FLT98 with full frame ASI6200MM

[symmetal]

Finally got the FF spacing reasonable. I bought the WO 68III FF as it's designed for full frame and screws directly onto the rear of the focuser (M92 fitting) which should help with tilt as everything in the image train is now screwed together. Zwo filter wheel with 7 Astrodon 2" filters. Skipper Billy may recognize his WO guide scope he sold me. 😊 Previousy it was a dual rig setup with a ZS61 and a Zwo guidescope on top of the FLT98, hence the long carry handle. 😀

Not knowing the correct spacing, as WO seem to ignore scopes which aren't current, I took many test images but it was very critical. Half a mm meant corner stars went from elongated to displaying coma but not all at the same time. Eventually rotating the FF adjustment by about 10 degrees at a time got it close but you couldn't have sharp stars in the centre without poor corners. SGP autofocus goes for best average overall it appears. I tried using a tilt corrector after the flattener but it was no good as as soon as you corrected the tilt a slight alteration to spacing gave worse tilt. Without the tilt corrector the apparent slight tilt was consistant at different spacing which was better. You can't use the tilt correction on the camera as the camera is screwed to the filter wheel by 4 screws and requires removing the filter in front of the camera to access the screws.

CCDI tilt reports are not accurate as stars with some coma are reported as good while slightly elongated stars are bad. The coma looks worse visually. Focusing out by 5 steps gave sharp stars in the centre and apparently corrected the reported tilt but  3 of the corners were much worse.

Here's my current optimum spacing test done yesterday. 4 Ha images of 15 min, gain 0, stacked witout calibration. It's nice having a single CMOS image which doesn't show coarse blotchy noise and looks similar to a single Atik One 6.0 CCD image for the type of noise. Vignetting is slight, and with no amp glow darks may not be necessary, with bias being used instead.

The corners aren't great, but if binned 2x2 in software, as in the second image, they are much better. 😀

Full frame (9576 x 6388)

Binned 2x2

I'll keep making my fibre optic star field project, as hopefully it would have saved using 3 precious dark nights to get the spacing as it is.

Alan

[morimarty]

Hi Alan I have a similar setup as you. Flt98 WO guide scope Atik 460ex and a AFR4 flattener/reducer not as much real estate as yours I know. But I had trouble getting the spacing correct to get a good flatfield, But when I did get it right it wasn't consistant as there was definitely some Flexure in the imaging train. It wasn't until I saw an old post article from Steppenwolf who by the way as a similar setup to us saying how he had tightend up all the little screws on the adjustable mechanism in the flattener/reducer to make sure its flex free. So I did the same and married it up to the flat4 adapter which has a 64mm thread and since then it as been brilliant. So if I was you I would certainly check out the movement of your WO68lll flattener. The Asi 6200 chip is massive so it will be asking quite alot from your flattener. The Flt98 as an image circle of 44mm. So you could get some vignetting. I think I would put the WO guide scope on top of the Flt and lighten the image train as much as possible and take short luminance subs say of 10 seconds to eradicate any guide issues and work from there. Once you get some consistancy you can start adding to the rig again. Hope you get it sorted I will look forward to seeing your images.

here is my rig

 

[symmetal]

29 minutes ago, morimarty said:

Hi Alan I have a similar setup as you. Flt98 WO guide scope Atik 460ex and a AFR4 flattener/reducer not as much real estate as yours I know. But I had trouble getting the spacing correct to get a good flatfield, But when I did get it right it wasn't consistant as there was definitely some Flexure in the imaging train. It wasn't until I saw an old post article from Steppenwolf who by the way as a similar setup to us saying how he had tightend up all the little screws on the adjustable mechanism in the flattener/reducer to make sure its flex free. So I did the same and married it up to the flat4 adapter which has a 64mm thread and since then it as been brilliant. So if I was you I would certainly check out the movement of your WO68lll flattener. The Asi 6200 chip is massive so it will be asking quite alot from your flattener. The Flt98 as an image circle of 44mm. So you could get some vignetting. I think I would put the WO guide scope on top of the Flt and lighten the image train as much as possible and take short luminance subs say of 10 seconds to eradicate any guide issues and work from there. Once you get some consistancy you can start adding to the rig again. Hope you get it sorted I will look forward to seeing your images.

