Imaging at 0.78 arcsec pp

[Craig a]

Ok so is it possible?

I have been imaging with my 533mc pro and 130pds although I like the wider fov I have also got my Quattro 10inch Newtonian and as I’m a galaxy and planetary nebula imaging fan would like to stick the 533 on that purely for the longer fl the Quattro gives me, I understand at 0.78 arcsec pp I’m oversampling and not gaining any resolution because of Uk seing normally being poor, also I understand guiding would need to be pretty good so I would be doing away with the guide scope and buying an off axis guider to help with that, would I be mad going for it as I love my Quattro to bits and have done a lot of work to it and Carnt bear seing it in the corner of the spare room gathering dust!

[vlaiv]

533mc pro is OSC camera, and if you use super pixel binning mode, you'll be effectively imaging at twice that - so 1.56"/px.

That is reasonable sampling rate for 10" scope if your guiding is somewhere around 0.6-0.8" RMS.

Main issue will be small FOV of course and your main targets will be planetary nebulae and smaller galaxies with that combination.

In fact, it's not even that small:

This is M13 - it fits nicely in FOV. So does M81:

I would say  - just go for it!

[Craig a]

Ok thanks, super pixel mode? Do I use that when stacking or debayering? Or when capturing? Never used it so not sure how to use it? 

[vlaiv]

39 minutes ago, Craig a said:

Ok thanks, super pixel mode? Do I use that when stacking or debayering? Or when capturing? Never used it so not sure how to use it? 

It is just method of debayering.

Instead of interpolation, it just uses bayer matrix of 2x2 pixels of different color (RGGB for example) to produce single pixel with all three colors - R, G and B.

It is available in DSS and I'm sure it is also available in other software. With DSS, you just need to select it:

As you see - resulting image size is divided by two, so you'll end up with images that are 1504px x 1504px in size. Hence, resolution will be two times smaller as well - instead of 0.78"/px it will be 1.56"/px. In reality, because of Bayer matrix, you are sampling at twice lower rate than pixel size suggests, so this is convenient way to get actual sampling rate of color camera - which suits you better.

[Craig a]

Makes sense, abit off topic here sort of but the guide cam I’ve got is the Orion starshoot would that be sensitive enough to use on a off axis guider at f4? I’ve read elsewhere that it is no good on an off axis guider because it Carnt pick up faint stars 

[vlaiv]

14 minutes ago, Craig a said:

Makes sense, abit off topic here sort of but the guide cam I’ve got is the Orion starshoot would that be sensitive enough to use on a off axis guider at f4? I’ve read elsewhere that it is no good on an off axis guider because it Carnt pick up faint stars 

I think you won't have any problems with sensitivity, just remember - with OAG and fast scopes, you need to pay attention to distancing in order not to stop down aperture with prism itself.

What OAG will you be using? Do you know the size of pick off prism?

Let's say it is something like 8mm. At F/4, you need to be less than 30mm from prism to guide camera (and similarly to imaging camera as these two are about the same in distance) to fully exploit aperture.

I guess that your imaging train will be something like CC -> OAG -> camera. If you need to dial in CC / sensor distance - put extension rings between CC and OAG and make OAG closer to imaging camera as there is already something like 17.5mm from T2 to sensor.

[Craig a]

I was looking at the zwo OAG v2, it is 8mm prism 

https://www.firstlightoptics.com/zwo-accessories/zwo-off-axis-guider-oag-v2.html

The camera came with all the spacers to get 55mm back focus I just thought I could swap the 16.5mm spacer and put the OAG in its place, both the same thickness like shown in the pictures on the link, didn’t realise that would change with a faster scope? 
but I spose I could swap the 21mm I have closest to the camera and put the OAG closest to cam and put the 21mm between the cc and OAG does that make sense?

