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Astrophotography with Celestron 8SE


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I've used my 8SE for viewing and now want to try astrophotography. I'm looking at ZWO cameras and would like to be sure that these make sense with the 8SE with its 80 inch focal length. Looking at Telescopius with some ZWO cameras shows an excessive magnification for some typical DSO's. Does this need some kind of focal length reducer, or is a lower focal length scope required? I appreciate the supplied mount with the 8SE isn't the best for this, but I'd like to see what is possible.

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With the Alt Az mount you will get some field rotation, so short subs will help.... 80 inch fl, you're the first I've heard saying it in imperial.. you can get a focal reducer that will help but imaging with a SCT has quite a few hurdles to get over... Don't suppose you have a DSLR that you could use with it at first, then if you wish to proceed it further then you can, but a shorter focal length scope will be far easier and using a german equatorial mount

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15 hours ago, pike said:

I've used my 8SE for viewing and now want to try astrophotography. I'm looking at ZWO cameras and would like to be sure that these make sense with the 8SE with its 80 inch focal length. Looking at Telescopius with some ZWO cameras shows an excessive magnification for some typical DSO's. Does this need some kind of focal length reducer, or is a lower focal length scope required? I appreciate the supplied mount with the 8SE isn't the best for this, but I'd like to see what is possible.

If you spend a little time sat down with a cuppa or a glass of beer and browse through similar threads on starting DSO  imaging or astrophotography you'll see that three things are mentioned as being preferable or essential 

  • Mount
  • Aperture
  • Focal length

Whilst an Alt/Az mount is capable of being used for imaging they suffer from field rotation.  The idea it to try and get as sharp an image as possible through stacking multiple images, with as few defects or trailing of stars as possible.  So any images stacked over a full night will have rotation in them which is another factor the stacking software has to deal with, and depending on the severity you may find the software rejects most, if not all of them as a result.  So an EQ mount is more suited as it cancels out field rotation, and is easier to guide for long exposures.

Aperture and focal length are tied together, and define the focal ratio.  Your scope is F10, which is really good for luna and planetary imaging as you get high magnification with long focal lengths, and being 8" aperture lots of detail.  However, when it comes to faint DSO's a faster, short focal length is desirable.  Most would say that F5 or F4 are the "ideal" focal ratio, so for an 8" that would be 1m  focal length.  Now adding a focal reducer to scopes with longer focal lengths can work, but ideally you want to put as fewer lumps of glass between the camera and the target.  But it still gives a workable solution.

Personally,  I would start with the DSLR route.  Look for an old Canon camera (D450 or similar) that can be picked up fairly cheaply, and a suitable T ring to fit it to the scope.  This way you can dip your toe in the water without wasting a lot of cash on dedicated and sensitive astro cameras only to find that the equipment you have is restrictive for the needs, and whilst a handful of bright DSOs are fine, the limitations of the hardware can be found when turning to the fainter targets.   

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On 13/03/2022 at 19:50, pike said:

I've used my 8SE for viewing and now want to try astrophotography. I'm looking at ZWO cameras and would like to be sure that these make sense with the 8SE with its 80 inch focal length. Looking at Telescopius with some ZWO cameras shows an excessive magnification for some typical DSO's. Does this need some kind of focal length reducer, or is a lower focal length scope required? I appreciate the supplied mount with the 8SE isn't the best for this, but I'd like to see what is possible.

I’ve used the OTA on an AVX mount with a 0.63 reducer giving 1280mm focal length at f6.3.

got good images just failed to get decent post processing- recently move to PixIndight so will see what that does for me.

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I think we might as well get the terminology right at the outset because it does help to get things straight and avoid confusion.

Focal length (and nothing else) determines the size of the object's image as it is projected onto the camera chip. Longer FL = larger projected image. It does not necessarily mean a larger final image. We'll come to that. The objects you will photograph are bigger than any picture you'll make of them so there is no 'magnification' anywhere in this story! See resolution, later.

On a given chip FL also determines the size of your field of view but chip size can vary, increasing or decreasing your field of view.

Chip size of itself has no effect whatever on the size of the object on your final image. The small chip imposes a natural 'crop' but a larger chip can be cropped to give the small framing of the smaller chip.

Resolution has nothing whatever to do with pixel count and the term 'crop factor' is meaningless in AP. 

