Getting started is hard going.....

[eddie3000]

Well yes. It is. To start with, I don't have enough time to play with my telescope. And when I get the chance to go out, I simply make mistakes and encounter problems, and to learn from my mistakes I have to wait for the next time I get to play with my telescope, typically months apart.

I have a problem I don't know how to solve, though I think I know what the cause is.

When I attach my dslr to the focuser, I have the impression that the weight of the dslr makes the focuser flex slightly. I end up with a perfectly in focus area, and other out of focus areas. I can refocus on the out of focus areas, and the lose focus in the first area.

I have a Skywatcher mn190, which is supposed to have a flat field right? Shouldn't I be able to get focus all across the frame? Maybe I'm missing some theory?

If the focuser is the problem, apart from getting a new focuser, is a workaround to this problem? Thanks.

Besides this problem, last time I went out I didn't get enough exposures of m31. Can I just take more exposures any other day and combine them? Is it a good idea to get previous exposures from a different camera and combine them? What would the aprox. necessary total exposure time be for M31 to get a decent image? And hour?

[brantuk]

Sounds to me like you need a field flatenner. I wouldn't know which one though for your scope. There aren't many refractors that don't need a flatenner.

[eddie3000]

I thought that mak-newt scopes had a flat field already. Or maybe it doesn't provide enough flattening and needs more?

I really don't know, but still think the problem might be the focuser because the out of focus area seems to be in line with gravity. If the camera is set up vertically, as in like a portrait steup, the out of focus appears on top and on the bottom of the frame, but not on the sides. If i put the camera as in a landscape position, the out of focus is less evident ('cause it's cropped out), but it's still there. I'll try and post some pics later this evening.

[eddie3000]

Here is a pic of what I get, hopefully it'll help with the veredict. So you reckon I need a field flattener? It isn't the flexing of the focuser?

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Uploaded with ImageShack.us

[eddie3000]

Sorry to insist, but does anybody know whether this is a non-flat field issue or what?

[brantuk]

Hi Eddie - sorry mate I thought you were using a refractor (missed the mn190 bit) - with Mak-Newts I honestly haven't had experience. I'll see if I can find someone that can help you

[eddie3000]

Thanks. I guess many people are out on holiday now, so maybe not too many people around.

I probably haven't posted in the right forum, and I'm sorry the title is misleading too.

I'll do better in the future.

[brantuk]

No probs Eddie - you're in the right place - I've got a mate who had an MN190 - not sure if he's online but he will answer here eventually

[Deneb]

The MN190 does not need field flattener, as you know getting good collimation on a Mak Newt is critical. I would suggest make sure the focuser is actually screwed on tight, as I found on the MN190 I had the focuser screws were not fastened properly on the ota.

Looking at the stars it does seem they are out of focus as well.

How have mounted the scope on the Tube rings, I mean what side is the focuser on?

A pic would be nice...

Nadeem.

[ollypenrice]

I haven't used a 190 but let's look at the issues in the image. On the left the stars are way out of focus and show the shadow of the secondary, I'd say. On the right focus is better but there is some distortion whch varies top to bottom. At the top, elongation runs NE to SW, so to speak, and at the bottom NW to SE.

I'm not an optics expert but I suspect there may be more than one defect at work here. Maybe not.

On the focus issue I think you most probably have droop in the focuser and/or distortion in the tube causing the chip to be at an angle to the light cone. Many 190 owners stiffen the tube up by adding a dovetail across the top and using long dovetails to set the tube rings further apart. Or so I believe. As Nadeem says, check the focuser itself, too. Is it tight and is the drawtube adjustment tight? There are usually little friction pads which can be tensioned but I don't know the focuser specifically.

Exposure time? I used 5 minute subs with a cooled CCD and took a total of 11 hours, half at F5.4 and half at F3.9. It started to look sound at around four hours though.

