My imaging nightmare - time to throw in the towel?

[lukebl]

Hi,

Can anyone shed any light on my guiding errors.

This is a highly-enlarged animated section from 30 x 1 minute frames. You can see that the image moves around all over the place during the exposures. It was taken on an SXVF-H9 camera, on a 250mm f/4.7 Newtonian, guided using PHD with an spc900 webcam attached to an ST80 guidescope. I am having real issues getting a decent image with my newly-acquired SXVF-H9 for various reasons, and am thinking that it's a darn expensive white elephant. Almost thinking of throwing in the towel. My NEQ6 also seems to playing up, and becoming increasingly noisy and clattery when slewing, and I wonder if that's part of the problem. I don't think flexure is the problem, as I've made sure everything is tight. I also made sure the balance is good, just heavy to the east in RA.

Could any PHD settings be adjusted to improve things?

here's the full frame. A 30-minute animation of Comet Garradd from last night:

[blinky]

Why not try and barlow up the ST80 to increase the focal lenght? I used to do that when imaging at 2m focal length.

[lukebl]

Why not try and barlow up the ST80 to increase the focal lenght? .....

Sounds good. That'll definitely be better, you think? I used to have a 90mm Mak with a focal length of 1250, and I tried guiding with that but I recall that it also had problems.

[foundaplanet]

Problems or not that is still one fine mini movie..

I hope you manage to solve the troubles but don't "throw the towel in".

Col

[lukebl]

Thanks gents. I'm probably not ready to throw in the towel just yet! Will have a go at barlowing the guidescope next clear night and see if that works. In the meantime, here's a reprocessed couple of images from the session.

Stacked on the comet:

and stacked on the stars:

On the original of the lower image I've managed to capture some galaxies down to at least 19th magnitude. Which was nice, for 1 minute exposures.

[Earl]

Have you eliminated stacking software issues?

[Carbon Brush]

You have got (I think) a really good image of the comet there.

If I got something that good I would be grinning. Not picking up a towel.

OK the scope isn't tracking 100% perfect for whatever reason(s). But look at what you are getting. Wow!

[Starlight 1]

I put it down to the mount have a look at the gears.

[lukebl]

Thanks all.

Have you eliminated stacking software issues?

I'm certain that it's not the stacking software. The individual frames just aren't right, with bloated and sometimes misshapen stars. I seem to have tracking issues and/or optical problems.

I put it down to the mount have a look at the gears.

You may be right, but I have no idea where to start with that one. The mount is only a year old, and hasn't been dropped or damaged in any way. How do I check the gears, or fix them if they're not right?

[Starlight 1]

I look like a gear is sliping ,I have not taking one apart but your may be my first one.

[Starlight 1]

You can just try first taking the scope and the weights off and moving the mount in both directions in a full circle just to check you haven't got a tight spot, if not it will be the gear looking at the video footage.

[MikeWilson]

Let's take a look at your PHD settings please sir!

[MikeWilson]

And tracking graph. Save an output for PECPrep.

Mike

[Whippy]

IMO, you're not making it easy for yourself at all. A 10" newt on an EQ6 guided by a webcam sounds like a receipe in frustration to me.

I would look into getting a smaller, shorter f/l imaging scope and a better camera for guiding, I'd wager a lot of your issues would disappear.

Tony..

[Glider]

So you are guiding on the comet?

Is it saturated?

[lukebl]

Thanks for the input, chaps.

As for being overambitious using a 10" Newt, well I've used it for the past year with my DSLR and it's worked really well. It's only since I started using a CCD/filter wheel that I started to get problems. I know it's not everyone's preference, but I primarily like the image scale of the 1200mm FL for capturing smaller objects which, as I say, has worked well for me in the past so I know it can be done. And if Yfronto can do it with similar kit, then so can I!

Guiding was on a nearby star, not the comet (the guide cam isn't sensitive enough to pick out the comet).

I'll have a go with PECPrep next session.

I wonder if it might be excessive Dec backlash. I read a thread on another forum where someone had a lot of backlash, so they increased the Max Dec Duration in PHD to 1500ms. Mine is only set to 150. Here are my current settings:

Not being particularly brave when it comes to DIY mechanics, I don't really want to take the mount apart, and anyway it's only a year old. Does anyone offer a maintenance service for NEQ6 mounts?

Of course, half the problem is that there are so few windows of opportunity for imaging at this time of year, that I don't have the time to investigate and eliminate all the potential gremlins.

[Starlight 1]

You can try going back to when it was ok then you know if it is the mount or not,

then add ccd and so on .

[Cjg]

Luke,

Just a thought, but why not visit the Norwich Astro Society one Friday? Some hardcore image'rs there who am sure would love to chat with you and may be able to assist?

Chris

[markarian]

Just a though: try guiding with Guidemaster instead. I had similar problems guiding my EQ6 with PHD. Tried Guidemaster and they just went away. I still can't get PHD to work for me but Guidemaster works all the time for me. Gives more immediate feedback on the quality of the guiding too I think.

Mark

[Uranium235]

If you had excessive dec backlash I think you would know about it becuase your graph would be sawtoothing quite badly. How does your dec graph look? Flat, oscillating or sawtooth? Ive had occasions where the graph has been flat, but the stars are still eggy - at which point I normally PA again and double check the focus.

Your PHD settings look ok, although my dec duration is never less than 200ms. Do you ever nudge your mount North before PHD calibration? Give it a try, it can make a big difference

[lukebl]

Hi all again.

Still having real problems getting a decent image with the old SXVF-H9. I am getting stars that aren't round, and I know that it's definitely not the guiding as it occurs even with short exposures. here's an extract from a short exposure:

Now, you would assume that there is something wrong with the collimation, so I put the stars out of focus to check on the airy discs, and this is what I could see:

Leaving aside the bad seeing, that looks OK as far as collimation goes, doesn't it? Someone please correct me if I'm wrong.

So if collimation is OK, then why do the stars go oval-shaped when in focus? Someone please help.

[blinky]

I dont think thats too bad, is the star sample you showed from the centre of the image? Do you use a coma corrector? Could be a spacing issue of the ccd is not square on the focuser.

[lukebl]

Thanks Blinky. Yes I do have a coma corrector, and I think I've cracked it at last. I went back to basics and took some images without either the coma corrector or the filter wheel and, bingo, the stars in the centre are nice and sharp and ROUND. So the problem is the coma corrector, or the distance from it to the sensor. I reduced the spacing by removing a 7mm spacer on the coma corrector, and that was better although not perfect. But, what the heck I thought, at least I've found the source of the problem!

[whirlpool5194]

You should have a distance of 55mm +/- 1 mm from the chip to the corrector so a few mm might not seem much but it really is that critical...glad your getting there though..

Wp

[lukebl]

You should have a distance of 55mm +/- 1 mm from the chip to the corrector so a few mm might not seem much but it really is that critical...glad your getting there though..

Wp

Yes, I knew about that distance, but I'm now wondering from where that distance should be measured. I had assumed that it was from the lip of the coma corrector as shown in the attached image, and that distance was exactly 55mm, but am I measuring from the wrong point?