TheSkySearchers

Posted: **Sun Jul 18, 2021 10:57 am**

I've been thinking of getting a camera for first wobbly steps in AP.
I think, I could try without guiding first (short exposures, bright targets), and maybe add the guiding gear later.
I understand that things are interdependent, but I haven't found a good description of how.

I have the tube (VX12, 300 mm, 1200 mm, F/4), and I think I'd like to select the camera in a way, that adding the guiding wouldn't become overly expensive or complicated.
What are the requirements for a guiding scope and guiding camera, if I went with something like ZWO ASI 183 MC Color?

Or maybe there's a better approach?
I don't like the idea of getting a DSLR. I'd rather handle things with a computer.

Posted: **Sun Jul 18, 2021 12:14 pm**

The Bintel Tools are good. https://www.bintel.com.au/tools/astrono ... 2b26af01d5

An ASI183MC would be slightly over sampled with that telescope but that's fine.

Starting with a DSLR is not a bad idea, it will connect to a computer just as well as a dedicated astro camera and will cost a lot less than a cooled astro camera.

As a rule of thumb, the guide scope focal length needs to be at about 1/3 of the imaging scope focal length.

Welcome to the rabbit (black) hole!

Regards

Graeme

Posted: **Sun Jul 18, 2021 12:42 pm**

Graeme1858 wrote: ↑Sun Jul 18, 2021 12:14 pm The Bintel Tools are good. https://www.bintel.com.au/tools/astrono ... 2b26af01d5

An ASI183MC would be slightly over sampled with that telescope but that's fine.

Starting with a DSLR is not a bad idea, it will connect to a computer just as well as a dedicated astro camera and will cost a lot less than a cooled astro camera.

As a rule of thumb, the guide scope focal length needs to be at about 1/3 of the imaging scope focal length.

Welcome to the rabbit (black) hole!

Regards

Graeme

I got an impression that the pixel sizes of the cameras have some effect here too?
And how about guide scope aperture (resolution)? Does that have any effect?
And how about the F-numbers?

Posted: **Sun Jul 18, 2021 1:25 pm**

I was thinking, if the focal length of the guide scope is short, the image would be smaller, which requires camera with smaller pixel size to get to the same guiding accuracy as with a guide scope with longer focal length.

And the accuracy needed depends on the main scope and main camera?

Am I making any sort of sense here?

Posted: **Sun Jul 18, 2021 2:12 pm**

astronomy.tools has a calculator that will show FOVs of different objects for different camera / scope combinations. You can overlay them on one screen shot so it's easy to compare.

IF I try EAA/AP, I'm torn between a lower end ZWO to start and a Canon T7 DSLR. The Canon is not very expensive and I could use it for regular photography also. Eventually, I would need some kind of tracker / tracking mount. Or I can save my cash for a Sharpstar 94mm.

Posted: **Sun Jul 18, 2021 3:13 pm**

pakarinen wrote: ↑Sun Jul 18, 2021 2:12 pm astronomy.tools has a calculator that will show FOVs of different objects for different camera / scope combinations. You can overlay them on one screen shot so it's easy to compare.

IF I try EAA/AP, I'm torn between a lower end ZWO to start and a Canon T7 DSLR. The Canon is not very expensive and I could use it for regular photography also. Eventually, I would need some kind of tracker / tracking mount. Or I can save my cash for a Sharpstar 94mm.

My mount (CEM120) accepts, at least, ST-4.
What I'm concerned, is camera/guide scope/guide camera combination given the main scope.
On the other hand, if the guiding is good, it doesn't need to be upgraded, if I want a better main camera.

On the second thought, the main camera may not be so important in this "equation" after all, as long as the guiding is "good enough"?
It just wouldn't be smart to pay a lot for a very accurate camera, if the guiding is rough.

Posted: **Sun Jul 18, 2021 3:24 pm**

I recommend an ASI120MM mini with OAG for autoguiding. This is eventually best considering your focal length and potential flexibility of your aluminum tube. I have a QHY and it works but check the reviews of the ZWO ones.

