Am I doing wrong or is it my mount?

[davy999]

Hi all

I have done some test imaging with the kit listed in my signature (using Atik not SPC cam)

The problem is the pictures are very poor.

I have made a homemade fine focuser, whilst this is not ideal it is much more sensitive that the rubberised knob on the 6SE. I can get perfect focus using an EP but struggle to get good focus with CCD but I am working on that.

The problem I have is that I have read, and been told, that with the mount I have I cannot do long exposures i.e. 20 min subs etc.. and I fully understand that, focal rotation etc.. but when I try imaging the picture on screen is never perfectly still. The mount is level as I have checked with a decent spirit level so I don't think this is at fault. I use manual 2 star align and goto works great, it finds the objects I request, sometimes a slight tweak is needed but they are usually in view of larger EP's straight from GOTO with just slight tweaks to centralise them.

The image seems to move around on screen sometimes in a slight circular motion, this is apparent if I take a long image. When I say long I don't mean 20 mins, anything over about 10 seconds and all the stars appear as small wiggly worms (best way I can describe it) The image isn't doing full rotations whilst i am viewing it just seems to turn slightly one way then the other hence the wormy appearance.

I know the scope / mount isnt ideal for DSO but I had hoped for better results than I am getting.

I can get recognisable images of things like M11 or M15 but only using a max exposure of about 2 secs.

The two attachments show one image of a second or so and one of a much longer exposure, sorry I cannot tell you how long because I was doing so many tests and not writing anything down.

Any help or criticism is very welcome, if you think I should put everything in a box and never open it again, maybe then I wouldnt be too happy

Thanks for any feedback that is forthcoming.

Dave <image consultant to the photography magazine "What the hell is that?">

[jgs001]

Dave, I think you're up against the motor drives, and the long focal length of the scope... the long focal length is far more demanding on the mount... and the drive motors may not be able to track well... It might be balance related... if you too close to getting the balance spot on, then the drives aren't necesarily engaged fully... you want it a little tail heavy

[davy999]

Hi John

So am I right in thinking moving the OTA back a little further in the dovetail? I have it fairly close to fully forward as I thought with the Atik Titan the weight would be too tail heavy, I will give it a go tonight if is clear and gets dark enough for a few shots.

Thanks for the suggestion.

I should note that I have a focal reducer that was delivered today so I will also be experimenting with that. I will definitely try your suggestion tonight and see if there is any change, this time I need to make a note of exposure times and image numbers so I can compare and remember what the heck I am doing )

[rwg]

Hi Dave,

on these mounts, if the balance is too far off the clutch on the altitude axis can slip - this might be what you are seeing in your first image. I usually pick up my scope by the dovetail with all equipment attached to find the balance point, then get that point in the centre of the dovetail jaws. You could also try tightening the altitude axis a bit - instructions on nexstarsite I think.

I've managed to get 30 second exposures using an SLT mount (not as solid as the SE) and a ~1kg SLR body on the back, although this was with a 650mm focal length scope. The focal reducer should help you here.

Robin

[jgs001]

Robin has already said what I was gonna say about finding the balance point... but yeah slightly tail heavy, it keeps the gears engaged.

[davy999]

Do you mean tightening by a spanner or by settings in the controller?

I havent really looked deep into settings other than standard use as I was afraid I would mess it up totally and never see a DSO ever again!

[rwg]

With a spanner You'll have noticed that you can push the OTA up/down by hand and that there is a certain force where it starts to move - you might want to increase that a little.

Here's the instructions... http://www.nexstarsite.com/Book/Updates/N58Altitude.htm

That being said, balance it first - tighten if all else fails!

Robin

[rwg]

Perhaps I should just say that you don't need to strip down the whole axis to adjust the clutch tension - just remove the cover plate and tighten the big nut slightly.

R

[EA2007]

Agree with JGS001.

I had the NexStar 4SE and I had similar issues. Its not that you have a bad scope or a particularly bad mount, its just that the mount isn't equatorial and its not the strongest.

Unless you have the wedge for the mount which I don't think comes as standard (it comes with a built in version on the 4SE and the 8SE I think), then you can rule out long exposure imaging as like you said field rotation appears.

Also the slow focal ratio (think its an f/12 ?) won't help.

I would recommend maybe purchasing a wedge or possibly trying to make one yourself.

By the looks of it your second image is okay. I had a similar shot of M13 through my 4SE and I think the main issue is vibration dampening. The first few images I took, I exposed for 30secs+ (as I was using the wedge) and was walking around and had trails like in your first shot. The latter shots I took, I stayed as still as possible and the image wasn't trailing. Likewise, you can probably only get away with a few seconds unless you have the wedge.

Keep persevering with it though. maybe go for some widefield images instead by removing the OTA and attaching the camera onto the mount somehow. You should be able to track for long enough to get an okay image.

[davy999]

I will try balancing first as the scope is quite new and I don't wanna chance messing up my warranty at the moment.

If all else fails.. out comes the spanner )

[davy999]

I see the wedge is discontinued and is an expense I cannot really afford as I have only just bought the scope camera etc... so unless a cheap one appears on ebay then that aint an option at the moment.

When I start imaging the laptop is just inside the flat, scope is outside. whilst imaging i am sat very still in my chair so I dont think I am causing any particular vibrations myself.

Will see how I get on tonight! )

[jgs001]

The nut in the alt axis really doesn't need much, my slt needed less than a quarter turn. But if your close to balance I'd think you'd be ok.

[davy999]

Ok

I think the focal reducer has made a big difference to the images, exposures are still short but i can see more structures in objects like M27 than i could before. Maybe the moving back the OTA also helped as it seemed a little more stable.