Optimum guide scope

[Robculm]

Hi, I'm wondering if my current guide scope set up is optimum for my scope / camera. Sorry, I know there are lookups for this somewhere, but don't recall where.

I have the 200PDS, with x0.9 reducer (coma corrector) so focal length 900mm.

EOS800D, so pixel size 3.72um.

ASI 120 mini camera (3.75um pixel size).

ASI mini finder/guider, 30mm aperture, 120mm focal length.

I selected the guide set up without much thought, mainly going for low weight (on my HEQ5pro), but now I wonder if it's really optimal.

Many thanks,

Rob

[Stuart1971]

So,you are imaging at a pixel scale of 0.85 “/pixel and guiding at an image scale of 6.45”/pixel, and if we use the maximum 4x rule, then no, your guide image scale is much more than 4x, in fact it’s over 7x….the imaging scale, you really need a longer focal length guidescope…..sorry if it’s not what you wanted to hear….but you should be aiming for a guiding image scale about 1-3 times the imaging image scale….👍🏼

Edited February 10, 2022 by Stuart1971

[Robculm]

Hmm, thanks Stuart.

So something like this would be a better match:

https://www.firstlightoptics.com/guide-scopes/sky-watcher-evoguide-50ed-guidescope.html

Cheers,

Rob

[Stuart1971]

10 minutes ago, Robculm said:

Hmm, thanks Stuart.

So something like this would be a better match:

https://www.firstlightoptics.com/guide-scopes/sky-watcher-evoguide-50ed-guidescope.html

Cheers,

Rob

That would give you a guiding pixel scale of 3.2”/pixel, so much better, and just under the 4x…..but only just……so maybe with your scope and camera you want to be looking for a guidescope with around 320-360mm or thereabouts….focal length

but that one you linked to would be much better….👍🏼

[Robculm]

Thanks Stuart, I will do a little more research. If I'm going to fork out that much I'd better choose something really optimal. Any suggestions welcome.

The other concern is how much a larger guide scope will overload my HEQ5! I'm hoping to upgrade it later in the year, but not for the time being....

Cheers,

Rob

[Stuart1971]

5 minutes ago, Robculm said:

Thanks Stuart, I will do a little more research. If I'm going to fork out that much I'd better choose something really optimal. Any suggestions welcome.

The other concern is how much a larger guide scope will overload my HEQ5! I'm hoping to upgrade it later in the year, but not for the time being....

Cheers,

Rob

The other option is to get the one you linked to, and add a cheap 0.5x reducer to the front nosepiece of the guide camera 

Something like this, and that will turn that guidescope into approx 360mm, which would be perfect….

https://www.firstlightoptics.com/astro-essentials-eyepieces/astro-essentials-05x-1-25-focal-reducer.html

[Stuart1971]

My mate actually uses this and so do many others….and swears by it, it’s 250mm focal length…so still use the reducer….

https://www.amazon.co.uk/Telescope-Guidescope-Helical-Finderscope-Astronomical-default/dp/B08CZL8C91/ref=sr_1_6?crid=1HGDO79JAH83D&keywords=60mm+telescope&qid=1644535653&sprefix=60mm+telescope%2Caps%2C59&sr=8-6

[Robculm]

Hi Stuart, so as a starting point I could use the reducer (https://www.firstlightoptics.com/astro-essentials-eyepieces/astro-essentials-05x-1-25-focal-reducer.html) on my current set up?

 

[Clarkey]

 

15 hours ago, Robculm said:

Hi Stuart, so as a starting point I could use the reducer

Don't forget, if you use a 0.5x focal reducer you are increasing the focal ratio. F8 will be quite slow for guiding.

I have used the standard SW finder with a c-adaptor with the asi120mm for guiding my 200p. It works fine. 4x lower pixel ratio with phd2 will not be a problem. (I have also used an ST80 to guide an RC8 which is similar in scale).

[Stuart1971]

54 minutes ago, Clarkey said:

 

Don't forget, if you use a 0.5x focal reducer you are increasing the focal ratio. F8 will be quite slow for guiding.

