Image error - any ideas?

[kosh]

Hi all,
Just having a go with the new 8" f4 newt last night at the flame horsehead region, and I have got this weird streak opposite of alnitak.
It almost looks like a reflection of its diffraction spikes. Also, I have this strange shape gradient on the bottom of the image, which also appears on another image (of the tarantula neb ) which is completely different part of the sky, but same place in the image. I'm not sure if they are the same thing or not.

The flame region is a stack of 11 x 180sec iso1600 on a 450D.
The Tarantula is a single image of 60secs at iso1600.
I still don't have the coma corrector either.

These problems may be common, but if someone can tell me what they might be I would love to hear it.

Thanks.

[cometcatcher]

The streak is from Alnitak. It's brighter on one side for 2 reasons, 1 it's better exposed on the right and 2 Alnitak is off center making it elongate aberrations to the other side.

The gradient is most likely unwanted light entering the front end tube of the newt. Adding a cardboard tube extended from the front like a dew shield should help.

[kosh]

Thanks kevin.
In regards to the streak, is there anything I can do about that? Is it exposed more on one side due to the position in the frame or the collimation or something else?
Ill make a dew shiels up and see if that helps.
I'm just worried that there is no hardware defect with the new ota. Hopefully its all normal.
Im new to newts :)

[cometcatcher]

I think perhaps the coma corrector will shorten that off axis streak once it's installed. You'll still have 4 diffraction spikes, but of more reasonable length.

[kosh]

Thanks kevin.
I don't actually mind the spikes, just not when they mysteriously appear on the other
Side of the image. :)

[cometcatcher]

I can't guarantee it won't happen with the coma corrector. Alnitak is a very bright star in an otherwise dim field and as such hard to tame, but hopefully the corrector will help.

I have a bunch of newts but none this fast so may end up getting one myself yet at some stage.

[doppler]

Hi kosh, I am having a similar problem with my 10" f 4.5 newt. Visually the collimation looks good but as soon as I put a camera on the difraction spikes on the stars are off center and the stars on one side of the image are elongated but round at the center of the image. When the weather clears I am going to try and collimate the scope using the camera in liveveiw at 10x magnification. I have an 8" f6 and have never had any problems collimating it. maybe fast scopes are really that much harder to get right.
Cheers Rick

[raymo]

Sounds more to me like the weight of the camera is is causing the focuser to sag, so that the camera's sensor is not perpendicular to the light path.
Just a possibility.
raymo

[doppler]

That is distinct possibility in my case, my scope is an older meade starfinder and the focuser is not the best. I will try and tighten it a bit and see what happens.

Cheers Rick

[kosh]

Hi Rick,
I have collimated the scope pretty well ( I think ). I used a laser collimator which I have checked in a home-made jig and it is spot on. I don't know how much more precise I can be unfortunately. I am pretty sure that the elongated stars are due to the coma, pretty severe at the faster end such as f4. Im also waiting for the weather to clear so I can take another shot of this complex but with Alnitak in the centre, I am hoping that the spikes etc will be more uniform. Just freak me out to see a diffraction spike all the way across the image!



Well i'd never even thought about that, thanks ray. Come to think of it, I did loosen the camera up so that I could rotate it for framing purposes. Perhaps I didn't tighten it properly. How much "sag" would it take to make a difference? 1mm, less?

[raymo]

Sorry I can't help with the amount of sag, having never measured it;
but I'd imagine that you wouldn't need much for it to become noticeable.
Of course, it may not be that at all, it just seemed to be a logical possibility. I hope it is, because at least it wouldn't be too hard to fix.
raymo