtonian and DSLR

[rocketandroll]

Hi all

So, I managed to get my DSLR attached to my new Newt today for the first time... and hit a snag.

I have a SW T-adaptor for my Eos 500D but in between the thread for the T-adaptor on one end and the 2" mating tube that goes into the focuser on the other end there is a larger diameter 'shoulder' that adds about 2" or more to the length of it.

With that in place I can't get the camera in far enough to achieve focus.

I took the adapter out and manually moved the camera in and out towards the focuser aperture and it appeared to be in focus about 15 - 20mm (air gap between camera and focuser face) from the focuser.

I spoke to FLO and put on order one of their 2" t-adapters which states it only adds 4mm to the optical train.

First Light Optics - FLO 2-inch T mount camera adapter

FLO have subsequently said however this may not solve the problem?

I am keen to know if I am missing something here? Surely it's just an issue of needing to get the camera chip the right distance from the secondary... in this case, about 30mm closer than my current adapter allows.

Is there some other problem that I'm missing or, given the distances specified, should this work for me?

The enclosed diagram (sorry for the poor quality, done in about 2 minutes to explain the problem to FLO) is meant to show the problem with the current set up... and the proposed set up with their shorter adapter which, as far as I can see, shortens the distance from the chip to the focuser by 46mm or so.

Anyone else had problems getting focus with a DSLR and a Newt?

Oh, and it's an Orion Optics VX8L btw, 1.2m fl, f6, 8" dia.

Advice glady accpted!

Ben

[Stephen]

Welcome to newts (again)

As you've worked out, yes you've just got to get closer. I think FLO might be suggesting this won't get you far enough - but if your maths add up, great!

What sort of t-mount has a 2" extension on it though? The mind boggles

[rocketandroll]

...this one does:

First Light Optics - Skywatcher 2 inch T Adapter

:-)

It's always been fine before on fracs and my RC which needed as much out-focus travel as possible... but for the Newt it's useless.

I didn't measure exactly but yes, as a rough guess, anything that adds less than 20mm to the distance from the face of the focuser to the front face of the camera will be fine.

FLO adapter on it's way via (hopefully) next day delivery.

Forecast is for a clear night Sunday and I desperately want to get first light on this thing on M42.

Still no Bahtinov for it (doesn't seem anyone makes them) but I'll have to use the old 'take a photo and review it on the screen, tweak, and repeat' method :-)

Ben

[Stephen]

Good point, I didn't think of fracs.

The guy who makes the masks will make one to order for you, I think FLO can pass the message along, but you can always drop him a line via his eBay shop if not.

[Deneb]

Sorry Ben, have you tried maybe something like a low profile eos t ring, then thread a m42 nosepiece into the t ring.

Nadeem.

[FrankieValley]

Hi Ben,

Hopefully your adapter will work. If not, you could try this, which doesn't have a shoulder - TS Adapter auf 2' mit Ringklemmung für Skywatcher Newtons

An alternative (and something that will give you a flat field) is this. Again there is no shoulder. Scroll down the page for the user manual - Skywatcher coma corrector for F5 or faster Newtonians :: Adapters - Photo Visual :: Altair Astro

[dark knight]

what you need is one of these

SLR CAMERAS

2" Direct SLR Camera Adapter

Just screw the correct "T" Adaptor for your camera to the top and you have a direct connection between camera and telescope.

This is how I do it with a Nikon DSLR and Skywatcher 200p Newt

[dark knight]

Here's another suitable adaptor

Altair-Ex Low Profile T-Thread Camera Adapter (2" barrel) :: Adapters - Photo Visual :: Altair Astro

[spaceboy]

Hi Ben

Before you go any further you need to consider coma when using a Newt to image. I over looked this when purchasing my DSLR adapter and now my images now suffer from coma as well as unguided star trails. A coma corrector connects directly to a T2 adapter and if you still struggle to achieve focus after this then a low profile focuser will have to go on Santa's list as with out one your images will never be up to your expectations.

[RikM]

I used to use one of these.

Now I use one of these.