I have used the standard SW finder with a c-adaptor with the asi120mm for guiding my 200p. It works fine. 4x lower pixel ratio with phd2 will not be a problem. (I have also used an ST80 to guide an RC8 which is similar in scale).

I think you have that the wrong way round, a Barlow will make it f8 a reducer does the opposite hence the term reducer…..👍🏼

But it’s a Barlow he needs not a reducer, I did have that the wrong way round….

Edited February 11, 2022 by Stuart1971

[Stuart1971]

16 hours ago, Robculm said:

Hi Stuart, so as a starting point I could use the reducer (https://www.firstlightoptics.com/astro-essentials-eyepieces/astro-essentials-05x-1-25-focal-reducer.html) on my current set up?

 

No, ignore that, I had that the wrong way round, I was tired last night when I replied…..you would need to increase the focal length of the one you have, so a Barlow would be needed, so a 2x Barlow would make it 240mm, and a 3x would make it 360mm, but then your f ratio would increase

I think your best to either go for the 50mm one you linked too last night, and try that, it should be ok, as it’s much better than you have ATM, and it’s generally regarded quite highly for guiding….

[Clarkey]

3 hours ago, Stuart1971 said:

I think you have that the wrong way round, a Barlow will make it f8 a reducer does the opposite hence the term reducer

I was thinking Barlow but wrote reducer. It must be catching. D'oh.🤪

[alacant]

23 hours ago, Robculm said:

wonder if it's really optimal

Hi

What is wrong with the images you obtain? Fat or trailing stars perhaps? Are these related to guiding? If not ...

 

Edited February 11, 2022 by alacant
Translation

[Robculm]

Thanks Stuart & Clarkey for the clarification.

Hi Alacant,

It's not really that I'm trying to solve a specific problem, just trying to optimise all possible aspects of the set up.

      

I lose some subs, like left above with elongated stars, the right one I would keep as good. But my guess is it's more associated with the worst periodic error points in the worm gear cycle and the tracking can't 'keep up'.

On nights of good seeing my guiding is <0.5" total, but on bad nights (as it was for the above images actually), it's closer to 1" total.

Here's another example:

Actually I'd say this is quite representative of most of my subs.

What I would say, is that on my finished images, I struggle to get the really crisp 'pin point' stars that I see in your posted images!!! Maybe it's my processing, but here's an example:

I can shrink down the centre a little in the process, but it still leaves the fuzzy outer blob! So if I'm not careful I then get a hot centre, dark circle, fuzzy outer.

Anyway, as I said, it's mainly just an exercise in trying to ensure I have everything optimised as much as possible to achieve best possible images from a fairly 'basic' set up!

Cheers, Rob

[Stuart1971]

As you are imaging at 0.85 “/pixel, in theory your guiding should be at least 0.85 RMS, but some say it should be half of your imaging resolution which would be just above 0.4 RMS, but at the end of the day you want round stars….have you tried applying PE, periodic error correction for your mount….? Also there is an algorithm in PHD2 for the RA drive called Predictive PEC, give that a try too, it may well sort you out….

[Robculm]

I'm using the AAP for guiding, direct (EQMOD) to the mount, so don't think there's an option for this. I have tried PEC on the mount back when I was using the handset and recall it being a right palaver!

As I mentioned, longer term I'm likely to follow the advice of a mount upgrade (probably the EQ6R).

[Stuart1971]

39 minutes ago, Robculm said:

I'm using the AAP for guiding, direct (EQMOD) to the mount, so don't think there's an option for this. I have tried PEC on the mount back when I was using the handset and recall it being a right palaver!

As I mentioned, longer term I'm likely to follow the advice of a mount upgrade (probably the EQ6R).

AAP…?? What’s that…?

[Robculm]

Asi air pro

 

[alacant]

Hi

If you wish to be certain the guiding is optimal, the best option would be to use an OAG. This has the advantage of eliminating the effects of tube flexure, less than adequate dovetail connection and poor mirror cell mechanics.

