200ds vs 150ds AP differences and finderscope to guidescope adapter

[Stefan87]

Hi everyone

I recently decided to purchase a 200ds for my HEQ5 as I've heard from quite a few people that with a good balance it can work quite well. I wanted the 200 so I could do some Planetary imaging as well as a bit of visual here and there. Mostly AP.

I have a few questions though and I do apologise if they are a bit silly.

I've been trying to wrap my head around aperture vs f/ratio for imaging and it seems there are differing opinions out there. From what I understand aperture is important for visual and f ratio is important for imaging. A fast scope will capture more light. So will there be any difference besides FOV between the 150ds and 200ds Newtonian for deep space astrophotography? If their f ratio is the same then does that mean they both father the same amount of light with a different FOV. Logic kind of says the larger aperture with smaller FOV On the 200ds would mean a brighter image for same exposure time regardless of f/ratio. Just a bit more difficult for guiding. Am in the right track here?

 

Also, has anyone tried adapting the 9x50 finderscope on the 200ds as a guide scope? I feel that saving weight is a great idea in my case and I'm struggling to see a reason not to adapt  the existing finderscope to a guidescope. I plan to purchase an ASI120mm mini for it. Is this a good or bad idea? 

 

Thanks very much in advance

 

Stefan

[Cosmic Geoff]

I suggest you get the book "Making Every Photon Count," the  essential guide to deep space imaging, which may save you from making some expensive and unsuitable purchases.

By 'ds' I assume you mean the PDS imaging version?

A 200mm Newtonian is reckoned to be near the limit for a HEQ5 for visual use.  For deep space imaging, a small refractor or a smaller Newtonian like the 130PDS would be a better match for the HEQ5.

With the same f-ratio, a bigger scope just gives a bigger image - the exposure times etc remain the same.

Trying to use one scope for everything never ends well.

[vlaiv]

3 hours ago, Stefan87 said:

I've been trying to wrap my head around aperture vs f/ratio for imaging and it seems there are differing opinions out there. From what I understand aperture is important for visual and f ratio is important for imaging. A fast scope will capture more light. So will there be any difference besides FOV between the 150ds and 200ds Newtonian for deep space astrophotography? If their f ratio is the same then does that mean they both father the same amount of light with a different FOV. Logic kind of says the larger aperture with smaller FOV On the 200ds would mean a brighter image for same exposure time regardless of f/ratio. Just a bit more difficult for guiding. Am in the right track here?

You are missing the third component in that equation - that is pixel size / imaging resolution.

Two same F/ratios only have the same speed if they are used with same camera / pixel size. If you want to compare two systems, best measure is aperture at resolution. This means - you fix your working resolution and then compare apertures. Sampling resolution - or arc seconds per pixel is very important metric.

What camera do you plan to use with either of these?

[AstroMuni]

4 hours ago, Stefan87 said:

So will there be any difference besides FOV between the 150ds and 200ds Newtonian for deep space astrophotography

Most DSO imagery is done over extended periods of time, so you can effectively gather that additional light using more images. Hence you will come across many AP folk who use smaller aperture refractors and get good images.

[ONIKKINEN]

I tried to adapt my TS-optics 50mm finder to a guidescope with an adaptor and hated every second of trying to work with it. Difficult to focus and even then not very good because of the F-ratio being around 3.2.

I ended up buying this:https://www.teleskop-express.de/shop/product_info.php/info/p7073_TS-Optics-Deluxe-60-mm-Guiding-Finder-scope-with-micro-focusing.html

Its a bit of money but worth every penny. Its also lighter than the 50mm finder i had at first.  Doesnt have to be this exact one but a dedicated guidescope is imo money well spent.

For the scope i would advice to get the smaller one. 200mm newtonians are wind magnets and require a lot from a cheap mount. Less trouble (note less, not no trouble) with the 150.

[malc-c]

From Personal Experience:

The HEQ5 / 200PDS (you can use a standard P, but the PDS is more suited to imaging) has been a default "standard" entry to "serious" imaging.  However, if you are exposed the 200P acts like a sail and the mount can struggle.  Place it in an observatory, or sheltered spot and that will make all the difference.  A 150 PDS will be more stable on an HEQ5, and gives you the option of using a larger guide scope.  My original setup in the observatory was an HEQ5 / 200P (modified secondary)  / ST80 as a guidescope / QHY5 / modified 400D Dslr.  I found this was right on the limit for the HEQ5.  I since sold the ST80 and converted the stock 9 x 50 finder into a guidescope using a ring adapter, and this works surprisingly well.

