Does imaging with a smaller sensor require a longer FL guide scope?

[maxchess]

I am experimenting with guiding, and want to understand what I need to guide my ES102  with a ASI 1784 on an HEQ5-Pro.  I have an Orion SSAG mono guide camera and its 50mm guide scope, fl= 162mm. I also have an ST80 fl=400mm.

I am having a bit of trouble guiding with the ST80 using PHD2. I get large guiding errors. I am considering going back to the Orion 50mm. However when I calculate the FOV for the ES102 + ASi1784MC  I see it is  0.6° x 0.4°, which is equivalent to  DLSR on a 2100mm focal length scope! Therefore I wonder if the 50mm/162mm FL will cope.

In the discussions on Focal Length for guiding I only find references to matching the FL of the main and guide scope, not the FOV, but surely its the FOV that affects the apparent angular motion and therefore the required guiding accuracy? Or have I missed something.?

[vlaiv]

Small guide focal length can mask off guide errors due to lack of precision in determining true guide star position. This is probably reason why you see increase in guide error (provided it is expressed in arc seconds - if in pixels then main reason for large guide error is simply higher guide resolution).

Here are some rules that can help you figure out what would be better solution:

1. You need your guide RMS expressed in arc seconds to be about half of that of you primary imaging resolution (so indeed it is not FOV that matters, but rather imaging resolution). For example you are imaging with ES102 and ASI178 (I presume not ASI174 - can't tell from your post, it's probably typo as there is no ASI1784 as far as I know). So you image at 0.69"/pixel it I'm correct (714 FL + 2.4um pixel size of ASI178).

This means that you need your total guide RMS to be 0.35" - that is simply something you can't do with HEQ5 (reliably anyway). Stock HEQ5 will have guide error order of 1-1.5" total rms. Modded and tuned HEQ5 can go as low as 0.5"-0.7" on most nights. With my HEQ5 (completely replaced bearings, tuned, belt mod, Berlebach planet tripod, sturdy dual saddle losmandy/vixen scope clamp), lowest that I've seen in guide precision was around 0.38"-0.4" and that lasted couple of minutes - any wind, or a bit poorer seeing, and it gets back to or over 0.5".

Either use reducer / flattener to bring your focal length down, or bin your images in software.

2. Let's say that you image on resolution of 1.2"/pixel - you need your guide precision to be at least 0.6" RMS total. In order to reliably measure such error you need your guide setup to be able to measure with some precision at least 1/3 of this. So we are looking at guide precision of around 0.2". Current guide algorithms operate with precision of around 1/16 to 1/20 of single pixel. So you need your guide resolution less than about 0.2 * 20 = 4"/pixel.

With 162mm focal length that would mean pixel size of 3.2um. Orion SSAG has pixel size of: 5.2um - so you will simply not have enough precision with this setup to measure such guide error.

Let's see calculation for ST80:

With orion SSAG it will have resolution of 2.68"/pixel - or have centroid precision of around - ~0.14". That is enough for good error estimation down to around 0.5" total RMS - which you should consider as lower limit of what HEQ5 can do at all (tuned and modded - in obsy or when there is no wind).

Good combination for maximum effect would then be ST80 + Orion SSAG. Only thing left to do is get your mount to behave good (consider modding / tuning if you want to go for that level of performance), or settle with the fact that your average guide rms will be around 1" and there fore you need to image at around 2"/pixel - which means at least binning x2 with ASI178 or even binning with focal reducer.

Hope this helps.

[Steve 1962]

I can't see how FOV can affect your guiding.

On my old HEQ5 I used a 70mm f5 to guide a 100mm f9 and visa versa, and used the 70mm f5 to guide a 200mm f5 and a 100mm f4.9 - never had a problem

I would have thought that a 50mm guide scope or yoru ST80 should be able to  guide an HEQ5 adequately - what do your actual subs look like??

Steve

[RichLD]

32 minutes ago, Steve 1962 said:

what do your actual subs look like??

+1

If your subs have round stars then that's that as far as I'm concerned. I've had guide graphs that look horrendous but have ended up with nice round stars on 1200s subs using the Orion SSAG and 162mm FL guidescope with a 900mm Newt. Go for it!

[vlaiv]

2 minutes ago, RichLD said:

+1

If your subs have round stars then that's that as far as I'm concerned. I've had guide graphs that look horrendous but have ended up with nice round stars on 1200s subs using the Orion SSAG and 162mm FL guidescope with a 900mm Newt. Go for it!

Round stars - not best metric to judge guiding - that simply means that error is random enough (equal enough in each direction) - it does not characterize size of error.

Round tight stars is better metric. Again term tight does not lend itself to comparison, so probably best metric would be star Gaussian fit - FWHM and roundness parameters ...

[RichLD]

Just now, vlaiv said:

Round stars - not best metric to judge guiding - that simply means that error is random enough (equal enough in each direction) - it does not characterize size of error.

Round tight stars is better metric. Again term tight does not lend itself to comparison, so probably best metric would be star Gaussian fit - FWHM and roundness parameters ...

Yes, that's correct. I was trying to convey that we can get too caught up in calculations rather than concentrating on the end product and what it looks like. My old metalwork teacher used to say "if it looks right it is right" - I for one am making pictures, not scientific analyses. Not disagreeing with your methodical and scientific approach at all here, just looking at things from a different point of view ?

You're never going to beat the seeing anyhow, which is shocking here at the moment!

Rich

 

[vlaiv]

6 minutes ago, RichLD said:

Yes, that's correct. I was trying to convey that we can get too caught up in calculations rather than concentrating on the end product and what it looks like. My old metalwork teacher used to say "if it looks right it is right" - I for one am making pictures, not scientific analyses. Not disagreeing with your methodical and scientific approach at all here, just looking at things from a different point of view ?

You're never going to beat the seeing anyhow, which is shocking here at the moment!

Rich

 

Quite right about that - nothing wrong with visual presentation of round stars, even if that are a bit bloated (whether it is due to poor seeing, or guiding being less than "tight") - one can always resample down image to account for this and present it at lower resolution to avoid it looking too blurry (I often do that in poor seeing). It's just a question of sampling it at high resolution in the first place - going with high resolution if it makes no sense just impacts SNR in negative way - hence my rant about matching main pixel scale to guide performance (or improving guide performance to match wanted pixel scale).

[maxchess]

Thanks all, for some really helpful advice. I have only been imaging a year and just started exploring guiding. Before that I was doing up to 120 secs unguided and getting (for me) decent results. My best image was https://astrob.in/373575/0/    160 x 60" unguided and I am interested in your comments on what is achievable with my HEQ5 Pro if I guide it. At present I am trying just to get sub 2 arcsec guiding but seem to be getting bad backlash related errors errors.  Once achieved it sounds like I need to stick to my DLSR through the ES102.