Thanks morimarty. 😀

Previously I had the WO Flat 4 reducer/flattener and an Atik One 6.0 fitted which is the same sensor as the 460ex. The flat 4 was attached using the 2" nosepiece rather than the M92 to M64 adapter plate you have which is better. It had a little tilt though not enough to really matter and I wondered how stable the moveable flattener lenses were. It's a great flattener as you don't have to rotate the camera to adjust the spacing distance. WO seem not to use that method anymore and just use the screw adjustable rear plate on the flattener. I thought the M42 rear mount would cause too much vignetting with the 6200 and filter wheel so splashed out for the WO 68-III flattener with the M92 front and M48 rear mount. It's a x1 flattener which is better with the big sensor as you get plenty large enough FOV (3.3 x 2.2 deg) at native focal length with an image scale of 1.25"/pixel.

The flattener does give slight star distortion when out of focus though, even in the centre as I wondered why I had elongated stars in the centre at one time while stars further out were round and sharp. A tiny focus adjustment will correct the centre but at the expence of the edges where stars go from elongated to coma as you pass through the optimum spacing distance with a mixture of the two if the edges are out of focus as will generally be the case. Where I am at the moment the stars on the right show elongation while those on the left are transitioning from elongation to coma. The centre stars are slightly elongated while those further right are round and sharp so the guiding at 15 min was OK. There is left right tilt but it's small and consistent. Inserting a tilt corrector was a bit of a nightmare as at zero tilt correction it actually gave considerable tilt so every time you adjusted the spacing by a fraction the tilt changed considerably. So I removed the tilt corrector.

If I image a smaller target it's worth manually focusing out a few steps after SGP has done its autofocus, to give round sharp centre stars and accept the poor edges.

I was using 15 sec L images when doing the spacing tests. Normally you do many short subs with CMOS but the read noise at zero gain is 3.5 electrons. There is no unity gain setting as at zero camera gain it is 0.78 e-/ADU. The 10*RN^2 sky background ADU where I am requires around 240 secs with L and I guess around an hour with Ha so longer subs are preferable. It cuts down on the number of 116MB images too. 

It has an ADC processing trick at gain 100 which it claims has almost the same dynamic range as zero gain but with only 1.5e- noise so I'll try that at some stage which means shorter subs. Gain 0 or 100 are the only gain settings recommended with this camera.

My guide scope is now off to one side as the imaging PC box is to the right of the FLT98 and so the guide scope helps balance it. 🙂

Alan

[Jarek]

Hi, im quite new here but noticed setup similar to mine... Im using WO FLT98 with FF68III flattener and ASI1600.

Could you help how to calculate bacfocus for FLT98 and FF68III? Only i have are approx values... 

all best

Jarek

[symmetal]

1 hour ago, Jarek said:

Hi, im quite new here but noticed setup similar to mine... Im using WO FLT98 with FF68III flattener and ASI1600.

Could you help how to calculate bacfocus for FLT98 and FF68III? Only i have are approx values... 

all best

Jarek

Hi Jarek,

I started off with around 68mm like the GT102 but had quite bad elongated corner stars pointing towards the centre so kept adding distance until the corner distortion was minimized. I also had to add a tilt adjuster, as the 6200 is a full frame sensor, and I couldn't get left and right stars in focus at the same time. The final total back focus distance is now 78.5mm, which seems quite a bit longer than for the scopes listed on the WO 68III flattener page, but seems to work.

Your 1600 is a smaller sensor so the flattener spacing is not so critical for you but this may be helpful as a starting point. 🙂

Alan

[david_taurus83]

Great image BTW!

Is that a galaxy in the background? Top left.

 

[symmetal]

8 hours ago, david_taurus83 said:

Great image BTW!

Is that a galaxy in the background? Top left.

 

Thanks David for spotting that. It's actually in the top right. 🙂. I kept enabling the catalogs in Stellarium and it was identified with the PK catalog of Planetary Nebula. It's PK 084+01.1, mag 16.5 😀

Alan

Edited October 6, 2020 by symmetal

[Jarek]

22 hours ago, symmetal said:

Hi Jarek,

I started off with around 68mm like the GT102 but had quite bad elongated corner stars pointing towards the centre so kept adding distance until the corner distortion was minimized. I also had to add a tilt adjuster, as the 6200 is a full frame sensor, and I couldn't get left and right stars in focus at the same time. The final total back focus distance is now 78.5mm, which seems quite a bit longer than for the scopes listed on the WO 68III flattener page, but seems to work.

Your 1600 is a smaller sensor so the flattener spacing is not so critical for you but this may be helpful as a starting point. 🙂

Alan

Thanks a lot! I was going to have ca. 69mm and than get CCDInspector to experiment with. Yes, as 1600 has much smaller chip distance should be not so critical as for 6200. Also im trying to avoid tilt adjuster... 

Clear skies!