Moved to a bigger Sensor - Eggy Stars, advice sought!

[CKemu]

 

Finally got first light on my ATIK490EX (See above!) I recently purchased the camera and the EQ6R-Pro, and honestly super happy with the results, despite clouds rolling in, 27mph gusts of wind, I managed to calibrate the mount, get it guiding and take this single exposure of NGC 6888 in H-Alpha (20 min).

So did my usual pan around the image to check on the stars/focus etc and noticed that in each corner of the image, my stars are...well eggs.

I wondered if I would run in to this issue as I've gone from my 414EX to this camera which has a bigger sensor.

Now is this going to be fixed by simply buying the Skywatcher 100ED field flattener, or am I going to have to worry about spacing, my optical train is currently:

Skywatcher Esprit 100ED > ATIK EFW2 (Filter) > 490EX

If I am going to have to consider spacing, any advice will be greatly received! I've been out of the imaging loop for a year now thanks to work, weather and my 414ex developing faults.

[symmetal]

As well as being 1.4x larger across, the new sensor has pixels around half the size so the field curvature effect of the focused image will be much more apparent with the new sensor.

This will be fixed by buying the field flattener, but this means you do have to worry about spacing but it's fairly straightforwand. 🙂

You will need 55mm between the rear mounting face of the FF and the sensor of the 490EX.

Atik 490EX front plate to sensor = 13mm +/- 0.5mm

EFW2 thickness = 22mm

So the extra spacing required between the rear of the FF and the filter wheel is 55 - 13 - 22 = 20mm

The filter glass thickness does effect this spacing though and you need to add 1/3 the filter thickness to the original 55mm quoted for the FF. For 2mm thick filters this is around 0.7mm making 55.7mm spacing required. Therefore your 20mm spacing mentioned above should be nearer 20.7mm.

However, as your new sensor isn't that large compared to say an APS-C size sensor the FF spacing isn't so critical so you can probably forget about the extra 0.7mm and the 20mm extra spacing should be fine.

The FF has an M48 rear thread and I assume your EFW2 is M42 so the thickness of any M48 to M42 adapter used will count as part of the 20mm.

So your new optical train will be

Skywatcher Esprit 100ED > Field flattener > 20mm spacing > ATIK EFW2 (Filter) > 490EX 

Hope that helps 😊

Alan

[Skipper Billy]

That's just about the clearest, most succint explanation of how to get the spacing right I have read in a long time !  

[CKemu]

15 hours ago, symmetal said:

As well as being 1.4x larger across, the new sensor has pixels around half the size so the field curvature effect of the focused image will be much more apparent with the new sensor.

This will be fixed by buying the field flattener, but this means you do have to worry about spacing but it's fairly straightforwand. 🙂

You will need 55mm between the rear mounting face of the FF and the sensor of the 490EX.

Atik 490EX front plate to sensor = 13mm +/- 0.5mm

EFW2 thickness = 22mm

So the extra spacing required between the rear of the FF and the filter wheel is 55 - 13 - 22 = 20mm

The filter glass thickness does effect this spacing though and you need to add 1/3 the filter thickness to the original 55mm quoted for the FF. For 2mm thick filters this is around 0.7mm making 55.7mm spacing required. Therefore your 20mm spacing mentioned above should be nearer 20.7mm.

However, as your new sensor isn't that large compared to say an APS-C size sensor the FF spacing isn't so critical so you can probably forget about the extra 0.7mm and the 20mm extra spacing should be fine.

The FF has an M48 rear thread and I assume your EFW2 is M42 so the thickness of any M48 to M42 adapter used will count as part of the 20mm.

So your new optical train will be

Skywatcher Esprit 100ED > Field flattener > 20mm spacing > ATIK EFW2 (Filter) > 490EX 

Hope that helps 😊

Alan

That's fantastic, thank you very much for the help with that. Really clear and helpful

[ollypenrice]

On 15/09/2022 at 11:48, Skipper Billy said:

That's just about the clearest, most succint explanation of how to get the spacing right I have read in a long time !  

That's what I thought as well!

Olly