Skywatcher coma corrector with CCD and filterwheel

[Pompey Monkey]

Afternoon all,

I have been using the Skywatcher coma corrector with my Canon DSLR for a few months and, although some of my images are not brilliant, I have nice sharp stars that stay almost completely round to the edges of the field. So all good here

I now, however, am the proud owner of an SBIG STF-8300M CCD and a Starlight Xpress USB filter wheel. The kit that I bought (second hand) has a 2" nose piece included and I can fit it all together and see that it will work in the 150P-DS. Although I have not yet checked that there's sufficient back-focus yet, my calculations are optimistic.

The question is: How do I fit the SW coma corrector in the optical path? Do I just screw in the 2" adapter and mount the CCD/FW nose piece in this? Or do I need to fit the CC directly to the FW (necessitating yet another adapter)? Is there a critical distance between the CC and CCD sensor? If so, how do I achieve this?

Looking forward to your answers!

[Tinker1947]

There will be a back focus measurement for the CC, I don't know what it is, Baader CC is 55mm, so deduct the sensor to flange distance, then deduct the Filter wheel thickness, whats left needs a spacer if its over 20mm Baader Varilock 29 works from 20mm - 29mm with a locking ring to keep the distance precise, there is also a 12mm - 16mm adapter and a 29mm - 45mm Varilock shown on the FLO accessories web page... 

this image shows the Baader CC, Varilock, CCD with the distance set at 55mm, round stars right to the corners...

[Lonestar70]

Hi PM,

You may struggle with this I am afraid but it depends on what adaptors you may have for the scope-side of your filter wheel.

The backfocus distance for the SW Coma Corrector is 55mm and with the full format sensor of the SBIG this will need to be within +/- 0.5mm.

This distance is measured from the bottom of the thread on the back of the Coma Corrector to the sensor plane.

The thread on the Coma Corrector is M48 x 0.75mm.

I believe the standard Starlight filter wheel is 33mm front to back including the normal adaptors... so you don't have much to play with.

Ask FLO if a suitable scope side filter wheel fitting is available with an M48 x 0.75mm internal thread which could replace the standard fitting... if such is available then the Coma Corrector can be screwed directly to the filter wheel and will minimise the optical path length of the fiter wheel assembly... if not available then you will need a converter adaptor the thickness of which will need to be added to the filter wheel thickness when calculating the spacing.

I am not aware of the body-sensor depth of the SBIG... possibly around 19mm so you won't have very much lee way.

I wish you luck in getting set up.

Best regards.

Sandy.

[Tinker1947]

Back Focus is 17.5mm for the Camera

https://www.sbig.com/products/cameras/stf-series/stf/stf-8300m/

[Pompey Monkey]

Let's see if I've got this right:

On paper, the filter wheel is 29 mm:

But if I measure from the CCD face to the front of the adapter on the telescope side of the FW, I get 37.2 mm. This is a female T-2 (M42 x 0.75 mm) - because it says so on the nose piece box 

The back focus for the camera is 17.5 mm (0.69"):

I need a total of 55 mm from the flat face of the SW CC so i need a spacing of 55 - (17.5 + 37.3) mm = 0.2 mm. 

The thread on my SW CC is male M48 x 0.75 mm. 

So even with a T2 to M48 adapter: http://www.altairastro.com/altair-m48-to-t2-adapter.html which adds another 1 mm to the optical path I will be 0.8 mm over.

So if I get the SX FW M48 adapter to screw directly into the CC, this should work with a 0.2 mm error provided that the height of the M48 adapter is the same as the current T2 adapter. I am struggling to find this critical piece of information.... 

Does anyone know this?

[Tinker1947]

With Filters inline you may get a extra 1mm, somebody with the knowledge will come along soon...

[Lonestar70]

PM,

I think you will find the SX adaptors all maintain the 29mm thickness but you can email Altair Astro to double check.

Don't forget to order the FEMALE thread version to take the MALE SW Coma Corrector thread.

http://www.altairastro.com/starlight-xpress-male-m48-face-plate-for-sx-filter-wheels-starlight-xpress-t2-face-plate-for-sx-filter-wheels-choose-male-or-female.html

This would result in 29mm +17.5mm = 46.5mm... so you would need to add 8.5mm on the camera side... not the best option really since you need to keep the filters as close to the camera as possible to avoid vignetting issues.

Perhaps a better solution would be to get the Male thread version of the SX adaptor and locate a short 8.5mm all Female M48 x 0.75mm extension tube which could then be fitted between the Coma Corrector and the SX adaptor.

I believe TS (Teleskop services) may have a suitable extension tube.

Note... the above does not take account of the effect the filters have on the optical path length of the filter wheel but I seem to remember this increases the optical length of the filter wheel assembly by something like 0.7 x thickness of the filter, therefore the 29mm physical length would be increased by this amount which will reduce the length of the required extra extension by the same amount.

Hope you get sorted.

Best regards.

Sandy.

[Pompey Monkey]

PM,

I think you will find the SX adaptors all maintain the 29mm thickness but you can email Altair Astro to double check.

Don't forget to order the FEMALE thread version to take the MALE SW Coma Corrector thread.

http://www.altairastro.com/starlight-xpress-male-m48-face-plate-for-sx-filter-wheels-starlight-xpress-t2-face-plate-for-sx-filter-wheels-choose-male-or-female.html

This would result in 29mm +17.5mm = 46.5mm... so you would need to add 8.5mm on the camera side... not the best option really since you need to keep the filters as close to the camera as possible to avoid vignetting issues.

Perhaps a better solution would be to get the Male thread version of the SX adaptor and locate a short 8.5mm all Female M48 x 0.75mm extension tube which could then be fitted between the Coma Corrector and the SX adaptor.