Unless you are going to mess about with CCD cameras and barlow lenses, forget about using the 200 / 150 PDS for planetary work.  f5 reflectors just don't have the magnification for planetary work.  You will get decent luna images, but Jupiter will still just look like a large star...

There is no "one size fits all".  Serious planetary imaging needs not only long focal lengths such as F10 / F12, but also large aperture, which is why large refractors or SCT are favoured.  But these scopes need focal reducers etc to make them suitable for DSO's and then the results may not be as good as a 150 / 200 P....  Bad analogy is that you wouldn't use an F1 race car for rally racing.

 

 

[Clarkey]

I have used my 200p to mess about with a bit of AP on the HEQ5, but as others have said it is really pushing the mount. A 150PDS would be better suited.

With regards to the finder being used as a guider, I have used mine (with an ED80) to guide and I found it fine. Focus is a bit of a faff, but once set it can pretty well be left alone.

[Stefan87]

First of all I want to say thank you to everybody for helping me! I know this question has been asked before, and I have spent time reading about this, but I have found many differing opinions and it really does make it confusing for a noobie. 

17 hours ago, Cosmic Geoff said:

I suggest you get the book "Making Every Photon Count," the  essential guide to deep space imaging, which may save you from making some expensive and unsuitable purchases.

By 'ds' I assume you mean the PDS imaging version?

 

Thanks for the suggestion. It certainly seems to be a popular read! I've tried to find this book in Australia and could find it on Amazon, but they want 900 bucks! Do you know of a decent source for this book?  I'd prefer to receive it quickly if I were to purchase it. 
I'm referring to this scope. It appears to be just a "ds" model. This store says it is specifically for AP and sell it as a bundle with the HEQ5 as well for this purpose. It seems to be a bit lighter coming in at 8.5kg.

https://www.sirius-optics.com.au/telescopes/saxon-200ds-8in-astrophotography-newtonian-ota.html

17 hours ago, vlaiv said:

You are missing the third component in that equation - that is pixel size / imaging resolution.

Two same F/ratios only have the same speed if they are used with same camera / pixel size. If you want to compare two systems, best measure is aperture at resolution. This means - you fix your working resolution and then compare apertures. Sampling resolution - or arc seconds per pixel is very important metric.

What camera do you plan to use with either of these?

It seems I have alot more to read before making this purchase. I have never even considered this. I will be using a Nikon d5300 for a while, but will eventually upgrade to a dedicated astrophotography camera (they are ****** expensive!)
I am very keen to wrap my head around this, but I'm not sure where to start. I'm guessing what you are talking about is if you zoom right in on a stacked and stretched image of a "small" target like a galaxy, this would be the amount of pixels the galaxy is spread across? So the 150ds will have a much worse resolution on small targets than a 200ds? Is this metric only relevant when considering these smaller targets? 
I was comparing both using the d5300.

 

9 hours ago, ONIKKINEN said:

I tried to adapt my TS-optics 50mm finder to a guidescope with an adaptor and hated every second of trying to work with it. Difficult to focus and even then not very good because of the F-ratio being around 3.2.

I ended up buying this:https://www.teleskop-express.de/shop/product_info.php/info/p7073_TS-Optics-Deluxe-60-mm-Guiding-Finder-scope-with-micro-focusing.html

Its a bit of money but worth every penny. Its also lighter than the 50mm finder i had at first.  Doesnt have to be this exact one but a dedicated guidescope is imo money well spent.

For the scope i would advice to get the smaller one. 200mm newtonians are wind magnets and require a lot from a cheap mount. Less trouble (note less, not no trouble) with the 150.

Thanks for your input. We are still going through spur of the moment lockdowns here in Australia so these fiddly tasks are keeping me sane. I plan on 3d printing an adapter and trying this out with the stock finder. I am so shocked at how light that guidescope is for 60mm!! The SVbony 60mm is 1kg! Which I believe is a copy of the Orion? This one is a similar weight to the ZWO 30f4 mini guide scope! Unbelievable.
 

