Using PHD2 with long FL Scope + Ultrastar


I have only used PHD2 with a widefield (SVQ100) scope until just recently. With the SVQ100 I have had no problem with finding stars or the brightness of stars.

Recently I have started to use a C11 Edge @ 2800mm. I an finding that the stars are a lot dimmer and fewer of them. I have been having “Star Mass Loss” issues as well. I have tried binning the Ultrastar to 2x2 but it doesn’t seem to help a lot.

I am using 5 sec exposures…should I up that to ten seconds to try and get a brighter star? What settings would help improve the Star Mass at this FL and configuration?

A focal reducer will help, it will increase the field of view so giving you more stars and may make them brighter. Also applying a dark frame may improve the signal to noise ratio.

Don’t know if this will help since I am using an IF ONAG, but I have a 2127 mm f/l scope using phd2 and ultrastar. The inventor of the onag suggested using 15 second exposures and I’ve had no problems with guiding under reasonable seeing conditions.

The whole purpose of guiding is to introduce as few corrections as possible to the mount ( chasing the seeing).  For my operations, the 15 sec. works well, but it depends on a lot of things such as the mount, flexure, ect.  BTW, I don't live where seeing is optimal.

What are you using for guiding? onag or off? If you are using a guide scope as off axis, then the longer f/l will cause fits if it is not rock solid. I’ve tried that method with limited success on long f/l. It worked well with the fsq I have. MMOAG was a complete waste of money for me as I didn’t have a rotator, hence the reason I went with the IF ONAG.

Mirror flop is another issue with the celestrons.

I use a 12"LX200 Classic with a Starizona 7.5 SCT Corrector. This puts the effective FL around 2182 mm for the image camera. I also placed a small 0.5x reducer directly in front of my OAG mounted Lodestar guide cam. This puts the Lodestar at around 1091 mm. The setup can guide fine with 2 second exposures.
I’ve had this arrangement forever, and recently wondered if it was really doing its job. I pulled the 0.5x reducer and found the Lodestar’s FOV was indeed smaller and less effective. So back with the 0.5x.
Even so, finding a suitable guidestar can be challenging depending on the target’s location, at least with my sky.
Hope this helps.

Thanks for the suggestions.

I am using a SX OAG with the Ultrastar. The C11 Edge mirror is locked as I am using a Moonlite focuser. I will try the 15 sec exposure and see how that works.

I guess I should also up the “Search Region” to 50 or 60 pixels? Change the “Star Mass Detection” to maybe 100% instead of the default 50% or just uncheck this box?

Good grief - the whole purpose of guiding is to keep the star in one place on the sensor - so if the star moves a bit in one second then you should make a correction promptly. I have no idea where this idea of “making as few corrections as possible” comes from. In any control system you don’t want to make more corrections than are needed - but would you say that when driving a car you should adjust the steering “as few times as possible”? Should you resist and only make a change every 15 seconds while driving down a busy road with turns?

I guide EdgeHD11 at f/10 on cge-pro with corrections every second - and I think any mount would do well with such corrections - as long as you have a good guidestar and low latency in the corrections. My resulting fwhm’s are in the low 1 arc-second range - and it is not a so-called high-end mount.

So for the OP - some oag’s will work better than others for finding faint stars - and it’s important to set them up so they can get as much light from the star as possible. EdgeHD has a well corrected field so you should be able to focus a faint guidestar onto a small spot of the detector and guide well with it. But with a relatively small sensor you would be better off if you actually selected a guidestar ahead of time and rotated the OAG to find it. This takes some practice - but will allow you to have a much brighter star than you would have found by random chance.

A key aspect of oag setup is to have the guide sensor as close to the prism as possible - so that no spacers are needed on the imaging camera side.

So before you worry about software settings and go to 15 second exposures (!!) I recommend making sure the oag is set up well - and if need be learn to rotate the oag for the best guidestar possible.

I have a pyxis rotator on my EdgeHD11 so it is easy for me to pre-select a guidestar and dial it in - but I used SCT’s with oag for years before the pyxis and did it all manually.


Well Good Grief Frank,

I realize that you are the metaguide guy with an agenda and your arguments with Gaston are well known on CN, so why not keep them there? That’s why I bought the IF ONAG and not your system. If you have to correct every second, then there is a fundamental problem with the system mount, flexure, PA, backlash, PEC, ect. The longer your guide intervals and hopefully, the smaller your corrections, the better your system. A longer guide interval also allows you to capture dimmer stars and not chase the seeing.