The diagonal of the ASI183MC is 15 mm. I suggest you consider the ASI2600MC with a diagonal of 28 mm. It is a superior camera with an appropriate FOV for your 1200 mm, and also more forgiving, any errors will be perceived half when the image is viewed full size. Buy once not again later on.

Posted: **Sun Jul 18, 2021 5:07 pm**

SkyHiker wrote: ↑Sun Jul 18, 2021 3:24 pm I recommend an ASI120MM mini with OAG for autoguiding. This is eventually best considering your focal length and potential flexibility of your aluminum tube. I have a QHY and it works but check the reviews of the ZWO ones.

The diagonal of the ASI183MC is 15 mm. I suggest you consider the ASI2600MC with a diagonal of 28 mm. It is a superior camera with an appropriate FOV for your 1200 mm, and also more forgiving, any errors will be perceived half when the image is viewed full size. Buy once not again later on.

Something like this OAG? https://www.astroshop.eu/off-axis-guide ... t2/p,33139
(COVID seems to have made many OAGs unavailable.)
The 183MC is now 674 eur and 2600MC pro 2299 eur.
Have to ponder...

Posted: **Sun Jul 18, 2021 5:58 pm**

That one might be fine however it does not have a helical focuser that mine has, it is the QHY medium autoguider https://agenaastro.com/qhy-off-axis-gui ... oag-m.html . If you intend to use filters in the future make sure that the OAG is appropriate for the filter size. QHY has a small / medium / large size, I forgot what the criteria are but they list what camera they can be used with.

I like having the helical focuser for making small adjustments though I'm not sure if it is necessary. The ASI120MM mini that I have is narrow enough that I can hold it between thumb and fingers with the thumb resting on the OAG so with loose screws I could probably move it manually accurate enough. When it is slightly out of focus I get V-shaped stars in the OAG, when I focus it, they are round again. It will take some effort to become familiar with it.

One more advantage of an OAG is that it is attached to the imager so it becomes one combined piece of equipment that you don't have to worry about as opposed to having a separate guide scope. I use a lanyard on the combination wrapped around the focuser knob to keep it from falling.

The ASI2600MC seems to not need taking darks at all when cooled properly according to others. Taking darks is a PITA because they take a long time (flats and biases are quick). It is 16 bit. So I recommend it, and for all the other reasons I mentioned.

Getting back to your original argument, I know someone who has an SBIG and he mentions that one major advantage is that it can stack 10 second images live. For him this eliminates the need for an autoguider, which is what you had in mind. He has an iOptron CEM120 like yours and an 11" SCT. I don't think the ZWO cameras can do this yet. Maybe it is possible using SharpCap (I don't know because I image on Linux). Check this thread: https://www.cloudynights.com/topic/7321 ... -stacking/

Posted: **Sun Jul 18, 2021 6:41 pm**

I have an astro-buddy that has a 183 and recently bought a 294, then immediately sold the 183 since he considered the 294 that much superior. And it is 22mm diagonal right in between the discussed cameras.

As to guiding, I have been using OAG for 5+ years and recently went back to guide scope for this season. Coupled with using my cmos camera which requires shorter exposure to avoid clipping and the larger fov of the guide scope, I find it easier to find suitable guide stars.

BTW, I guide at 1000mm with 3.8micron pixels and shoot at 2500mm with 3.8micron pixels.

Steve

Posted: **Sun Jul 18, 2021 7:07 pm**

They say the guide pixel scale should be no more than 3x the imaging pixel scale.

Pixel Scale (arc-seconds) = 206 * Pixel Size (microns) / Focal Length (mm).

Posted: **Sun Jul 18, 2021 7:28 pm**

I've seen the posts over at CN that relay the information of three times your Imaging scale to your guiding scale as a rule of thumb. Here is a quote from Craig Stark from Starke Labs on the true guiding accuracy of phd2.