Resolution is determined by a combination of focal length and pixel size. It is measured in arcseconds per pixel, meaning how big a piece of sky lands on each pixel?  The same resolution can arise from a short focal length with small pixels or a long focal length with large pixels. So resolution (exclusively) determines the size of the object in the final image. Put more pixels under the telescope's projected image and you'll get a bigger image on your screen when viewed full size (1 camera pixel for 1 screen pixel.)

Photographic speed is not determined by focal ratio though folks frequently insist that it is. Speed (exposure time) is determined by how much light gets onto each pixel. A slow F ratio scope used with large pixels can put exactly the same amount of light onto each pixel as a fast F ratio with small pixels. We can make our pixels 'bigger' by binning them 2x2 or 3x3, etc.

The take away from this is that, by binning your F10 scope's data in hardware (CCD) or software (CMOS) you can speed it up. However, imaging at high resolution is less tolerant of error than imaging at low resolution and an alt-az mount will produce massive errors. I would say, Try your scope by all means but don't throw any money at it. Don't buy a reducer, don't buy a wedge, just try it as is. If you get the bug you will be very unlikely indeed to want to stick with the SCT for deep sky imaging and just as unlikely to find a buyer for an imaging-fitted SCT.

Olly

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On 13/03/2022 at 20:50, pike said:

I'd like to see what is possible.

I'm not sure what model you have, but we couldn't get anywhere with a DSLR and this model. We got the impression that for deep sky objects you'd need a well guided equatorial mount. It wouldn't hold focus and didn't follow objects accurately enough. Almost certainly our lack of understanding of this type of telescope though.

But hey, with a zwo, you could be talking of an ideal combination for stuff like the ring nebula and other small stuff like globular clusters.

Perhaps best not to invest at this stage but instead take it along to an astro club, hook up a zwo and see for yourself.

Cheers and HTH

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4 hours ago, ollypenrice said:

Chip size of itself has no effect whatever on the size of the object on your final image. The small chip imposes a natural 'crop' but a larger chip can be cropped to give the small framing of the smaller chip.

Thanks Olly. What I referred to as excessive magnification was the image size presented by telescope simulators such as Telescopius, when using a large focal length scope and various DSLR and astro camera sensors. This appears to show image size has an inverse correlation with sensor size for a given focal length. As such the focal length is the determining factor. This then suggests that a scope such as my 8SE with its large focal length is sub-optimal for all but the smaller DSO's. However that is by definition unlikely to work with the alt-az mount and field rotation This and the fact that the 8SE is almost at the mount's weight limit and the conclusion I draw is that using an 8SE for this purpose is a bad plan. This is especially the case as I did also consider a piggy-back mount to carry a camera on the 8SE. I did buy the 8SE for visual use so all is not lost. The next step is to look at alternatives or perhaps try a camera, alone, on the mount and see what can be achieved.

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2 minutes ago, pike said:

Thanks Olly. What I referred to as excessive magnification was the image size presented by telescope simulators such as Telescopius, when using a large focal length scope and various DSLR and astro camera sensors. This appears to show image size has an inverse correlation with sensor size for a given focal length. As such the focal length is the determining factor. This then suggests that a scope such as my 8SE with its large focal length is sub-optimal for all but the smaller DSO's. However that is by definition unlikely to work with the alt-az mount and field rotation This and the fact that the 8SE is almost at the mount's weight limit and the conclusion I draw is that using an 8SE for this purpose is a bad plan. This is especially the case as I did also consider a piggy-back mount to carry a camera on the 8SE. I did buy the 8SE for visual use so all is not lost. The next step is to look at alternatives or perhaps try a camera, alone, on the mount and see what can be achieved.

Unless you plug in the pixel dimensions, the simulator can only tell you how the target will be framed on the chip. This is not without interest, of course, because if it won't fit you'll need to do a mosaic.  However, it doesn't tell you anything more than that about the suitability of the setup or the final size of the object image. 

I think your decision is wise. I began by spending a lot of time and effort trying to set up a Meade LX200 for imaging but never managed to get a workable image from it. People do manage it but I knew it wasn't for me.

Olly

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  • 2 months later...

As Olly I found imaging with a 12 inch SCT disappointing even on a good mount as the Mesu 200. The amount of effort and tinkering made me spend more time troubleshooting than actual imaging, and the results were not that good for me.. SCTs are fine for visual, but for imaging I've switched to refractors. They do produce good results with planetary imaging though..most likely because planetary imaging is not very demanding on the gear 

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