Olly

[eddie3000]

I don't have a photo of my setup from the other day. This is a photo I posted on another thread. The dslr would be mounted straight on the focuser you can see on the right, and it would be pointing at M31 instead of the moon, which would be slightly more towards the right hand side of the photo. As you can see from the tube rings, the mount is at the right. It has just occurred to me that the problem could be a misaligned secondary mirror. Is that possible? Being a newbie I have never aligned or collimated my ota yet.

Olly! Wow! 11 hours! My mount can only really keep up with 10-15sec exposures without any wind!

[Deneb]

Turn your tube so that the focuser is facing the counter weight bar, as when you image the balance of imaging gear will be lot better.

You need to collimate the scope, thats essential..

Nadeem.

[eddie3000]

Ok! I'll do the collimation. Never done it before, but read a lot about it. I'll have to overcome my fears.

Putting the dslr as you suggest makes sense as long as the object is fairly near the horizon, but how do I look through the view finder in order to focus? The mount, telescope and the dslr together are taller than me. I s'pose I'll have to take a small ladder or a stool.

There are many different issues in here as Olly pointed out. I guess I'll have to address them one at a time. Given all the spare time I have (sarcastic hahaha:D) I'll practice doing the collimation process some time soon, and when I get that right, I'll see what else is going wrong.

One more thing, I have read that collimation is best done in the dark. Is it too tricky to do it during the day?

Thanks for your help folks.

[ollypenrice]

You can collimate in the daytime. Cheshire or Laser will work. However the change in temperature may mean it will be out again by the time you want to image.

You absolutely must master collimation if you use a fast reflector.

You would be much better using your camera through a PC. Yopu need to see what you are doing. Do you have a Bahtinov mask? If not, get spending!! (Or make one.) You can't focus by looking at stellar images and thinking they look OK. It isn't like that, imaging focus being hyper-critical..

Olly

[Deneb]

Something that will help you regarding collimation:

Collimating a Newtonian

Nadeem.

[MartinB]

I agree with collimation being important but the principle problem shown in the M31 image is due the DSLR's chip not being square to the focal plane. Far and away the most likey cause, as has been mentioned, is sag in the imaging train or the camera being misaligned within the drawer tube.

I have an MN190 with the original, single focus knob, Skywatcher Crayford. It is a bit agricultural but it handles much heavier loads than your DSLR without a problem. I suspect that the T adaptor isn't fitting snuggly in the drawer tube for some reason. Have another look and satisfy your self that everything seems a good fit, not all T adaptors mate well with all drawer tubes. If in doubt the best bet is to get a Hotech 2" T adaptor First Light Optics - Hotech 2" SCA T-Adapter

They aren't cheap but your pics are only as good as the weakest link.

The worst case scenario for you is that the drawer tube itself isn't square. You can check this using a laser collimator to collimate and then rotating the collimator in the draw tube and checking collimation again. If the scope appears to not be collimated, it will be down to the draw tube not being square. I get a small variation with my MN190 and hotech laser collimator but analysis of my images using CCDInspector software still shows pretty tight collimation. The message is not to get too bogged down in the details, it's easy to suffer paralysis by analysis in this hobby!

You could always do some daylight testing using a distant brick wall or something similar.

[ollypenrice]

You can't beat direct experience and Martin has it. Looks promising to me.

Olly

[eddie3000]

Thank you Martin for your experience. My mn190 has the new focuser, I don't know how it stands up to the previous one regarding sturdiness.

I'll take a look at how my t-mount and 2inch adapter fit together with the focuser tube. The 2 inch adapter I use is the one that came along with the telescope.

Nevertheless, I will collimate the scope anyway because I've had it for about a year now and it hasn't been touched. I'll check all that has been said here. I'm afraid I won't have time until maybe next week. This is my first time doing this fairly delicate operation, so I think I should find the time and do it calmly. I will get a collimator to help me in the process.

Thank you all for you time and advice. My beautiful images of m31 are now a step closer to reality.