Canon DSLR and Tunnel Effect?

[MushroomBill]

Hi All

It was nice to get out last night and have the chance to take a few pictures. Attached is one of my pics but over-brightened to demonstrate the tunnel effect that I get.

I suspect it is something to do with the way I connect my camera to the telescope. On this occasion the Canon 500d was connected with a T-ring to 1.25" nosepiece and then into my SW ST102.

I've noticed it to a lesser extent when using a T-ring into the Celestron T adapter onto the back of my SCT.

Is it just because the connector shape allows more light to the centre of the sensor than the edges?

Probably of more use then...

1. how do you stop it? Use a different connection method - 2"?

2. can it be post-processed out easily? How?

Many thanks

Bill

[Photosbykev]

It's vignetting in the optical path, the best way to remove it is to take flats when you take the original subs. Most astro biased software will combine the subs and flats to remove the gradient. There is section on how to obtain the flats here Jim Solomon's Astrophotography Cookbook but a google search will show up a lot of other sites with similar information

[Psychobilly]

Does the focuser on your ST102 support "direct connect" ?

A lot of the more recent SW ones do. This allows you to connect the DSLR to the focuser using just a T ring , this will give the maximum claer aperture to the Cameras sensor and should reduce the vignetting.

You will almost certainly still need flats but there should be a noticeable improvement...

Peter...

[studdedsole]

GradientXterminator will help you process the vignetting out until you solve the problem fully, or if you are only able to minimise it, is a good ongoing solution.

GradientXTerminator Download

First 30days are free if you want to go to the trouble of trying it, use the tutorial as a guide to the application which is short but effective.

Tony

[philj]

From experience I was doing the same with a canon 300d and an ST120 when I 1st started taking pics through a scope I got similar results. Trying to flat frame extract will work ish but you will loose alot of data because the vignetting is so bad. If you can do what Pete says and still achieve focus then thats the way to go. I couldnt reach focus that way because an extention tube was required so I went the 2" camera adapter route which sorted it.

Philj

[MushroomBill]

Thanks for your replies.

I took some darks at the time and have reprocessed my image with them - its better but the effect is still noticeable.

Kev - thanks for the guide - useful stuff.

Tony - I'll look into GradientXterminator.

Peter - yes it does allow the direct connection of the T ring. I'll try this way asap.

Phil - I've got a 2" nosepiece for the canon too - I'll give that a whirl too.

I think I shot myself in the foot with the 1.25" nosepiece but I did it to try using a 1.25" filter on the end. It obviously didn't help that much!

Many thanks again, your time is much appreciated.

[Keiran]

As said before, the easiest way to over come this is to dp flats.... You can either make a light box (many on SGL have) or buy a flat panel light screen like this First Light Optics - Gerd Neumann Aurora Flat-Field Panels

to help reduce it, you need to connect straight to T thread or a 2" nose.

other than that the image is looking good, you will notice a huge differance when you image without the moon too

[Psychobilly]

If you need an extension then checkout astroboot for a T extension they can usually be found for very little money... and if you buty 3 items P&P is free.... I usually make up the order with the cheapest things I can find...

AstroBoot

Watch out though the "boot" can become addictive...

Peter...

[MushroomBill]

Peter - I'm already guilty of the boot addiction!

I've already got some extension tubes that I bought thinking I might need them one day. They've been useful with the DSI and focusing through the 102ST already.

The real problem with the boot is that there are so many things that might be useful that it seems a shame to ignore them!

For the flats I direct the scope towards an outside light and cover with a white cloth. Fortunately there are no other astronomers in the local area that this light pollution will affect - although it is a small lantern not a halogen security light. I figured this would be similar to the light box.

Thanks again.