9 hours ago, malc-c said:

I since sold the ST80 and converted the stock 9 x 50 finder into a guidescope using a ring adapter, and this works surprisingly well.

Unless you are going to mess about with CCD cameras and barlow lenses, forget about using the 200 / 150 PDS for planetary work.  f5 reflectors just don't have the magnification for planetary work.  You will get decent luna images, but Jupiter will still just look like a large star...

I completely understand why you would do that. I think that guidescope is well over 1.5kg? May I ask what adapter you used? I plan on 3d printing one I found on thingiverse. Appears to be worth a shot considering most places are charging 50 bucks for an adapter. 
I will certainly be purchasing a powermate for the odd occasion I wish to do planetary. If I end up getting the 150 then maybe a I will need a 3x or 5x powermate. I will be 3d printing a bracket to use a webcamera for planetary. It sounds like a fun thing to play around with. But my main focus will be DSO, particularly galaxies.  I love a good nebby too 

This has prompted me to think of another issue. Dew. Where I live is basically zero wind most nights, however dew can be quite an issue. This is one of the reasons I'm moving away from SCT. Do the guide scopes require an extended dew shield when pointing towards zenith? Do any other newt owners on here think that dew control will still be an issue with a newt? Part of the reason I'm changing from SCT to newt is to avoid expensive dew heaters and the associated power requirements and extra setup time. Right now I'm using a low powered hair dryer every 20 minutes on dew heavy nights and it's becoming a chore. I love the idea of "set and forget" until before I go to bed, and this is my ultimate long term goal. 

Once again, I really do appreciate your help. Thanks again!

Stefan

 

[malc-c]

Stefan,

It was a long time ago, but something like this t-adapter was used.  The problem you may have in 3d printing such a ring is the resolution of your printer.  The threads on the inside of the finder are incredibly fine.  Personally for £29 it's probably better to juts buy one. 

Edited August 10, 2021 by malc-c

[alacant]

7 hours ago, Stefan87 said:

I'm not sure where to start

Hi

By far the best place is in person at your local astro club. You'll be able to witness someone take a video of say Jupiter, using a 200p or a 150p and a Nikon 5300. 

Gut reaction: if all you want is planets, the 150pl would be better. But go and see it, do:)

Cheers and HTH.

[vlaiv]

7 hours ago, Stefan87 said:

It seems I have alot more to read before making this purchase. I have never even considered this. I will be using a Nikon d5300 for a while, but will eventually upgrade to a dedicated astrophotography camera (they are ****** expensive!)
I am very keen to wrap my head around this, but I'm not sure where to start. I'm guessing what you are talking about is if you zoom right in on a stacked and stretched image of a "small" target like a galaxy, this would be the amount of pixels the galaxy is spread across? So the 150ds will have a much worse resolution on small targets than a 200ds? Is this metric only relevant when considering these smaller targets? 
I was comparing both using the d5300.

It is quite a complex topic, but here is some basic info to get you started.

You are quite right - arc seconds per pixel tells you the scale of the image - or how many pixels across does your target have. It depends on focal length and pixel size and formula is:

sampling_rate = pixel_size * 206.3 / focal_length

where pixel size is in micrometers and focal length in millimeters.

Another important point is that while you can sample at any resolution - it is not wise to do so. After some point - you will be over sampling, or using too many pixels per arc second - too many pixels per target. This is because there is limit to what can be captured in terms of detail.

This limit depends on context. For planetary imaging it is set by telescope aperture alone (and wavelength used), but for deep sky imaging it is set by telescope aperture, seeing influence and mount performance (tracking / guiding precision).

It is rather complex matter how these three interact to produce level of blur, but for all intents and purposes - you can measure star FWHM as pretty good indicator of sharpness you achieved. Good sampling rate for image with given FWHM stars is FWHM / 1.6

If your image has 3.2" FWHM stars - then you need to sample at 2"/px. Using lower sampling rate (higher numerical value of arc second per pixel is lower resolution / lower sampling rate / coarser image) is called under sampling and is perfectly fine (some people complain about blocky stars - but that is consequence of image viewing software and not capture process), while using higher sampling rate is called over sampling and is generally a bad thing because you spread light too much thus lowering SNR without any benefit of capturing additional detail - as there is none, it is blurred away by combination of seeing, aperture and mount performance.

