Best guidescope and camera combo..

[LightBucket]

Hi all

i have been using an Altair 80mm maxiguider for a couple of years now for guiding  both my 8” SCT and my 80mm frac, with a standard lodestar guide cam, but to be completely honest never been that happy with the guiding, so am lookimg for some advice..

For my two set ups is there a one fits all in the case of guiding, if so what combo of guidescope and camera, I realise the lodestar pixels are pretty big and wonder if that is part of my issues...

I have both the 80mm Altair maxiguider and the 60mm version too.

imaging camera is Atik 383L+ mono, imaging scopes are 8” SCT @ f7, plus 80mm triplet @ f6, (not both together..lol) 

Thanks in advance..:)

William 

 

[fwm891]

I'm using a TS 80mm guide scope with a Lodestar with no problems. Rather than using the short rail that came with the TS guide scope I took the rings off and put them as far apart as a could on the main OTA. That took out all the vibrations coming from the single stalk under the supplied guide scope rail.

I've also added a pale yellow filter (a UV/IR cut filter would be better) before the Lodestar to reduce and focus difference between the red/blue which helps slightly in sharpening up the guide star.

[vlaiv]

What sort of guide issues do you have?

What mount are you using, and what do you expect out of guiding?

If it's something indeed related to guide setup, then have you considered OAG?

[LightBucket]

I have an EQ6 Pro belt modded...

The issues I have have been varied, lack of stars, just general bad guiding graphs with too many corrections and yet PA is excellent, just not happy.

I have used an OAG in the past with the SCT but just could not get on with it..

What sort of problems would I get with the size of the pixels on the lodestar on the 60mm guide scope, (not really tried that) as it will give a large pixel scale, and don’t thing that would be too good, also would a 60mm guide scope guide the 8” SCT ok, or is it a bit small..?

[vlaiv]

Let's see what would be guide resolution here.

Lodestar has 8.4um pixels (actually 8.2x8.4um, but let's take "coarser" one).  Not sure which one is your guide scope, but let's go with 60mm FL 225mm - that one seems to be 60mm AA guide scope.

That would give 7.7"/pixel guide resolution - way small I would say. Let's go with safe 1/16 precision - that means that centroid algorithm is capable of determining star position to ~0.48" accuracy. Not good at all - star can appear as if +/- 0.48" away then it really is.  Depending on your target RMS, you want that number to be at least 3-4 times smaller than target RMS.

So if you for example want 0.6" RMS (and that would be quite doable with modded / tuned EQ6) - you need above precision to be something like 0.15" or 3 times less than it is now. So with Lodestar (which has large pixels) you want about three times more guide focal length - or about 675mm. Other option would be to get new guide cam with small pixels - something like 1/3 of Lodestar. That means camera with less than 3um pixels - something like 2.4um or 2.9um (but first would be better).

I wonder why would you have a lack of guide stars. Such large pixels should be really sensitive to stars (almost whole star fits onto one pixel). What sort of guide exposure do you use?

Btw, in order to have something like 0.6" RMS on EQ6 - you need to really tune the mount (besides belt mod) - this means almost 0 backlash, very stable tripod or pier, and very smooth running mount (good bearings, proper greasing and tension on anything inside).

[LightBucket]

11 hours ago, vlaiv said:

Let's see what would be guide resolution here.

Lodestar has 8.4um pixels (actually 8.2x8.4um, but let's take "coarser" one).  Not sure which one is your guide scope, but let's go with 60mm FL 225mm - that one seems to be 60mm AA guide scope.

That would give 7.7"/pixel guide resolution - way small I would say. Let's go with safe 1/16 precision - that means that centroid algorithm is capable of determining star position to ~0.48" accuracy. Not good at all - star can appear as if +/- 0.48" away then it really is.  Depending on your target RMS, you want that number to be at least 3-4 times smaller than target RMS.

So if you for example want 0.6" RMS (and that would be quite doable with modded / tuned EQ6) - you need above precision to be something like 0.15" or 3 times less than it is now. So with Lodestar (which has large pixels) you want about three times more guide focal length - or about 675mm. Other option would be to get new guide cam with small pixels - something like 1/3 of Lodestar. That means camera with less than 3um pixels - something like 2.4um or 2.9um (but first would be better).

I wonder why would you have a lack of guide stars. Such large pixels should be really sensitive to stars (almost whole star fits onto one pixel). What sort of guide exposure do you use?

Btw, in order to have something like 0.6" RMS on EQ6 - you need to really tune the mount (besides belt mod) - this means almost 0 backlash, very stable tripod or pier, and very smooth running mount (good bearings, proper greasing and tension on anything inside).

Thanks for that, interesting..

Is this for imaging with 80mm frac or 8” SCT...or does it not make a difference... (sure it does)

[vlaiv]

Just now, LightBucket said:

Thanks for that, interesting..

Is this for imaging with 80mm frac or 8” SCT...or does it not make a difference... (sure it does)

In general you want your guiding to be the best possible regardless of scope used for imaging. However, since guiding error is one of the sources of blur, and sources of blur add in similar way as other noise (one can think of blur being noise in star position) - and that is as addition of linearly independent vectors - at some point we get into zone of diminishing returns. Best way to describe it is via right angled triangle - as one side gets longer compared to other - hypotenuse to longer side difference gets smaller. 

