Best flattener for William Optics Megrez 72?

[mint_mark]

Hi all,

I have a William Optics megrez 72 that I use with my astrotrac for imaging (with a CLS filter). It's my first scope and at the same time I bought the William Optics flattener III since that was the one recommended for the scope (on the William Optics web site).

However, I am becoming disenchanted with it because the best stars I can get in the corners (of an APS-C sized sensor) are like this

What can I do to improve the situation? As I focus the shape of the stars changes, but they are never round. One obvious thing I can try is not using the flattener... never actually tried that but I guess it would be worse.

Does anyone know of a better flattener? Maybe this Skywatcher one? The scope is 430mm and f6.

Reducers/Flatteners - Skywatcher Field Flattener

Will it be better or the same? Maybe that's just the way things are at this price level...

Any suggesstions greatly appreciated...

Mark

[Whippy]

I'd initially go with your suggestion of trying to image without the flattener or alternatively double check the spacing between the flattener and your chip. A lot of flatteners have to be spaced pretty accurately (between 1-2mm) for them to work properly.

Tony..

[mint_mark]

I'd initially go with your suggestion of trying to image without the flattener or alternatively double check the spacing between the flattener and your chip. A lot of flatteners have to be spaced pretty accurately (between 1-2mm) for them to work properly.

Tony..

I thought someone might suggest that... the flattener attaches to the camera adapter with a T thread, so I could put something between to stop it screwing together so much... that would increase the distance a little. I can't see a way to decrease the distance.

It could be clear tonight, so maybe I'll give it a try.

[acme]

Mark,

I have a Megrez 72 FD and I use the Skywatcher field flattener, I don't have the same issues you describe. I use mine with a 1000d and it seems to be quite flat with no bloated or elongated stars arround the edges.

I use the Astronomik CLS CCD clip filter also...

Acme

[Whippy]

Just looking here: William Optics - Flattener III, The suggested distance is 56mm between flattener and chip.

[mint_mark]

Mark,

I have a Megrez 72 FD and I use the Skywatcher field flattener, I don't have the same issues you describe. I use mine with a 1000d and it seems to be quite flat with no bloated or elongated stars arround the edges.

I use the Astronomik CLS CCD clip filter also...

Acme

Thanks Acme, that gives me some confidence that another combination will work. It sounds like we have very similar setups

Maybe I'll order one of those... do you think they'll take the old one in part exchange?

[acme]

I doubt it Mark, hold on to it until you try the skywatcher

then if your happy with it you can sell the WO one.

Acme

[mint_mark]

Just looking here: William Optics - Flattener III, The suggested distance is 56mm between flattener and chip.

I'm pretty sure I found that page the last time I tried to fix this... I just tried measuring from the sensor plane mark on the camera forward by 56mm... it's hard to tell precisely but it looks bang on to me.

Anyway, there is nothing for me to adjust. Is there a standard depth for T-adapters or is it just down to luck?

I'm going to try with some bits of wooden drink stirrer between the flattener and the T-adapter. This will increase the distance by about 1mm. If that makes it worse then I'll try the Skywatcher flattener.

[mint_mark]

Alright then, here are the results of some more testing. First up we have a similar part of the sky taken with the flattener III and the improvised pieces of drink stirring stick introducing an extra ~1mm between the flattener and the sensor.

I think the focus was slightly better this time (the stars are less tangentially elongated) but the flattener still gives me its classic "swimming manta ray" shape.

Next up we have a different part of the sky with no flattener, so a longer focal length (since the flattener also reduces by 0.8).

This is the bottom left corner and in general there is a "warp speed" effect around the edges. Some edges look weird too with stars elongated in different directions quite near each other. I checked the centre stars to make sure it wasn't trailing and they were OK. Focus wasn't brilliant through high cloud last night, so not the best test.

So my conclusions for now are

• With the flattener is easily better than without the flattener. I haven't been missing anything.
  
• The corner star shape is very sensitive to focus.
  
• Changing the distance to the sensor by 1mm didn't make any difference. There is little scope for adjusting it further.
  
• I can try the Skywatcher flattener that Acme uses. I will lose the 0.8 reduction, but I think that will be OK.
  

Thanks for listening.

[Psychobilly]

The FFIII on the Megrez 72 seems to be extremely focus critical... as some night I get a lot better stars than others without changing anything else...

Peter...

[mint_mark]

OK, I just wanted to complete the tests in this thread... I did buy the skywatcher flattener and had a chance to try it out. Here is the result...

This is the top left corner of a 3 minute exposure at ISO 1600 through an Astronomik CLS filter.

I think this is a big improvement on the WO flattener III. It's not perfect, but at least the stars are oval and not cross shaped This flattener doesn't reduce, so I am at f/6 and have to expose for longer, but I seem to have managed 3 minutes which I'm also pretty pleased with.

Thanks everyone for your suggestions and advice... my next upgrade might be a new camera with a better LCD, so I hope to improve my focus accuracy.

Mark