Smaller telescope apertures are suitable for lower sampling rates, for example - 80mm and smaller telescopes can deliver 2"/px or coarser sampling (like 3"/px, or 4"/px).

100-120mm usually gets you ~1.8"/px

150mm can produce 1.5-1.6"/px

and 200mm can produce around 1.2-1.5"/px

Many people use 1"/px or higher sampling - but realistically - you need 8" or larger, perfect steady sky and excellent mount to achieve that.

My recommendation would be to aim for 1.8-2"/px if you choose 150PDS and ~1.5"/px with 200PDS.

 

[Cosmic Geoff]

Stefan - "Making Every Photon Count" is available from FLO the forum sponsor for £20.

7 hours ago, Stefan87 said:

Where I live is basically zero wind most nights, however dew can be quite an issue. This is one of the reasons I'm moving away from SCT. 

A dew shield should be considered an integral part of a SCT, just as on a refractor. Manufacturers don't like to supply them as standard because it makes their product look big and ugly.🙂

What targets do you have in mind? For many famous objects, a small widefield refractor would be far more suitable than a 200mm Newtonian.  A 200mm Newtonian would be more suited for imaging planets, planetary nebulae and small distant galaxies. 

[Stefan87]

10 hours ago, vlaiv said:

 

Smaller telescope apertures are suitable for lower sampling rates, for example - 80mm and smaller telescopes can deliver 2"/px or coarser sampling (like 3"/px, or 4"/px).

100-120mm usually gets you ~1.8"/px

150mm can produce 1.5-1.6"/px

and 200mm can produce around 1.2-1.5"/px

Many people use 1"/px or higher sampling - but realistically - you need 8" or larger, perfect steady sky and excellent mount to achieve that.

My recommendation would be to aim for 1.8-2"/px if you choose 150PDS and ~1.5"/px with 200PDS.

 

Wow thanks for this reply. Your explanation of this to a total noobie makes sense! I really do appreciate the effort you made to explain it. I have some further reading to do! Thank you again. 

 

10 hours ago, Cosmic Geoff said:

What targets do you have in mind? For many famous objects, a small widefield refractor would be far more suitable than a 200mm Newtonian.  A 200mm Newtonian would be more suited for imaging planets, planetary nebulae and small distant galaxies. 

Hey mate my all time favorite thing to image is without a doubt a galaxy. This is the main reason I wanted the 200. Although I do love a good nebby one of the reasons I decided against an ED80 was that I felt it was a bit too widefield for my taste. Although now I've been going through astrobin and I've realised that these diffraction spikes can get UGLY! For example, the horsehead nebula has stars so bright nearby that the diffraction spikes take over the damn image! I have suddenly been put off purchasing a newt. I feel that diffraction spikes are nice when they are small, but when they reach the kind of scale like the stars around the horsehead that take over the image with their spikes then it actually seems problematic.


Ahh [removed word] I don't know what to do. Now I'm considering an OAG and dew strips for the 6se. 🤔 This is a tough decision that I clearly need to give alot more thought to.

[Stefan87]

Thank you everybody for your input. I really do appreciate your help. Top humans!

[wimvb]

8 hours ago, Stefan87 said:

Hey mate my all time favorite thing to image is without a doubt a galaxy. This is the main reason I wanted the 200. Although I do love a good nebby one of the reasons I decided against an ED80 was that I felt it was a bit too widefield for my taste. Although now I've been going through astrobin and I've realised that these diffraction spikes can get UGLY! For example, the horsehead nebula has stars so bright nearby that the diffraction spikes take over the damn image! I have suddenly been put off purchasing a newt. I feel that diffraction spikes are nice when they are small, but when they reach the kind of scale like the stars around the horsehead that take over the image with their spikes then it actually seems problematic.

That's why I went from a 150PDS to a SkyWatcher MN190 Maksutov-Newtonian. But I wouldn't recommend this scope to a beginner. And it DEFINITELY WON'T FIT on a HEQ5 mount. Here are the two next to each other.

The MN190 is about the same size as a 200PDS, and if you are imaging from an  exposed location, will act as a sail in the wind. With these large reflectors, you should also look into off axis guiding, eventually.