That, along with binning 2x2 will go quite a way to more pinpoint stars.

Cheers

 

Edited February 12, 2022 by alacant

[Stuart1971]

23 minutes ago, alacant said:

Hi

If you wish to be certain the guiding is optimal, the best option would be to use an OAG. This has the advantage of eliminating the effects of tube flexure, less than adequate dovetail connection and poor mirror cell mechanics.

That, along with binning 2x2 will go quite a way to more pinpoint stars.

Cheers

 

What does binning make guiding better, as I thought smaller pixel sensors for guiding do a better job, I have been using a lodestar X2 with 8.3 micron pixels and found it not so good and have now got a camera with 3.75 micron pixels to try, if I bin that I am practically back where I started……🤔🤔

[alacant]

1 hour ago, Stuart1971 said:

does binning make guiding better

Guiding at 900mm focal length, yes. Otherwise with an asi120 and such a reduced field of view, you'll struggle to find suitable guide stars, especially during the up and coming galaxy season. ASI have an efficient bin in firmware.

In the quest for tighter stars, I'd also volunteer that unless the OP has really good seeing, binning the imaging camera would help too, albeit with a dslr, in software.

Cheers

Edited February 12, 2022 by alacant

[powerlord]

Sorry if I'm missing something, but seeing is never gonna be as good as 0.85" surely, so I'd have thought if imaging lower than seeing, go with seeing? I. E. Prob around 1.5-2"..so 6.45"/pixel guiding fine?  Saying that I stopped using my 30mm minis as they just made things too dark. I know use of of those 50mm svbony 190mm you linked to, and an old 50mm SW finder with a 3d printed adapter on the end (182mm). I use them whatever I'm imaging with - whether 135mm, 500, 750, 1300 or 2300. Seems to work fine.

I sometime get those jerks in stars, but it's likely an ra or Dec spike rather than anything else -  check out phd2 log -  maybe increase aggressiveness to pull it back quicker?

Stu

[alacant]

20 minutes ago, powerlord said:

old 50mm SW finder with a 3d printed adapter

As good as any.

TBH, the only time we've noticed much difference (and even then mostly at 1000mm and over) is when switching between guide telescope and OAG; more usable frames to stack.

[ollypenrice]

Before changing anything you can simply look at your guide RMS to see how your guiding matches up with your imaging pixel scale. This requires your guider to be told the FL of your guide scope and the size of your guide cam pixels. If your guide software can't give you this, switch to PHD2 which can. The rule of thumb is that the RMS should be about half the image scale.

But here's the thing: if you are getting some good subs and some bad ones, changing your guider is unlikely to cure this. On the bad subs, something has clearly defeated the guider. The same 'something' will, in all probablity, defeat any guider.  Optimizing the guider is not really going to be about getting the infamous round stars, it will be about getting smaller stars and better resolution everywhere.

Most of us have experience of using a perfectly matched autoguider which did not give optimal guiding. I think that you are in danger of joining this club if you don't get to the bottom of your present problem!

Olly

[alacant]

17 hours ago, ollypenrice said:

in danger of joining this club

+1. May I suggest a different strategy?

Leave the guide telescope until last; it's most likely fine as it is. Working from the mirror cell upwards:

• fit six 1.6mm wire springs. 3 replacements, plus three passive springs over the locking screws. Leave the latter loose.
• seal the primary mirror to the cell using three generous blobs of neutral silicone sealant to coincide with skywatcher's cork
• set the tube rings at least 50cm apart on a Losmandy dovetail
• tie the top of the rings with a rigid aluminium box secion
• remove the rubber o-ring spacers in the focuser
• drill and tap a third screw to the focuser collar
• replace any metal or metal/plastic guide telescope retaining screws with all nylon types

Now you can start to think about the guide telescope. Our preference is direct mounting to the top rail mentioned above.

Cheers

Edited February 13, 2022 by alacant