Edited December 6, 2020 by Craig a

[vlaiv]

23 minutes ago, Craig a said:

I was looking at the zwo OAG v2, it is 8mm prism 

https://www.firstlightoptics.com/zwo-accessories/zwo-off-axis-guider-oag-v2.html

The camera came with all the spacers to get 55mm back focus I just thought I could swap the 16.5mm spacer and put the OAG in its place, both the same thickness like shown in the pictures on the link, didn’t realise that would change with a faster scope? 

That sounds like a good plan - put OAG close to sensor, just make sure you have enough space to bring your camera to focus.

There are really only two things that you need to be careful with.

First is length of prism stalk.

You can't bring your guide camera closer to prism than this distance. This is also minimum distance from prism to your imaging sensor.

Second thing is total distance between prism and guiding sensor. This is limited by speed of your scope if you don't want to loose light for OAG. Here is diagram to help you understand what is going on:

In top image - prism is closer to sensor and can pick off whole cone of light. In bottom part of the diagram, prism is further away - and since light beam is wide - it only pick off part of it - effectively reflecting less light. This acts as aperture stop and can cause significant loss of light if you have fast system and move prism too far away.

Critical distance is easy to calculate - F/ratio * size of prism. In this case it is F/4 * 8 = 32mm. With 32mm distance, only central point gets 100% illumination. You want a bit larger illuminated field so you need sensor to be 30mm or less.

If you can't place camera close enough - don't sweat it, I use OAG at F/8 on my RC scope and still manage to get guide stars.

[Craig a]

Thank you for explaining this to me, off axis guiding is a new endeavour for me but I believe olly when he says every newt should be guided this way to prevent differential flexure, so that 30mm you speak of it this from prism to guide cam sensor? Or prism to be less than 30mm away from main imaging camera sensor?

looking at zwo website it looks as if the prism holder is 31mm long? Would that mean if I’ve got less than that from prism to main cam sensor I’ve got no chance of reaching focus with the guide cam? 

Edited December 6, 2020 by Craig a

[vlaiv]

1 hour ago, Craig a said:

so that 30mm you speak of it this from prism to guide cam sensor? Or prism to be less than 30mm away from main imaging camera sensor?

It will be the same.

Guide sensor will be roughly (1mm give/take) the same distance from prism as will be imaging sensor. This is because the beam that would otherwise converge on imaging sensor is diverted at 90 degrees - and it will converge after the same distance, but this time on guide sensor (that 1mm give or take is due to field curvature and such, we are taking beam at extreme edge of the field).

1 hour ago, Craig a said:

looking at zwo website it looks as if the prism holder is 31mm long? Would that mean if I’ve got less than that from prism to main cam sensor I’ve got no chance of reaching focus with the guide cam? 

If prism holder is 31mm long, then yes, you'll have to put OAG slightly further away from imaging sensor than those 31mm to get focus on guide sensor.

This means that you won't place guide camera at optimum distance for prism size and speed of your scope, but don't worry, it will still work even if it has a bit less light. It will be as if guider is operating at - F/5 or F/6. A bit less light then it would otherwise receive from F/4 scope, but still plenty of light to find guide stars.

 

[Craig a]

Ok thank you for clearing that up for me 

[alan potts]

19 hours ago, vlaiv said:

It is just method of debayering.

Instead of interpolation, it just uses bayer matrix of 2x2 pixels of different color (RGGB for example) to produce single pixel with all three colors - R, G and B.

It is available in DSS and I'm sure it is also available in other software. With DSS, you just need to select it:

As you see - resulting image size is divided by two, so you'll end up with images that are 1504px x 1504px in size. Hence, resolution will be two times smaller as well - instead of 0.78"/px it will be 1.56"/px. In reality, because of Bayer matrix, you are sampling at twice lower rate than pixel size suggests, so this is convenient way to get actual sampling rate of color camera - which suits you better.

Must give that a try on the 183mc, never seen or heard of it before.

Alan

[tomato]

Just to give you a flavour of what you might expect, this is M81 with an Esprit 150/ASI 178 binned 2x2 so imaging at 0.94 arcsec per pixel. I used an OAG on a Mesu 200 and achieved 0.5” total RMS guiding so just about OK. Looking forward to seeing the results.