Tilt or Poor Back spacing? Advice needed................

[Dave kay]

Hi Folks,

I'm looking for some advice on my images below. I took two images of the Sadr Region with my new Askar FRA400 and two with my Redcat 51. I took an initial image with each scope and then rotated the camera to get a second image. 

All four images show oval stars, but in different corners. I'm not sure what to make of the issue?

My first thoughts are tilt on the camera? 

my second thought is poor back focus? --> I'm less certain of this because I was under the impression that both scopes didn't require an accurate back spacing as they are both Petzval designs. 

Both scopes were on my EQ6-R mount and I used a ZWO asi294mm camera (bin1, gain 120, -10 degrees, with suggested 55mm back spacing) and an astronomik 6nm Ha filter. I have also attached an image of my guiding on the night. 

Any help / advice will be much appreciated as I'm a bit vexed about the whole issue. 

Thanks in advance. 

Sadr Region 300s Ha (Redcat 51) rotated with spacers.tif Sadr Region 300s Ha (Redcat 51).tif Sadr Region 300s Ha (FRA400) rotated.tif Sadr Region 300s Ha (FRA400).tif

[michael8554]

1)  It's impossible to compare the star shapes in these images because they are all 300 secs exposure, so will show differing guide errors.

Take one short exposure with each at max gain, stretch if necessary, post a jpeg, 90MB tifs aren't necessary for this analysis.

2)  "both scopes didn't require an accurate back spacing as they are both Petzval designs. "

"I used a ZWO asi294mm camera (bin1, gain 120, -10 degrees, with suggested 55mm back spacing)"

Confusion here. As you said, the OTA's don't need a Backspacing for best star shape.

And cameras don't have Backspacing, so no need for a "55mm Backspacing".

That's only necessary if there's a 55mm backspace FF or FR involved.

Just focus the camera.

Michael.

[Dave kay]

1 hour ago, michael8554 said:

1)  It's impossible to compare the star shapes in these images because they are all 300 secs exposure, so will show differing guide errors.

Take one short exposure with each at max gain, stretch if necessary, post a jpeg, 90MB tifs aren't necessary for this analysis.

2)  "both scopes didn't require an accurate back spacing as they are both Petzval designs. "

"I used a ZWO asi294mm camera (bin1, gain 120, -10 degrees, with suggested 55mm back spacing)"

Confusion here. As you said, the OTA's don't need a Backspacing for best star shape.

And cameras don't have Backspacing, so no need for a "55mm Backspacing".

That's only necessary if there's a 55mm backspace FF or FR involved.

Just focus the camera.

Michael.

Thanks for the input Michael.

I'll take some short exposures when I get some more clear skies and I'll post the jpegs here. Should 10 secs be enough or would you go shorter? 

The two Petzval scopes I own have built in FF's in the rear of the scope. This is why I used the back spacing, suggested by ZWO, of 55mm from the back of the scope to the sensor on the 294mm. I then focussed both using a bahtinov mask, so both were bang on sharp focus. I've heard that some people don't bother with the 100% accurate back spacing whilst using scopes of this design, hence why I asked the question. 

I hope that made sense,

Dave. 

[michael8554]

So you are using ZWO's recommendation for Back Spacings for two different manufacturer's OTA's with built in FF's ?

Does WO recommend 55mm Backspacing?

Does Askar recommend 55mm Backspacing ?

As I said, 

"you said the OTA's don't need a Backspacing for best star shape.

And cameras don't have Backspacing, so no need for a "55mm Backspacing".

That's only necessary if there's a 55mm backspace FF or FR involved.

Michael

[Dave kay]

1 hour ago, michael8554 said:

So you are using ZWO's recommendation for Back Spacings for two different manufacturer's OTA's with built in FF's ?

Does WO recommend 55mm Backspacing?

Does Askar recommend 55mm Backspacing ?

As I said, 

"you said the OTA's don't need a Backspacing for best star shape.

And cameras don't have Backspacing, so no need for a "55mm Backspacing".

That's only necessary if there's a 55mm backspace FF or FR involved.

Michael

Having done some more reading I'm going to buy a simple m48 to t2 adapter so I can screw the filterwheel straight onto the scope from now on. There's no need to mess about with the 55mm. 

I'm not sure that this will have any effect on the issues in my images, but it's worth a try.

Dave. 

[symmetal]

Unfortunately backspacing has different meanings depending on what it's referring to.

The WO diagram for the Redcat 51 does show the image plane placed 54.7mm behind the rear plate of the scope. This is also  a common backspacing distance used for field flatteners and coma correctors as the spacing distance is critical for these items, and it conveniently enables a DSLR with T-Adapter to be fitted to the rear of such correctors, and the required 55mm spacing between the corrector rear and the image plane of the camera sensor is achieved.

