Guidescope Recommendation for RCOS 20" ? - Forums

Topic starter

17 days ago

I work at our local observatory where we are fortunate to have an F8.2 RCOS 20" on an AP GTO3600 mount. We would like to upgrade from our current 80mm F6 guidescope to something with a longer focal length. With a focal length of 4115mm, I don't believe that the "normal" rule-of-thumb ratio of 20-25% of the guidescope/main scope focal length applies, because I believe that the guiding accuracy will be seeing-limited. (but I am open to being corrected). However we'd still like to be higher than our current 480mm. We currently achieve total RMS guiding errors in the 0.4' range.

Does anyone have experience with this, and if yes could you make a recommendation for a guidescope that we could use? We have several older ZWO cameras that we could use with the guidescope.

We did consider an OAG, but ruled it out because we have Optec Perseus 4-port mirror in the imaging train and we would need separate OAG's for each of our main cameras.

I should add that our mount does not have the Precision Encoder

Thank you!
Neil

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andrea tasselli

Frequent contributor

17 days ago

Image scale is what counts here, not focal length per se. If your current guide scope can be coupled with a camera (and modern one too) to give 1/5 the image scale of the main imaging camera you're all set and don't need much else. If you can find an old achromatic refractor such as the 4" Vixen f/8 - f/10 or some of the Russian ones that would be sweet too, as at 4" that won't really be that sensitive to seeing which would be felt mostly as scintillation. I have guided with this setup at 2800mm and it was sweet although not light.

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John Hayes

Frequent contributor

17 days ago

Neil, a guide scope is not the way to go on this telescope. You really should be guiding through the telescope to avoid problems with mechanical flexure. You should look into using and off-axis guider. There are plenty of them on the market. One of my favorites is the sagitta OAG from Optec. I use one on my refractor and it works well.

John

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NeilM

Topic starter

17 days ago

Many thanks for the quick replies, Andrea and John!

Andrea, I like the idea of an older F8-F10 and this would likely be an inexpensive (or free) experiment for us.

We use this telescope for:
- Public Star Parties (we capture single subframes or we do live stacking and display the results on large screen TV monitors for the public to see
- Astrophotography
- Research

We have three cameras on the scope : CMOS Mono, CMOS Color and a CCD. Our image scale changes depending on which camera we are using and to what degree we are binning. That said, we can calculate the most commonly used image scale and base the guidescope on that - thanks for that suggestion.

John, we discussed an OAG and rejected it for the following reasons:

- We are pretty certain we can't install an OAG between the telescope and the Perseus 4-port mirror. That means installing three OAG's - one in front of each camera. We're not sure we can handle the complexity of that
- We have two sets of filters in front of the mono camera: UBVRI for the research team and LRGBHaOiiiSii for the Astrophotography team (and obviously we don't stack any filters). We're nervous about how an OAG would work behind the filters.
- Often when we are imaging at that focal length there seems to be very few suitable stars for guiding in the FOV. We thought that a separate guidescope would give us more flexibility for selecting suitable guide stars.

Is our thinking incorrect here?

And one more question if you don't mind… If we do try with a longer focal length guidescope, is there an easy way (other than poor guiding) to determine if we are indeed seeing significant mechanical flexure?

Thank you!
Neil

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John Hayes

Frequent contributor

17 days ago

- We are pretty certain we can't install an OAG between the telescope and the Perseus 4-port mirror. That means installing three OAG's - one in front of each camera. We're not sure we can handle the complexity of that
- We have two sets of filters in front of the mono camera: UBVRI for the research team and LRGBHaOiiiSii for the Astrophotography team (and obviously we don't stack any filters). We're nervous about how an OAG would work behind the filters.
- Often when we are imaging at that focal length there seems to be very few suitable stars for guiding in the FOV. We thought that a separate guidescope would give us more flexibility for selecting suitable guide stars.

Is our thinking incorrect here?

And one more question if you don't mind… If we do try with a longer focal length guidescope, is there an easy way (other than poor guiding) to determine if we are indeed seeing significant mechanical flexure?

BWD is always a consideration but so is achieving acceptable guiding. It sounds like you have a lot of stuff stacked up on the back of that scope. What is the specified BWD? (I've configured a 24", f/8 RCos system and it has 10.1" of BWD.) No matter how rigid the scope might seem, at arc-second levels, everything is made of rubber. That's why guide scopes are not a great approach with larger scopes. A good seeing monitor can be useful for determining if you are at maximum performance. The FWHM of your images should "mostly" match the seeing numbers--at least when wavefront tilt is the primary contributor to seeing. If the big scope is always showing worse numbers, it's likely that you have a problem. It helps to have a rotator with an OAG but with a sensitive camera you can almost always pick up guide stars without rotating. In any OAG installation, you want the pick-off mirror to be as large as possible and as close to the filters as possible to get the largest possible field and to avoid vignetting. Unfortunately, a 4-port selector with a fold mirror is not very compatible with any kind of through the telescope guiding. Here's another option for a 4-port adapter that takes up a lot less BWD: https://www.astronscientific.com/. If it were me, I'd might consider redesigning that thing to incorporate a quick-change port with four different interchangeable instrument packages.

John

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andrea tasselli

Frequent contributor

17 days ago

And one more question if you don't mind… If we do try with a longer focal length guidescope, is there an easy way (other than poor guiding) to determine if we are indeed seeing significant mechanical flexure?

That's quite simple; if your guiding graphs shows no anomaly and it is consistent around the zero correction line but the imaging results show consistent (same direction with respect to the horizon) trailing then you have mechanical differential flexure. But a well designed attachment can be made to avoid it.

NeilM

Topic starter

17 days ago

Thanks John!

BWD is not an issue for us. I don't have the number with me but I'll look it up. The scope is quite forgiving in terms of exact BWD and we use spacers between the Perseus and the filter wheel(s) to get it more or less correct. There is plenty of room for an OAG and rotator.

We have definitely had issues with the FWHM numbers being higher than the seeing predicts - but we believe that has been due to other issues which we hope are now mostly resolved.

Yes, we do have a lot going on with this scope! We have literally dozens of (trained) volunteers working with this telescope each month, which is why we wanted to limit any required mechanical changes. Right now each team can do its work without physically touching the telescope. All changes are made through software and we have strict, documented procedures that all operators are required to follow. (The most important of which is to ensure that telescope is parked correctly before we activate the automatic roll-off roof!). I know that this isn't ideal but it is a good compromise for us and ensures that we maximize the uptime of the scope.

Based on your suggestions I'm going to see if we can test an OAG, at least on the mono imaging train (the one that gets the most use)

with kind regards
Neil

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NeilM

Topic starter

17 days ago

andrea tasselli:
And one more question if you don't mind… If we do try with a longer focal length guidescope, is there an easy way (other than poor guiding) to determine if we are indeed seeing significant mechanical flexure?

That's quite simple; if your guiding graphs shows no anomaly and it is consistent around the zero correction line but the imaging results show consistent (same direction with respect to the horizon) trailing then you have mechanical differential flexure. But a well designed attachment can be made to avoid it.

Thanks Andrea!

Neil