Suggestion: Guide scope paired with 102mm F7 Refractor

[Amajed]

Hi

Which guide scope do you recommend to guide a Explore Scientific 102mm f/7 Essential Apochromatic FCD100 ED Triplet Refractor Telescope.

Mount is Sky-Watcher EQ6-R

 

Guiding Camera ZWO ASI224MC

 

I already own ZWO Mini Guide Scope

but from my understanding that it's not recommended to use with a telescope longer than 500mm. is that true? if yes, which guide scope should I get?

also of the guide scope needs an accessory to mount it on the telescope please let me know.

 

Thanks

 

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A side question, does this telescope need a flattener?

[drjolo]

I used to guide 130/740 triplet with 50mm guider and ASI290 mono camera (on EQ6 mount) and it worked, but with larger guidescope 60/240 the results were significantly better. I improved guiding further using Fringe Killer filter, because achromatic guidescopes give bloated stars, and with Fringe Killer star diameter were reduced by about 30-40%. 

[vlaiv]

I really depends on your target resolution - what camera are you going to use with main scope and what resolution it will be working on ...

Here are some pointers:

- you want your guide total RMS expressed in arc seconds to be at least half of your imaging resolution (or resolution that you want to present your image at). It is always better to have more precision, but 1/2 is kind of rule of thumb for good results (that does not mean that you can't guide with less precision - only stars will be a bit larger and image just a bit blurrier).

- EQ6R should be able to guide in 0.5-0.7" RMS reliably. In order to guide to this precision, your guide scope needs to be able to "resolve" star movement down to, let's say 0.2-0.3". Current centroid algorithms can achieve precision of 1/16 - 1/20 of a single pixel. 

- With ZWO mini guider you are guiding at 6.45"/pixel resolution - this means that your guider is able to resolve star position down to 0.3-0.4", and my guess is that with this precision your guide RMS will be around 0.8".

So if you are imaging at 1.6"/pixel or more, even this small guider should be ok. With 714mm focal length this means pixel size of ~6.4um. It is most likely that your camera does not have this large pixels, so your options are to go with bigger FL guide scope or to use binning to bring down your final resolution (you can use super pixel mode for DSLR or bin your images in software if using CMOS cameras - they usually have pixel size around 4um).

What would be appropriate guide scope if you decide to go for it? Anything with FL about double the current one will be good, so you are looking for guide scopes with around 240mm FL.

Alternative to guide scope would be OAG - this way you remove guider precision from equation since you will be guiding at same focal length as imaging.

50 minutes ago, drjolo said:

I used to guide 130/740 triplet with 50mm guider and ASI290 mono camera (on EQ6 mount) and it worked, but with larger guidescope 60/240 the results were significantly better. I improved guiding further using Fringe Killer filter, because achromatic guidescopes give bloated stars, and with Fringe Killer star diameter were reduced by about 30-40%. 

Have you tried IR pass filter? IR part of the spectrum is supposed to be much less susceptible to seeing, and it will have similar effect to fringe killer (as will plain #8 yellow).

I have one and did use it on one or two occasions, but not long enough to see if there is significant difference to guide quality.

[drjolo]

13 minutes ago, vlaiv said:

Have you tried IR pass filter? IR part of the spectrum is supposed to be much less susceptible to seeing, and it will have similar effect to fringe killer (as will plain #8 yellow).

I have one and did use it on one or two occasions, but not long enough to see if there is significant difference to guide quality.

I have not tried IR - it may be good idea, but with mono camera. Overall sensitivity limited to IR only will be maybe 20-30% (basing on ASI290QE curve), but should still be enough. However for IMX224 color chip IR pass filter probably will not work. Also simple achromat may have poor quality in IR band (it would need to work from 670 to about 900-1000nm for mono camera). 

[vlaiv]

41 minutes ago, drjolo said:

I have not tried IR - it may be good idea, but with mono camera. Overall sensitivity limited to IR only will be maybe 20-30% (basing on ASI290QE curve), but should still be enough. However for IMX224 color chip IR pass filter probably will not work. Also simple achromat may have poor quality in IR band (it would need to work from 670 to about 900-1000nm for mono camera). 

I actually tried IR pass with OSC ASI185 - it does not have IR/UV cut front glass cover - just plain AR coated. I think most of OSC sensors used in guide / planetary cameras do have significant sensitivity in IR due to their usage in security applications. If you look at ASI224 QE graph, you will see that above 800nm all three colors have same QE - so with IR pass filter (I used cheap ZWO IR) with response higher than 800nm will produce regular B/W image (almost no need to debayer).

You are right that achromat lens used for guiding will not have diffraction limited performance in IR part of spectrum, but I don't think it will be any worse than using full spectrum (or even violet removed). Camera sensitivity goes to approx 950nm (maybe 1000 in some models), so we are talking about effective 150nm "window" (or less) all on red side of spectrum. Achromats have focus shift from nominal position in blue and red part of spectrum, and difference on blue side is bigger than on red (assumed spectrum is centered at 550nm - green), so while there will be focus shift between 830 and 950, it will be the same as 550nm - 700nm shift, and you can center midway, so stars should be about the same size as filtered regular achromat.

Here is usual curve for doublet lens:

Notice focal shift - it is more pronounced to blue part of spectrum then it is on red (if so optimized by design, and usually it is - hence blue / violet / purple halo). If you look at graph, focal shift difference in 600-700nm range is about the same as it is in 450-600 with only center position shifted, if you follow the curve to 800-900 it will be about the same. This means that you can use IR pass with achromat to guide with no problems (apart from some loss in QE and different selection of stars - those that are strong in red/IR spectrum).