Vignetting - do I need larger filters?

[Jonk]

I have a Moravian G2-8300 with internal filter wheel, and screw in 1.25" filters.

The Moravian blurb says to use 31mm unmounted filters.

Simple question - by replacing the 1.25" screw in filters with 31mm unmounted as per the manufacturer such as these-> http://www.teleskop-express.de/shop/product_info.php/language/en/info/p8749_Baader-LRGB-CCD-Filter-Set-31mm-round---unmounted.html will I get rid of this vignetting, slightly reduce it, or will it be a waste of money?

It's a Star 71 II scope and I'm hoping someone can tell me if flats are sufficient for this amount of vignetting.

The reason I ask is because it looks fairly 'defined' to me.

Image is a 5 minute stretched lum of IC405.

[D4N]

I guess the test would be to take a flat with a 1.25" luminance or clear glass filter then another flat with no filter at all and see how it affects the vignetting.

[Jonk]

I didn't think of that, duh! I guess that would be a good idea, but that involves opening up the camera, which I can't do at the moment as it's busy!

That should show a difference though.

[Jonk]

Here's a 10 min 2x2 ha sub and the vignetting is there but not obvious.

[Horwig]

28 minutes ago, Jonk said:

Here's a 10 min 2x2 ha sub and the vignetting is there but not obvious.

 

Not much wrong with that, how much was the image stretched?

[Jonk]

Both were autostretched in PI just to show.

[Davey-T]

The actual size difference is very minimal Jon, doubt it would make any difference.

Good to see some images from the new scope, how are the stars in the corners ?

Dave

[Horwig]

20 minutes ago, Jonk said:

Both were autostretched in PI just to show.

In which case, I'd say that flats would cope with no problems

 

Huw

[swag72]

I use 1.25" filters in a Moravian 8300..... flats sort any issues

[Jonk]

I still have a slight guiding problem I think, but that could be the limit of the mount, Hopefully it'll be better as an when the pier arrives!

Here's the corners of a 5 min lum sub taken about an hour ago.

It's also manually focused, and it's very fiddly to do (I use a B mask) and once I've fitted an autofocuser, I'm hoping this will improve.

I've just bought SGP and going to order a Sharpsky autofocuser, as I've given up on the arduino.

[Jonk]

6 minutes ago, Horwig said:

In which case, I'd say that flats would cope with no problems

 

Huw

 

3 minutes ago, swag72 said:

I use 1.25" filters in a Moravian 8300..... flats sort any issues

Thank you both, hopefully I'll see this once I start to process these.

Sara - here's an ha flat.

The black mark in the top right corner has to be the filter mounting?

Is there any way to 'calibrate' the wheel position do you know, as it looks like it senses the positions using photo diodes or something?

[Xplode]

11 hours ago, Jonk said:

I still have a slight guiding problem I think, but that could be the limit of the mount, Hopefully it'll be better as an when the pier arrives!

Here's the corners of a 5 min lum sub taken about an hour ago.

It's also manually focused, and it's very fiddly to do (I use a B mask) and once I've fitted an autofocuser, I'm hoping this will improve.

I've just bought SGP and going to order a Sharpsky autofocuser, as I've given up on the arduino.

Looks like misscollimation, this seems to be a big problem with these scopes

[Jonk]

This is a new, tuned Star 71 II, so unlikely I would think.

I must be on the edge of the critical focus zone, hopefully an autofocuser and SGP routine will get it in the middle.

Doing it by hand isn't easy.

It could also be tilt I guess?

[Xplode]

Yes tilt will show up the same way as misscollimation.

Do you use connections that are all screwed?

Could be that the focuser just isn't stiff enough to hold the camera.

[Jonk]

Yes everything is screwed in M48, the tilt adjuster is fully home (I haven't got to adjusting this yet) and the focuser is only out by 14mm or so.

The vignetting question I think has been answered, so the autofocus / tilt is the next thing to get right.

Hard work this eh?!

[Jonk]

I did adjust the tilt to within 0.1 on ccd inspector in x and y the other day and the difference was clear in the next few test subs. Very pleased ir sorted the problem mentioned above.

Now my new sharp sky kit has arrived, I'll be getting that working and hopefully getting repeatable results.

