Back focus problem?

[Steve143]

Hi,

I'm imaging with an Altair Starwave 80 ED-R with the Starwave 0.8x Reducer V3 and a Canon 70D. In the front of the reducer I have either an Optolong L-Pro or L-extreme, depending on the target. I also have a 5mm spacer in the imaging train and my image field looks ok (see single exposure of M27 below). 

 

I've just added a ZWO filter drawer so I can easily change between my two filters, but expected this to create elongated stars as it adds 21mm to the imaging train. However, when I took some images of M31 the stars in the corners look like what you would expect if the sensor was too close but it is now 21mm further away. I've included an image of M31 below. I've also included a photo of my current imaging train. 

 

Could this be due to sensor tilt or do I need to add additional spacers to obtain a correct back focus?

Thanks. 

[AstroNebulee]

You need to add additional spacers as your M31 image is showing that the sensor is too close.

Lee 

[Steve143]

1 hour ago, AstroNebulee said:

You need to add additional spacers as your M31 image is showing that the sensor is too close.

Lee 

Thanks. That's what is looks like to me but before adding the filter drawer my images looked fine, so I can't understand now when I add in the 21mm from the filter drawer the sensor is too close.

[michael8554]

Hi Steve

The Starwave 0.8x Reducer V3 isn't that common, and may behave in an opposite way to other FRs.

You won't be able to get back to the 63mm backfocus with the filter drawer in that position, can it go in front of the FR without vigneting ?

Michael

 

[AstroNebulee]

2 hours ago, Steve143 said:

Thanks. That's what is looks like to me but before adding the filter drawer my images looked fine, so I can't understand now when I add in the 21mm from the filter drawer the sensor is too close.

Maybe as Michael suggests can you put the filter drawer before your reducer, though ideally I guess you'd want your filter closer to the sensor but could be wrong. 

Lee 

[Steve143]

3 hours ago, michael8554 said:

Hi Steve

The Starwave 0.8x Reducer V3 isn't that common, and may behave in an opposite way to other FRs.

You won't be able to get back to the 63mm backfocus with the filter drawer in that position, can it go in front of the FR without vigneting ?

Michael

 

Hi Michael,

I haven't tried it in front of the FR only because I thought any filter should be as close as possible to the sensor. I'll give it a go and see what happens.

Thanks. 

[Steve143]

1 hour ago, AstroNebulee said:

Maybe as Michael suggests can you put the filter drawer before your reducer, though ideally I guess you'd want your filter closer to the sensor but could be wrong. 

Lee 

That's exactly what I thought - the advice seems to say to have the filter as close as possible to the sensor. 

[AstroNebulee]

1 hour ago, Steve143 said:

That's exactly what I thought - the advice seems to say to have the filter as close as possible to the sensor. 

I did read this same thing yesterday on altair astro website as I've been trying my filter before the FF and in my 2inch filter drawer and think the image is less halos and sharper with it closer. So I've moved my filter back into the filter drawer again and will test further on next clear night. 

https://www.altairastro.help/info-instructions/faq/can-filters-be-placed-in-front-of-a-focal-reducer-flattener/

Lee