Edited August 11, 2021 by wimvb

[malc-c]

The problem when asking for advice on recommended kit for a particular purpose is that everyone will have different requirements, budgets, expectations, and needs.  I guess the first thing the OP needs to do is decide on whether he wants a reflector or refractor, given that he's commented on "ugly" diffraction spikes on stars.  He mentioned that he wants to image galaxies, again this is quite wide ranging.  On one end we have Andromeda that doesn't need demanding equipment to get a result, but then we have so many other attractive but fainter galaxies that need aperture and short focal lengths to capture them.  Now a fast 6" refractor won't be cheap, and if you enter the budget end you are back with a large OTA that requires a heavy mount like an EQ6.  With expectations, often people see the wonderful images in books and websites and expect to see the same thing the first time they attach their mobile phone to a £300 telescope, and then are really disappointed when the results are a small grey smudge (OK maybe I over exaggerated, but you get the idea).  

It's been mentioned that there is no "one scope fits all" and to give the OP proof of that here are a few images taken with a 200PDS on and HEQ5 from a town location with the setup in a DIY observatory.

This one is the Flame nebula in Orion, taken with a modified Dslr Canon D400.  I can't recall the number of subs etc as it was several years back and I've lost the notes.

Several years ago Jupiter was well positioned and close so I used an old Phillips SPC900 CCD webcam and two 2x barlows (to give an equivalent of f20 focal length) and after processing the resulting frames from the capture this was the best I got.

 

Now considering the equipment I used I'm quite happy with this result.  Attempts to sharpen the image of Jupiter resulted in a fake looking images with banding.  Now If I had used a dedicated planetary camera costing 10x the cost of the webcam then the results might have been a lot better.  A few nights later the seeing was better and this was the result after processing....still grainy and some artefacts caused by the cheap Barlow's but again, for the effort involved I'm quite happy with the result.

 

But I'm sure someone with a 120 Esprit, or better still an 8" SCT would get better, and I'm sure there are a lot of other SGL members who have produced better images.

The point I'm trying to make is that whilst it was fairly easy to get the image of the flame nebula, it took a lot of messing about and additional equipment to do planetary work with the same scope, and the results weren't as stunning.  That's not to say someone with the same set up, but in a nice secluded dark sky would get the same.  They my find the slightest breeze ruins the results, especially when you consider the magnification involved here.

 

We mentioned budget.  The above images were taken with what I would class as basic entry level cameras.  Canon have been associated with astrophotograpy for years as their sensors are less noisy and more sensitive than other brands ( the hot pixels of my Olympus dslr looks like a section of the milky way after 30 second exposures!! ).  400D - 450D's are quite light and small bodied.  Larger cameras with full frame sensors will be considerably larger and heavier, and all this extra weight should be taken into account when considering the mount.  At the other end of the scale, there are fast and sensitive CCD cameras designed for imaging.  These cooled CCDs have sensors designed for this purpose and as such don't come cheap.

With the mounts, at the bottom level we have the HEQ5 and EQ6 mounts.  These have the best resolution in skywatchers range around the £1000 mark.  Moving up you'll be spending three or four times that for the EQ8 and alternatives.  It's typically been muted that for imaging the load should be 2/3 of the published max payload weight.  So people will typically move an OTA up to the mount above the one that the OTA is normally bundled with.  Fore example the 150p is often bundled with an EQ3, so using one on an EQ5 will give a better option and more stable platform when used as an imaging platform.  Same for the 200p, which typically gets sold on an EQ5.  Stick it on an HEQ5 and it copes better under the same conditions.  Stick it on an EQ6 and you can bolt on larger guide scopes and heavier cameras.

You mentioned guiding.  OAG means it's using the main scope so its like having (in the case of the 200P) a 1000mm focal length guide scope which will give more precision.  However, it also places an additional obstruction in the light path, and some people get hung up on this.  Alternatives are to convert the 9 x 50 finder scope and used a cheap camera, through to a dedicated guide scope piggy backed on the main scope.  Both have advantages and disadvantages.  The OAG suitable for use with my Dslr so that I could still get back focus was around £130 so for me I opted to convert the finder using a £30 adapter.  