This new Star 71 II is certainly looking good. Question is do I keep the MK1 that I also have and go dual rig?!

[swag72]

10 hours ago, Jonk said:

.............. Question is do I keep the MK1 that I also have and go dual rig?!

definitely  

[gnomus]

11 hours ago, Jonk said:

I did adjust the tilt to within 0.1 on ccd inspector in x and y the other day and the difference was clear in the next few test subs. Very pleased ir sorted the problem mentioned above.

Now my new sharp sky kit has arrived, I'll be getting that working and hopefully getting repeatable results.

This new Star 71 II is certainly looking good. Question is do I keep the MK1 that I also have and go dual rig?!

I'm close to pulling the trigger on one of these Mark IIs.  It would be helpful to see one or two of those test subs just to confirm my decision.  Any chance?  Also, how awkward is it to adjust tilt?  I have absolutely no idea how to do this.  I would be concerned I would make things worse!  Could you give a brief account of the process?

Steve

[Jonk]

9 hours ago, gnomus said:

I'm close to pulling the trigger on one of these Mark IIs.  It would be helpful to see one or two of those test subs just to confirm my decision.  Any chance?  Also, how awkward is it to adjust tilt?  I have absolutely no idea how to do this.  I would be concerned I would make things worse!  Could you give a brief account of the process?

Steve

I'd have to dig them out if I still have them, I usually delete test shots after they've served their purpose.

For what it's worth, I am very happy with mine.

I had to adjust the focuser as it was sticky in places and now it's really smooth with no slop that I can feel. Quite easy to do once you pull it apart to see how it works.

I've just today, fitted a sharpsky focus kit too, so hopefully it will work well with SGP autofocus. It works well manually so far.

In terms of tilt adjustment, it is really fiddly but like anything, it gets easier the more you do it. I couldn't find any specific instructions, but its basically a push-pull method like a mirror collimation technique.

You will probably need a bahtinov mask or ccd inspector software to really appreciate the fine adjustments needed, but it certainly cured my slightly elongated stars in 1 corner.

When I get the scope back together with the camera, I'll see if I can remember to do a write up of how I did it.

 

[Jonk]

10 hours ago, swag72 said:

definitely  

I would really like to get another G2-8300 but it's the £s that I don't want to spend at the moment!

[swag72]

Go on ..... a dual rig with identical combo's makes ALOT of sense.....  

[Jonk]

It certainly does, but then the prices are astronomical (pun intended!).

It's taken me a long time to save the cash to buy what I have now, so maybe in another year or 2...

[Davey-T]

18 hours ago, Jonk said:

In terms of tilt adjustment, it is really fiddly but like anything, it gets easier the more you do it. I couldn't find any specific instructions, but its basically a push-pull method like a mirror collimation technique.

 

Hi Jon, was this a tested scope from FLO ? if so there doesn't seem much point in paying the extra if you have to adjust it yourself.

Dave

[Jonk]

Yes, but only the lenses.

The focuser and tilt adjuster are there to be tweaked by the user, depending on the camera.

One thing I did notice is that the rotator is on the wrong side of the adjuster (strange design choice), so to rotate a frame, the whole scope would need to be rotated in the rings, otherwise the tilt would need to be adjusted again to suit if the rotator was used. I just leave it tight now.

I made this mistake last time and now need to adjust it again.

The tilt adjuster has nothing to do with the lens alignment, just the ccd chip to the focal plane.

[ollypenrice]

Briefly back on flats, a stretched flat tells you next to nothing at all because the purpose of stretching is to emphasize contrast. What you need to do to analyse a flat is mouse over it in a suitable programme (I use Astro Art but I suspect most stacking programmes will allow this) and read the ADU (brightness) values in the corners as compared with the centre. My most vignetted rig shows about 25% fall-off in the corners and corrects happily with flats. It would be nice to have less vignetting but 'c'est la vie.' Half of our tandem uses 2 inch unmounted and the other half 2 inch mounted. I find the difference to be trivial but have often seen it stated that it's considerable. Not convinced.

You do have quite a bite out of the top right corner, though, and this could be filter wheel positioning. Simply looking down the tube can be very instructive since the filters and chip can be seen.

Olly