This is why there is a rule of thumb that guide RMS should be half or less of imaging resolution.

I think you should "optimize" guiding with respect to SCT - being longer focal length, and therefore probably having higher resolution (smaller "/pixel value). 80mm frac will certainly be fine with respect to this. Let's quickly crunch some numbers.

8" SCT at F/7 is ~1400mm FL, and KAF8300 is 5.4um - that gives 0.8"/pixel (which is oversampling, but let's not worry about that at the moment). Ideally you want your guide RMS to be 0.4". I don't think that is doable with EQ6 class mount (well not reliably, or on average, maybe sometimes on exceptional nights it can be done), so you need to settle for what the mount is capable of - and that would be around 0.6". This should be your "target guiding error".

Mind you, this is aiming for ultimate performance, and if you can't get there with reasonable effort - just don't worry about it - difference between 0.6" RMS and 0.8" RMS guiding on night of very good seeing will probably be less than difference between good and average seeing nights.

 

[ollypenrice]

A reflector is always best guided by OAG. Mirror movement is the reason. It's also known as mirror 'flop' but that rather overstates the case. If you follow the steps logically an OAG should work. Rotate the OAG so that the prism enters the light cone at right angles to the long side of the chip. This ensures that you can push it in as far as possible for brighter stars without it shadowing the chip. Use flats to find out the deepest you can go with the prism without it appearing. Then get the focus as good as you can. (This is not necessary or even desirable with a guidescope but with an OAG sharp focus will find more stars.)

I guide our refractors with simple ST80 guidescopes with a Lodestar in one, an Atik 16ic in the other. I find this allows me to image happily at 0.9"PP with the TEC140 from a guide setup working at 4.2"PP. This is using premium Mesu mounts with very slow PE, meaning I can use 4 second guide subs which stabilize the seeing effects. With our EQ sixes we do need to use shorter subs, not much more than a second, quite often.

While I would expect your guiding system to struggle with the SCT I would have expected it to work with the refractor. I'd be inclined to look elsewhere than the guidescope itself for ways to improve it.

Olly

[LightBucket]

I have the 80mm maxiguider too, which is 328mm fl, so guess that would help a bit but still nowhere near ideal with the lodestar, so I am considering swapping the lodestar for the ASI 120mm mini, with 3.75 pixels with the 80mm guidescope, which gives 2.3”/ pixel, and that seems a bit more like it..yes..?

[vlaiv]

15 minutes ago, LightBucket said:

I have the 80mm maxiguider too, which is 328mm fl, so guess that would help a bit but still nowhere near ideal with the lodestar, so I am considering swapping the lodestar for the ASI 120mm mini, with 3.75 pixels with the 80mm guidescope, which gives 2.3”/ pixel, and that seems a bit more like it..yes..?

That will certainly be improvement in guide precision, and if that is the reason for poor performance, you will be able to tell.

[mcbras]

Hi there folks!

I'm running in a small trouble with my autoguide setup. I've done some unguided astrophotography only with DSLR, but after a couple of years after I bought an used ATIK 16IC I decided to go guided.
My current setup for guiding is SkyWatcher Evoguide 50ED to be coupled with the used ATIK 16IC. Unfortunately I wasn't able to get anything in Artemis software or PHD2 guiding. I've ended up pointing the scope to the moon and I got moon shots but completely out of focus. I did all possible focus steps, even removing the 40mm extension and nothing, can't really understand if I need to gain more distance from the scope to the camera or the other way around. With a 25mm eyepiece, I got focus nearly at the end of the focus travel
Per manufacturer, the guidescope back focus is 60mm witha  focal lenght of 242mm

Any hints on this would be highly appreciated!

Thanks!

[Anne S]

I switched from a Starlight Xpress ex guidehead because of the guide scale I was getting. The pixels in the guidehead were the same as the Lodestar but permanently binned. Perfect for an OAG. I us the Altair 60mm finderguider. I now guide with an SX Superstar which has 4.65 micron pixels which I can bin if I need to. I also have an Altair GPcam which I use for my travelling setup which doubles as a solarimagint camera on our modded PST. That works well for both uses.

i did some calculations comparing imaging scale for my various setups which led me to ditch the guidehead. I was about 10x the resolution of my imaging setup and it was showing,

[michael8554]

Hi mcbras

Subtract the back focus of the Atik from 60mm and that's the length of M42 spacer you need, with the focuser mid-range.

Then try it during the day on a distant object.

You'll need a fast exposure and maybe stop the objective down with a cardboard mask if it's  too bright during the day. 

Then finetune on the moon and then use the Star Profile screen in PHD2 to focus a star's HFD to minimum.

Michael 

[mcbras]

18 hours ago, michael8554 said:

Hi mcbras

Subtract the back focus of the Atik from 60mm and that's the length of M42 spacer you need, with the focuser mid-range.

Then try it during the day on a distant object.

You'll need a fast exposure and maybe stop the objective down with a cardboard mask if it's  too bright during the day. 

Then finetune on the moon and then use the Star Profile screen in PHD2 to focus a star's HFD to minimum.

Michael 

Michael,

thanks for helping out. I'll try to do some tests like you said!

Regards!