I am able to guide with sub 1" using the IF ONAG system and 15 sec guide intervals with PHD2. No arguing that point Frank as the prove is there. Maybe I’m old school and to the OP I’ve not used a Celestron scope and whatever mount, but Rod Wodowski stated the following:

"Any mount that needs guide intervals of 3 seconds or less is one that I would call problematic. It’s very convenient to be able to use guide intervals of 7 - 10 seconds because it allows you to use dimmer guide stars. A mount that consistently delivers these longer guide intervals without causing guiding errors is an excellent mount, especially if it does so at long focal lengths (2000mm and longer).

So I’ll stand by my statements Frank, though I may be old school and perhaps you have reinvented the wheel with regard to autoguiding. If you have, congrats.

My apologies to the OP as I did not intend this to turn into an advertisement/arguement. You do have one of the best guide cameras out there. If you need further assistance with the celestron and guiding, PM me as I know someone who has used that scope with PHD2 and been very successful

Hi All,

Thanks and I do appreciate the different perspectives being offered. I should have mentioned that my mount is an AP 1100GTO, so its no slouch. I don’t even need to guide for the most part as the ASI1600 usually has me at less than a minute for LRGB, even at F10. But, I do want the “Dither” feature which is why I am trying to figure this all out.

Again Thanks

You do have a good mount there and the short exposures that the cmos cameras provide is a real plus for integration times. I’ve had the ap 900, 1100, and 1600 mounts. I sold the 1600 and now have a 10 Micron 2000HPS .

For the 10u mount unguided exposures of 1 hour plus have been documented in less than pristine seeing conditions by a friend who was using a TEC 160mm. I’ve never found the need to go longer than 30 minutes for NB with my FSQ. For the longer f/l 12.5", iDK, the best I was able to do was 20 to 25 min. unguided, and that was with FWHM of 2- 2.5 ( really good for here ). Thus the need for guiding. The IF ONAG provides a large fov, so finding a guide star has never been a problem.

I’m using SGP for the automation part and operating remotely via ethernet. SGP provides for 3-4 levels of dithering and integrates well with PHD2 for guided operations. Between the IF ONAG, 10u mount, and SGP/PHD2, my guiding is so much easier.

Hope all goes well with the guiding and dithering operations that you are embarking on.

I’m another ONAG user here.
As I see it, we have two rather different philosophies at work. Proponents of one seem not to appreciate the other. Mounts that are not precisely aligned, and of lesser mechanical quality, require frequent corrections but precisely aligned, “premium” mounts do not, although obviously focal length plays a role here.

I use a 2465mm FL, f/6.7 OTA on an AP1600 mount with an ONAG-XT and Lodestar X-2. Sometimes, although not “usually” I am faced with a decision. To use a very faint guide star or plod out to the observatory to adjust the X-Y stage of the guider to acquire a brighter guide star.
I have learned two things . . .
1/ I can guide and guide reliably at 15 secs with the ONAG and Lodestar X-2, (binned 3X3 IIRC) but . . .
2/ I do run into “lost guide star” issues if I go too low on the SNR.
In the 800 hours or so of imaging I have done with the ONAG as guider, on only two targets was I “forced” to adjust the X-Y stage to get a brighter star. There are times when I wish PHD2 allowed guiding at even longer integration times than 15 secs!

I can’t address how well a C11 on an AP1100 would do, but I would expect pretty similar results as the FL is similar to my OTA and the mount is of the same design and quality. The OTA is about a full stop slower though so guide stars will be fainter.

If your mount is of high quality, and precisely aligned, guiding at 15 seconds works very well!

You raise some good points here Terry and sounds like you may have an AGO 14.5" IDK. I’m looking to upgrade mine to that beauty from my 12.5" AGO.

You are correct that the AP mounts require a shorter guide interval. I think Roland recommended a minimum of 5 to 7 sec, if I remember correct. He was really down on short guiding intervals; otherwise, you just chasing the seeing.

