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Greetings everyone.

I recently scored a great deal on facebook marketplace and bought Skywatcher Skymax 180 mm Maksutov-Cassegrain Telescope (f/15) with EQ6R-pro mount. I understand that long focal length telescopes are more suitable for planetary imaging. However, as I am tight on budget, I would like to use the same scope for deep-sky astrophotography. I have Canon Rebel T5 DSLR camera that I am using for taking images. Without autoguiding and a decent polar alignment, I can get ~30 seconds long shots without any star trailing, but that's not sufficient for imaging objects like M51. I would like to be able to integrate longer, say few minutes, thus would like to purchase an autoguider.  Here are some specific questions I have.

1). The telescope comes with an 8 x 50 straight-through finder.  If I were to use it as a guidescope, what type of image integration time can I expect? Has anyone done autoguiding for f/15 scope with an 8x50 or other finderscopes?

2). If the integration time will be an issue with the 8x50 finder scope, I am open to purchasing an off-axis-guider. However, considering the narrow field-of-view of Mak-180 telescope, I am concerned about not having enough photons from guidestar for autoguiding. For instance, with my DSLR camera, I need to integrate 20-30 seconds to see the nearby stars in M51. What type of OAG and camera would I need to autoguide with my scope? Are there affordable cameras (~$200) that would do the job for me? Would ASI120MM Mini Monochrome (~$150) do the job? What about OAG?

I plan to use phd2 software for autoguiding rather than relying on the build-in guide port on the mount.  Instead of integrating for hours, I am planning to do DSS stacking of few mintues long multiple shots, hopefully this will put less strict requirements on the autoguider.

In case this information is relevant: with my current setup, I have no issue pointing my scope to a desired deep-sky object, track the object within the field-of-view of my DSLR camera for hours using the mechnical tracking of the EQ6 mount. For instance following is a single raw image of Ring nebula taken with 30 seconds shot.  But it is not enough for generating high-quality images. 

image.png.02a55b83b6a992fec357669a622a666c.png

 

Thank you in advance for your help.

 

Regards,

Jay

 

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Posted (edited)

I think you are setting yourself up for a lot of frustrations.   Disclaimer: I am not speaking from experience, just what I've read.... 

The focal length between a guide scope and the 180 is too vast for any meaningful guidance I think, so OAG would be the way to go, and as you say you may find it difficult to find a guide star in your target.  I do wonder if you would have significant vignetting too?  I don't know.  Personally I would be putting money into a more tried as tested astrophotgraphy setup.

I wish you luck and would be very interested in your results (as I own a 180 too).

 

EDIT: Having said all the above, this fella https://www.astrobin.com/r3f267/?nc=user looks to have had some success with guiding, doing a lot of 60s exposures on a EQ8.

Edited by scitmon

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The good news is that you seem to have a very accurate EQ6. They do vary.

Any guiding will be better than none but, as you understand already, an OAG is better suited to a scope with a mirror (especially a moving one) and to long focal lengths. Will you find stars? I wish I knew but, binned 2X2 I think you might. Set up the OAG prism as deeply as you can, so entering towards the long side of the chip.

I'm not sure what you mean, here: 'I plan to use phd2 software for autoguiding rather than relying on the build-in guide port on the mount.  Instead of integrating for hours, I am planning to do DSS stacking of few mintues long multiple shots, hopefully this will put less strict requirements on the autoguider.'  PHD will guide using the ST4 guide port on the mount or through ASCOM/EQ Mod using a special adapter. Personally I just use the old ST4 method but pulse guiding might work better for you. Nobody does integration of hours at a time. (Well, somebody will have tried it but 30 minute subs are considered very long.)  Your plan to stack lots of short subs is perfectly normal. You'll probably want to make them as long as possible but this will be down to experimentation.

Your combination of long focal length and small pixels is absolutely not the optimal choice but you know this. However, FLO will supply a new 130P OTA for £176. https://www.firstlightoptics.com/reflectors/skywatcher-explorer-130p-ds-ota.html  If I had to back either your guided Mak or the 130P unguided I'd put my money on the 130P. And you could add guiding later.

Olly

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Posted (edited)

The amount of data gathered in a 1 minute exposure at f4 will need 16 minutes at f11. This is why trying deep sky imaging at long focal lengths is an exercise in frustration. Your guiding will have to be spot on. You will need great weather...imagine the frustration of a cloud appearing at minute 14 of a 15 minute exposure and causing the guiding system to lose the guide star. 😡 Add to that an already noisy DSLR camera where the noise increases as the imaging sensor gradually warms up during the exposure. You may find that short sub-exposures on such a long focal system gathers virtually no data that's visible over the noise floor in the image.

 

I'm not saying that it can't be done. Likewise, you can do your morning commute on a pogo-stick. You'll get there eventually, but it'll be so painful that you're unlikely to do it again. Certainly not as a hobby.

 

Save a few quid up, get a second-hand 80mm refractor or a 130P Newt.

 

Edited by Zakalwe

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With that setup you're going to make things really difficult for yourself... unless you want to give yourself a mission 

If you're experienced in deep sky imaging , got deep pockets and want a challenge then yeah why not

If you're fresh as a daisy then I'd steer clear and use it for what it's good at,  planetary..

  • Thanks 1

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