Advice about spacing between reducer and camera

[Giles_B]

After some deliberation I jumped in feet first over Christmas and set myself up with some imaging rig. It's been a steep learning curve with some frustrating nights spent getting to grips with the new kit. I feel like I'm slowly getting there but I've still yet to image anything worth speaking of.

The rig I've assembled is based around an Altair 125 EDF, which is f7.8 but has a planostar m92 0.8 reducer which converts it to a f6.24 astrograph - https://www.altairastro.com/altair-wave-series-125-edf-apo-refractor-w-heavy-duty-m92-rp-focuser-454-p.asp.

I've paired this beast with an ASI585MC - which is far from using the full potential of the scope I know, but seemed a good place to start.

I've only now got the right m48-t2 adaptor to use the reducer, but assembling things tonight i realise I'm confused about the spacing from the reducer to the camera. The reducer spec states:

"The reducer comes with an M72 to M48 spacer pre-calculated to give a 55mm sensor to flange distance."

Does this mean I mustS make sure there is 55mm between the reducer and the camera sensor? And if it is not exactly this distance (I'm a couple of mm off), what happens - will I fail to reach focus, or fail to utilise the flattening of the reducer, or what?

 

[Elp]

This is what will happen to the imaged starfield if your spacing is wrong:

[Elp]

Your extension rings from the bottom of the thread of the reducer to the camera sensor (you need to exclude the thread lengths in this measurement, its where everything physically comes to a stop) (minus the front of camera distance to the sensor, on the 585 I think this is 6.5mm) needs to be 55mm long, so 55mm minus 6.5mm equals 48.5mm.

If you have a filter in the system you typically need to add 1/3 of the filter thickness to the total backfocus distance, usually people use +1mm for a filter thickness of 3mm but it depends on the filter.

[Budgie1]

Your ASI585MC should have an 11mm M42 spacer ring attached to the front of it, this makes the total distance from the sensor to the front of the spacer 17.5mm (see diagram below).

To reach your 55mm, ZWO have two other spacers in the form of a 16.5mm M42-M48 and 21mm M42-M42 spacer. Once added to the front of the camera this will give you your required spacing.

[Giles_B]

Thanks that's all really helpful - @Budgie1 I was wondering how I was going to get the 0.5mm T2 spacing!

I have a ZWO mini EFW inline, which adds 20mm between the T2 connectors - I'm planning on trying a variety of filters (L'extreme, Altair triband v2, OIII) but will try using the shorthand of +1mm for now @Elp

Overall with the 20mm EFW plus the 1mm filter and the 16.5mm, it sounds like I will need to find an additional 11mm spacer.

(Edit: sorry brain now working - I already have the 11mm spacer!) So I should get the right spacing with 20mm EFW + the 1mm filter + the recommended 16.5mm M48 to T2 (which I assume has a female M48 thread, but I'm checking this with FLO)

 

I'm assuming the example starfield distortion in the pictures is quite exaggerated, but will find out tonight - with my current heath robinson setup I'm about 1.5mm short, so I will closely examine the stars if I make any images during my experiments!

Edited January 17 by Giles_B
brain still asleep

[Elp]

The illustration is very close to what you see, it's obviously slightly less in real life only because out in space you don't get a uniform grid pattern of stars in an even distribution. Also the larger the imaging sensor the more you'll see it.

[Budgie1]

If you need to fine tune the spacing, you can get various packs of thin spacers covering 0.4mm, 0.5mm, 0.6mm, 0.8mm & 1mm. 

[Giles_B]

Thanks both. I've now got the 16.5mm on the way and some thin spacers to boot - I'll also treat tonight as a further experiment and not expect too much of my imaging 

[Giles_B]

Wonderful FLO - it arrived today so I'm all set up to enjoy another clear night - images will follow!

[Budgie1]

Excellent. 

Have fun & I hope it's warmer then the -7°C it was up here last night! 🥶