I also agree with you that I would go with even longer guide intervals, if PHD2 would allow that. At one time they had spoken of doing that. As far as I know, 15 sec is still the max. There has only been two times that I have had to move the X-Y platform to fine tune the guide stars. The IF ONAG has been a real epiphany for me with regard to guiding and a fabulous asset.

My agenda here is to help someone with OAG and guiding - to get best results with a given system. Before chasing issues with software it’s good to make sure the hardware setup is optimal in the first place, and to use good technique.

The claim has been made that the purpose of autoguiding is to make as few corrections as possible - and I think that is complete nonsense in a very obvious way - and it does not help the OP at all. It is also a statement made with no reference or authority. It’s a statement about a control system - and it makes no sense.

I think when there is a question about oag guiding - people with experience and good results with it are in the best position to answer - especially if it is not on a high end mount. High end mounts can tolerate a long time between corrections - but other mounts benefit and in fact need more rapid corrections.

I have been promoting oag use for many years now and my recommendations apply to any guide software. In the past there were very few oag users and when it was brought up, people complained that it was too hard to do and a guidescope was just as good. Over time more people have tried it and more camera makers are offering integrated systems with oag - and good results have been coming in from happy users. Many of the obstacles amount to myths and misconceptions - and I think it is good to dispel them when they have no basis and block progress.

In this case there is a new user who is getting things set up and having trouble - and that is normal. I am offering help based on my experience helping many people getting started - with oag - not something else.

Now that the OP has specified an AP1100 - he can in fact probably do ok with very long exposures and may not need to rotate the oag to find guidestars. But it is still important for him to optimize the optics of his setup so he is getting as much light from the star as possible. My advice to have the guide sensor as close to the prism as possible - so I would make sure there are no unnecessary spacers on the imaging side, and the ultrastar is inserted into the guide port as far as allowed - if that is possible. Also make sure the prism is oriented properly to receive light - which can be checked by looking in the guide port when aimed at blue sky.

Finally - focus the guidestar as sharply as possible - after the imaging camera has been focused.


Wow Frank,

Lucky I got you for the authority and no bias. Thanks for straightening me out. First you imply the 15" guiding interval is ridiculous. Now you come back and say well it’s an AP1100. I clearly stated that flexure, PA, pec, guide calibration, ect. could be a problem with his system. The whole point of guiding is to make as few corrections as possible and that only comes with a system that is well tuned on many different fronts. The basic rule is to use the longest exposure one can afford with a given mount and setup (i.e. mount quality, the use of PEC, quality of polar alignment, flexure, the use of point/speed models, …) since we do not want to “chase” the seeing or work with too low SNR values. What you seem to be say in a rather clumsy way is that the more corrections that are made to a mount the better the guiding. Running guiding intervals at 1sec intervals is ridiculous for a well designed mount.

A well adjusted, and well balanced mount along with the proper software configuration for things such as aggressiveness, guide exposure/calibration, PA, and lets throw in a good model for the mount, go a long way to creating a mount that can go autoguided for long periods of time with few corrections. Why use guiding at all, if you don’t have to? That’s why the encoder thing is now starting to sweep the astroimaging hobby. A good encoder based mount along with a good model makes imaging a joy. The new cmos based cameras and their short integration times are also making inroads toward unguided imaging. Longer focal length will require guiding whether encoders are used or not

There is no one size fits all when it comes to guiding, but what you propose is just chasing the seeing at 1 sec. Guide star limit magnitude gain versus exposure time is severely affected with this interval.

I’m sorry - but that key point you keep making: “The whole point of guiding is to make as few corrections as possible” makes no sense to me and I don’t know where it comes from. If you have a guide error - why not correct it as soon as possible so it doesn’t linger in the image?

This is a long standing notion and I don’t know where it started from - but it is an obstacle to getting good results with mid-range equipment - and in fact there is no reason rapid corrections would hurt even a very high end mount as long as they are accurate and timely - which they should always be.

I agree there is no “one size fits all for guiding” - and that is why general advice to go for very long exposures and rare corrections is I think harmful.

My general advice is to get the optics set up as well as possible so you are getting the strongest signal from the guidestar you can. And then make corrections as often as possible - as long it shows benefit in the guide error and the final star images. There is no reason to lean toward minimal corrections as optimal.

In the case of a high end mount - there may be less need for rapid corrections - but at the same time they should do no harm - as long as you tune the software properly and as long as you have a good centroid.