I believe TS (Teleskop services) may have a suitable extension tube.

Note... the above does not take account of the effect the filters have on the optical path length of the filter wheel but I seem to remember this increases the optical length of the filter wheel assembly by something like 0.7 x thickness of the filter, therefore the 29mm physical length would be increased by this amount which will reduce the length of the required extra extension by the same amount.

Hope you get sorted.

Best regards.

Sandy.

Thanks Sandy,

I'll do some more research and send off a couple of emails.

I agree that the SX adaptors are probably consistent, but perhaps it is a different consistent length for the males than for the females. The face to face distance on my set-up is most definitely more than 29 mm! I'll try to take a picture later today to show you the one I have.

Paul

[Pompey Monkey]

Here are a couple of pictures to explain my measurement of 37.3 mm between the flats on the FW.

The first shows the measurement:

The second shows how the female thread extends back below the flat face:

I would imagine that the male and female adaptors would all follow the same pattern, unless the females allowed the mating thread to potentially protrude inside the body of the FW.

I emailed Altair Astro with the question around lunchtime. I'm hoping for an answer at some poit over the next few days.

[Lonestar70]

Hi PM,

Not sure, but it looks like you may have a male to female (possibly T-2) conversion ring fitted.

If you look at your cross sectional drawing (a few posts above) this clearly shows a Female thread inside the adaptor (the side opposite the thumb wheels on the casing)  for the telescope connection (bottom side of the drawing).... which extends to the inside face of the casing.

Hopefully you will get some confirmation from Altair.

Keep happy.

Sandy.

[Pompey Monkey]

Hi PM,

Not sure, but it looks like you may have a male to female (possibly T-2) conversion ring fitted.

If you look at your cross sectional drawing (a few posts above) this clearly shows a Female thread inside the adaptor (the side opposite the thumb wheels on the casing)  for the telescope connection (bottom side of the drawing).... which extends to the inside face of the casing.

Hopefully you will get some confirmation from Altair.

Keep happy.

Sandy.

Sandy

By golly, I think you may be right!

I cannot, for the life of me, separate the two parts though.

Probably new adaptor/spacer time for me

Paul

[Pompey Monkey]

I posed the question about the adapter thickness to Starlight Xpress (UK) and got a super fast reply (on a Sunday, no less!). They are all 4.3 mm thick, giving a total thickness of 29.6 mm for the wheel.

Adding 29.6 mm to the back focus of 17.5 mm for the camera gives 47.1 mm

The CC requires 55 mm spacing between the flange and the camera chip so that leaves 7.9 mm. I then added 0.7 mm for the optical length of the 2.1 mm thick Baader filters to arrive at 8.6 mm.

So I've bought a SX female M48 FW adapter;

A 7.5 mm M48 extension ring;

And, mistakenly, two 0.75 mm thick M48 spacers instead of one 1 mm thick spacer. (Possibly because I'm a bit thick sometimes   )

So I should end up with 8.25 mm or 9 mm. Both are within the commonly quoted CC spacing tolerance of 0.5 mm, so I'll give them a go. If that fails, I'll get a 1 mm M48 spacer to get to 8.5 mm.

Wish me luck!

[Pompey Monkey]

Well, seeing as I just blew a grand and a half on a camera/filter wheel and filters, it seemed completely idiotic churlish not to get the correct spacing for the corrector. So I just ordered two 0.3 mm and one 0.5 mm M48 spacer from TE.

17.5 + 29.6  + 7.5 + (2*0.3) + 0.5 - 0.7 mm = 55 mm

https://www.google.co.uk/search?espv=2&q=17.5+%2B+29.6+%2B+%282*0.3%29+%2B+7.5+%2B+0.5+-+0.7+%3D+&oq=17.5+%2B+29.6+%2B+%282*0.3%29+%2B+7.5+%2B+0.5+-+0.7+%3D+&gs_l=serp.3...36636.36636.0.37766.1.1.0.0.0.0.159.159.0j1.1.0....0...1c.1.64.serp..1.0.0.CxKjUPr0ngA

   

[Tinker1947]

Well, seeing as I just blew a grand and a half on a camera/filter wheel and filters, it seemed completely idiotic churlish not to get the correct spacing for the corrector. So I just ordered two 0.3 mm and one 0.5 mm M48 spacer from TE.

17.5 + 29.6  + 7.5 + (2*0.3) + 0.5 - 0.7 mm = 55 mm

https://www.google.co.uk/search?espv=2&q=17.5+%2B+29.6+%2B+%282*0.3%29+%2B+7.5+%2B+0.5+-+0.7+%3D+&oq=17.5+%2B+29.6+%2B+%282*0.3%29+%2B+7.5+%2B+0.5+-+0.7+%3D+&gs_l=serp.3...36636.36636.0.37766.1.1.0.0.0.0.159.159.0j1.1.0....0...1c.1.64.serp..1.0.0.CxKjUPr0ngA

   

The distance increases with the use of Filters, my Baader 1.25" RGBL set increase the distance by 1mm, wider filter may increase this a bit more......

[Pompey Monkey]

The distance increases with the use of Filters, my Baader 1.25" RGBL set increase the distance by 1mm, wider filter may increase this a bit more......

Indeed.

That's why I have the - 0.7 mm term in the sum  

Linky: http://www.qsimaging.com/downloads/QSI-500-600-Series-Back-Focus.pdf

The calculation allows for the increase in physical separation to keep the optical length correct. The CCD is physically 55.7 mm from the coma corrector.*

*Assuming that the manufacturers supplied measurements are correct