Without any noise, PHD is accurate down to (on average) 0.004 pixels or 1/250th of a pixel. With a low amount of noise, the accuracy goes to 0.018 pixels or 1/56th of a pixel and with high amounts of noise it goes to 0.18 pixels or 1/5.5th of a pixel. Better stars and/or better guide cameras will get you more accuracy, but even with this very noisy star, we're still at 1/5th of a pixel accuracy.

So in the very worst of conditions your image to guiding scale can be as big as 5X.

Here is the article from Craig Stark. http://www.stark-labs.com/help/blog/fil ... curacy.php

Cheers

Posted: **Sun Jul 18, 2021 9:37 pm**

SkyHiker wrote: ↑Sun Jul 18, 2021 5:58 pm That one might be fine however it does not have a helical focuser that mine has, it is the QHY medium autoguider https://agenaastro.com/qhy-off-axis-gui ... oag-m.html . If you intend to use filters in the future make sure that the OAG is appropriate for the filter size. QHY has a small / medium / large size, I forgot what the criteria are but they list what camera they can be used with.

I like having the helical focuser for making small adjustments though I'm not sure if it is necessary. The ASI120MM mini that I have is narrow enough that I can hold it between thumb and fingers with the thumb resting on the OAG so with loose screws I could probably move it manually accurate enough. When it is slightly out of focus I get V-shaped stars in the OAG, when I focus it, they are round again. It will take some effort to become familiar with it.

One more advantage of an OAG is that it is attached to the imager so it becomes one combined piece of equipment that you don't have to worry about as opposed to having a separate guide scope. I use a lanyard on the combination wrapped around the focuser knob to keep it from falling.

The ASI2600MC seems to not need taking darks at all when cooled properly according to others. Taking darks is a PITA because they take a long time (flats and biases are quick). It is 16 bit. So I recommend it, and for all the other reasons I mentioned.

Getting back to your original argument, I know someone who has an SBIG and he mentions that one major advantage is that it can stack 10 second images live. For him this eliminates the need for an autoguider, which is what you had in mind. He has an iOptron CEM120 like yours and an 11" SCT. I don't think the ZWO cameras can do this yet. Maybe it is possible using SharpCap (I don't know because I image on Linux). Check this thread: https://www.cloudynights.com/topic/7321 ... -stacking/

I have a mini laptop running Windows 10 - just in case. All my other computers are running Linux (Debian and Mint at the moment).
Thanks for mentioning the filter size. Didn't come to my mind.

Posted: **Sun Jul 18, 2021 9:59 pm**

So if I "played for keeps", I should first check the maximum resolution of the scope and find the equal resolution main camera? Then I should find the needed guide camera resolution that's 1/5 of the resolution of the main camera (or better), and then find an OAG or guide scope that matches the guide camera resolution?
Is this the procedure?

Maybe with a guide scope I could get rid of the old sneaker I've been using as a counter weight for the finder scope...

What size of image should I get with 300 mm 1200 mm focal length scope?

Posted: **Sun Jul 18, 2021 11:52 pm**

SkyHiker wrote: ↑Sun Jul 18, 2021 5:58 pm Getting back to your original argument, I know someone who has an SBIG and he mentions that one major advantage is that it can stack 10 second images live. For him this eliminates the need for an autoguider, which is what you had in mind. He has an iOptron CEM120 like yours and an 11" SCT. I don't think the ZWO cameras can do this yet. Maybe it is possible using SharpCap (I don't know because I image on Linux). Check this thread: https://www.cloudynights.com/topic/7321 ... -stacking/

I mixed up SBIG and ATIK, the person I referred to uses an ATIK camera with that capability - sorry. See the reply in this thread, https://www.cloudynights.com/topic/7077 ... p=10193413 .

Posted: **Mon Jul 19, 2021 12:18 am**

turboscrew wrote: ↑Sun Jul 18, 2021 9:59 pm So if I "played for keeps", I should first check the maximum resolution of the scope and find the equal resolution main camera? Then I should find the needed guide camera resolution that's 1/5 of the resolution of the main camera (or better), and then find an OAG or guide scope that matches the guide camera resolution?
Is this the procedure?