The Redcat 51 'backspacing' figure of 54.7mm just means that if the sensor is placed this distance from the rear of the scope then the focus scale on the scope will read correctly. Useful if you have a DSLR with T-Adapter and when stars are in focus the focus scale will read infinity. If not at this distance you can still achieve focus but the focus scale will read incorrectly. Most astro scopes don't have a distance focus scale so don't quote an image plane distance, as it depends on what extras are attached to the scope.                

As you say, being Petzval designs, and as such a separate field flattener (with critical backspacing) is not needed, as long as you can achieve focus the stars should be corrected to the edges of the frame. 

Camera backspacing is not really backspacing, but is just the distance the camera sensor is behind the front mounting plate of the camera. Useful to know when you're adding extra spacing to achieve an actual backspacing distance required for a corrector etc.

Alan

Edited September 3, 2022 by symmetal

[michael8554]

To avoid this confusion, "Camera Backspacing" is often called "Flange Distance".

For instance all Canon EF lenses focus 44mm behind the lens's mounting bayonet.

That focus distance is fixed, so the camera's Flange Distance has to be 44mm.

Unlike a telescope, where any camera can be attached and focus found using the scope's focuser.

Michael

[ollypenrice]

Yes, the only issue with a Petzval design is, 'Can I get the chip onto the focal plane of the telescope with the hardware I have?' If the drawtube is too long or too short for your camera's chip distance (the distance between its front attachment point and its chip) you won't be able to. But it seems that you can, so all is good. 

However, mounting hardware can introduce tilt so the less of it you have the better - and it should all be screw fit.

As for chip tilt in the camera, there's a thread on how to tackle this. 

 

Olly

Edited September 4, 2022 by ollypenrice

[Dave kay]

10 hours ago, ollypenrice said:

Yes, the only issue with a Petzval design is, 'Can I get the chip onto the focal plane of the telescope with the hardware I have?' If the drawtube is too long or too short for your camera's chip distance (the distance between its front attachment point and its chip) you won't be able to. But it seems that you can, so all is good. 

However, mounting hardware can introduce tilt so the less of it you have the better - and it should all be screw fit.

As for chip tilt in the camera, there's a thread on how to tackle this. 

 

Olly

Thanks for your input Olly,

I'll have a read though the thread / watch the video and have a look at testing the tilt on my camera. 

I bought an adapter to shorten the imaging train behind the scope, hopefully this will help to.

Dave. 

[ollypenrice]

11 hours ago, Dave kay said:

Thanks for your input Olly,

I'll have a read though the thread / watch the video and have a look at testing the tilt on my camera. 

I bought an adapter to shorten the imaging train behind the scope, hopefully this will help to.

Dave. 

You can't shorten the train behind the scope. The camera chip needs to lie on the focal plane, which is a fixed distance behind the objective. What you can do is use a rigid extender which requires less of the drawtube to be extended. There has been a fad among some manufacturers to make shorter main tubes with longer longer drawtubes which sometimes leaves the drawtube under-supported when well extended.

Olly

[900SL]

On 03/09/2022 at 20:52, michael8554 said:

So you are using ZWO's recommendation for Back Spacings for two different manufacturer's OTA's with built in FF's ?

Does WO recommend 55mm Backspacing?

Does Askar recommend 55mm Backspacing ?

As I said, 

"you said the OTA's don't need a Backspacing for best star shape.

And cameras don't have Backspacing, so no need for a "55mm Backspacing".

That's only necessary if there's a 55mm backspace FF or FR involved.

Michael

Shirley some mistake here? My Redcat 51 requires 56mm backspacing to the sensor.. its a petzval. Whilst it isn't anywhere near as critical as a scope and FF to get it bang on, you wont get focus if you are far off that mark, and likely other issues too

[symmetal]

On 05/09/2022 at 17:27, 900SL said:

My Redcat 51 requires 56mm backspacing to the sensor.. its a petzval.

This Redcat 55mm or so backspacing 'requirement' is only if you want stars in focus and with the focus distance scale set to infinity. This will also allow you to focus on closer subjects as indicated by the distance scale on the scope, but for astro use this is not relevant. The Redcat is also marketed as a lens for terrestrial photography where the distance scale is useful, like on a camera lens. (Early versions of the Redcat had no distance scale markings anyway).

If you only have say 40mm 'backspacing' you can still focus the stars by setting the focus scale to a closer distance like 30m or something and you'll be fine. If you use much more than 55mm backspacing then you won't be able to focus at all. As long as you can focus the image with the Redcat, your 'backspacing' is automatically correct.

Unlike 'standard' astro telescopes where the lenses are fixed in place and you focus by moving the camera back and forth, the Redcat focuses by moving the complete lens assemblies back and forth so there is no fixed backspacing distance from the lens rear correcting elements anyway. 🙂

Alan