Working out extension tube requirments

[CedricTheBrave]

hi everyone

I have an ED80 with the 0.8 focal reducer.

the spec of which says the following

Recommended Starwave v3 0.8x Reducer Spacing:
Telescope model:    Sensor to reducer rear flange spacing (mm):    Focal Length with & (without) reducer (mm):
Starwave 80ED-R    63                                                                        448 (560)

 

so if the back focus of the ASI 290MC is 12.5mm, am I correct in saying that I will need a 50.5mm extension tube to mount it to the scope?

likewise if the Canon 700 is used with a T Ring that would need an 8mm extension?

hope I am going in the right direction!

 

many thanks

 

Dave 

 

 

[symmetal]

2 hours ago, CedricTheBrave said:

so if the back focus of the ASI 290MC is 12.5mm, am I correct in saying that I will need a 50.5mm extension tube to mount it to the scope?

likewise if the Canon 700 is used with a T Ring that would need an 8mm extension?

Yes and yes. 😀

However, the ASI290 is a small sensor at 5.6 x 3.2mm, so the reducer/flattener may not be necessary to get a useable flat field but if you do use the reducer/flattener you won't need to space it at exactly 50.5mm, and a mm or two either side should be fine.

The Canon 700 being an APS-C size sensor will need the extension spacing to be more exact. The Canon T2 adapters can vary slightly in their length by 0.5mm or so depending on the manufacturer, so the extension may need to vary slightly from 8mm to get optimum results. 🙂

Alan

[CedricTheBrave]

4 hours ago, symmetal said:

Yes and yes. 😀

However, the ASI290 is a small sensor at 5.6 x 3.2mm, so the reducer/flattener may not be necessary to get a useable flat field but if you do use the reducer/flattener you won't need to space it at exactly 50.5mm, and a mm or two either side should be fine.

The Canon 700 being an APS-C size sensor will need the extension spacing to be more exact. The Canon T2 adapters can vary slightly in their length by 0.5mm or so depending on the manufacturer, so the extension may need to vary slightly from 8mm to get optimum results. 🙂

Alan

perfect thank you

[adyj1]

5 hours ago, symmetal said:

The Canon 700 being an APS-C size sensor will need the extension spacing to be more exact. The Canon T2 adapters can vary slightly in their length by 0.5mm or so depending on the manufacturer, so the extension may need to vary slightly from 8mm to get optimum results. 🙂

Alan

Alan, 

Useful info. What should you look out for to be able to tell that the spacing is slightly off? 

Ady

[symmetal]

5 hours ago, adyj1 said:

Useful info. What should you look out for to be able to tell that the spacing is slightly off? 

At the correct spacing there should be tight round focused stars over the whole field especially in the corners. Without a field flattener the further from the focused centre of the image, the more distorted and out of focus the stars will be.

So examine the stars towards the corners of the image and adjust the spacing until they are as sharp and round as you can get them having refocused at the centre after each adjustment. You'll probably find that they may not be perfect, depending on the quality of the scope and flattener.

Sometimes you will have image tilt where the sensor image is not perfectly orthogonal to the scope centre axis, or maybe shifted to one side, so you'll find that some image corner stars are better than other corners. It can be quite a chore trying to get all corners looking the same so you may end up with a compromise. 🙂

Some image processing packages or plugins are able to correct for distorted star shapes to some degree.

Alan

[adyj1]

Alan, thanks for the great explanation.