Maybe with a guide scope I could get rid of the old sneaker I've been using as a counter weight for the finder scope...

What size of image should I get with 300 mm 1200 mm focal length scope?

A few things in addition to those: (1) Seeing is defined as a FWHM disc, while RMS is like a standard deviation so the disc would be, say, twice the RMS value. (2) For a color camera the pixel size of a color element (RGGB) is the relevant size, which is twice that of the physical pixel of the chip.

The subpixel guiding accuracy can vary by a factor 50 (0.004 to 0.2 pixel in Craig Stark's link) for zero noise vs a lot of noise. What matters much more is seeing, which is 2" on many nights. That throws all subpixel guiding arguments out the window because those are just a fraction of the seeing. This reference straightens things out: http://www.wilmslowastro.com/tips/autoguiding.htm . In other words the subpixel guiding accuracy is hardly relevant in most cases.

And the OTA resolution is typically less than the seeing, for instance the Dawes limit for a 12" is 0.39". This number might not be the limiting factor for guiding (like how Craig Stark illustrates for subpixel calculations) but it means there is hardly a point in trying to be better than that.

If you can guide to 0.5" that's pretty good.

Posted: **Fri Jul 23, 2021 6:24 am**

turboscrew wrote: ↑Sun Jul 18, 2021 9:59 pm
What size of image should I get with 300 mm 1200 mm focal length scope?

You need to include the sensor size in the calculation too. The FLO link or the Bintel links above will show you what you will get.

Regards

Graeme

Posted: **Fri Jul 23, 2021 7:22 am**

Graeme1858 wrote: ↑Fri Jul 23, 2021 6:24 am
turboscrew wrote: ↑Sun Jul 18, 2021 9:59 pm
What size of image should I get with 300 mm 1200 mm focal length scope?

You need to include the sensor size in the calculation too. The FLO link or the Bintel links above will show you what you will get.

Regards

Graeme

I meant, the actual image, not photo. I'd think that'd gives the idea of how big the sensor needs to be.

Posted: **Sat Jul 24, 2021 3:18 am**

Some good advice here. Your 120 mount is supposed to have a very low PE (3"?). I know you had some teething problems. If the mount is now working well and you want to start with shorter exposures of brighter objects, I suggest you buy the primary camera only. Try some lucky imaging without any autoguider, repeated short exposures, throw out any bad ones.

Examine the errors you are getting. Try to decide between drive precision and flexure. You may need to use an off axis guider or you may be able to use a guide scope. I would get the main camera first and investigate the mount's performance before deciding whether to go with off axis guiding or a guidescope.

Another approach is to buy a cheap guide camera (ASI120 or QHY5-ii) first but no guide scope. Use that to test the AP performance of the OTA, focuser accuracy, and the mount drive precision before investing a lot of money into a dedicated astro camera.

Cheers
Joe

Posted: **Sat Jul 24, 2021 10:06 am**

OzEclipse wrote: ↑Sat Jul 24, 2021 3:18 am Some good advice here. Your 120 mount is supposed to have a very low PE (3"?). I know you had some teething problems. If the mount is now working well and you want to start with shorter exposures of brighter objects, I suggest you buy the primary camera only. Try some lucky imaging without any autoguider, repeated short exposures, throw out any bad ones.

Examine the errors you are getting. Try to decide between drive precision and flexure. You may need to use an off axis guider or you may be able to use a guide scope. I would get the main camera first and investigate the mount's performance before deciding whether to go with off axis guiding or a guidescope.

Another approach is to buy a cheap guide camera (ASI120 or QHY5-ii) first but no guide scope. Use that to test the AP performance of the OTA, focuser accuracy, and the mount drive precision before investing a lot of money into a dedicated astro camera.

Cheers
Joe

I'm not sure if I got the mount problems fixed. I haven't got a chance to try it, because, when I got the tweaking done, it was already summer = warm and light nights.
Maybe I'll get a main camera first, and see where that takes me.

TheSkySearchers

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