I think you have gathered by now that there is no "one fits all" with any of this.  I've included images taken with budget equipment to try and illustrate what can be achieved with basic low end equipment, but I would suggest joining or visiting a local astronomy society (possibly at an observing evening) and look at what's on offer.  Speak to the owners, especially those who also image.  I have a friend who started off with the same equipment as me... he now has a dedicated two scope rig set up on a tuned EQ8 with imaging cameras costing more than I paid for my second hand Volvo V70.  He's has several images win awards and featured on the cover of Astronomy Now.... I now place him at the semi-pro armature end of the spectrum.  But then he knows how to get the results.  There are those who have all the gear and no idea......

I hope this somewhat rambling post has been helpful.  Good luck with your research and hopefully we'll see your first images very soon  

 

[Stefan87]

Malcom thank you ver

On 11/08/2021 at 20:44, malc-c said:

I hope this somewhat rambling post has been helpful.  Good luck with your research and hopefully we'll see your first images very soon  

Thank you Malcom for the information and sharing the pics! It's good to know planetary can be done with that arrangement and I think those pictures are totally acceptable for a hobbyist. Although as you said they certainly don't compare to someone with more appropriate gear for the job. 
I've been doing a lot of thinking and even more reading and I think I've reached a conclusion for now that I will get an OAG and dew strips for the 6se, and use this on the HEQ5. I've had no problems using the 6SE. Collimation is a breeze and dew is the only hassle which can be prevented with a heating system (using a low powered hair dryer every 20-30 minutes atm😅).  Knowing that the OAG can be used with other setups means it won't be wasted money and I can use this for the foreseeable future. Doing this, I can get a good guiding setup now using the money I told the Mrs I would spend on a new scope, then down the track I can get the new scope and enjoy two different FOV scopes for different targets! Thinking ahead!!

I have one more question for anybody who can help me. I am looking at OAG's and I have read that the larger prism (12mm x 12mm) is a great choice if you pair it with a better cam than the ASI120mm. So I will get the ASI290MM mini. This will be the most I spend on a guiding cam. There are 2 OAGs that I am aware of with the 12x12 prism. The ZWO OAG-L and the Celestron OAG. I really dislike the size of the Celestron and how much back focus it hogs in your image train. Reason being is that I'm concerned about buying this and not being able to use it in the future when I purchase a refractor or a Newtonian as a second scope to use with my DSLR (I'm beginning to lean towards a Saxon FCD100 102mm as a second scope and in the future I will get the ASI533mc). 

 

Has anybody got experience or know of any anecdotal reports comparing the two large prism OAG? Right now I can't see a reason to spend more on the much larger COAG besides the helical focuser, which seems like it would be slightly redundant if it only needs to be set up once. Would I be safe getting the ZWO OAG-L and the ASI290mm mini to use with a 6se and down the road with a Saxon FCD100 102mm refractor? 

I really appreciate the input and advice you guys have given me. The astro community is amazing and so many people are just so helpful, kind and patient. Thanks again for giving me your time. It is greatly appreciated

Stefan

[wimvb]

I don't have the large zwo oag, but their standard version with a helical focuser and the ASI290  mini. This works great and so far I've always had a few stars in its fov to choose from. Even multi star guiding works most of the time. Beware that a larger prism must be positioned closer to the main sensor. I mean, if you have the scope's illuminated field (circle) and inside that the area of the sensor (rectangle), then the rectangle of the prism has to fit between the long side of the sensor rectangle and the edge of the circle. If the prism rectangle isn't covered by the circle, you won't be utilising the full area of the prism. And you also have to make sure that the prism is clear of the sensor area, or you will see the edge in all your images. Finally, a larger prism means a thicker stem and therefore a thicker oag. This adds to the "backfocus".

[Stefan87]

On 14/08/2021 at 09:28, wimvb said:

I don't have the large zwo oag, but their standard version with a helical focuser and the ASI290  mini. This works great and so far I've always had a few stars in its fov to choose from. Even multi star guiding works most of the time. Beware that a larger prism must be positioned closer to the main sensor. I mean, if you have the scope's illuminated field (circle) and inside that the area of the sensor (rectangle), then the rectangle of the prism has to fit between the long side of the sensor rectangle and the edge of the circle. If the prism rectangle isn't covered by the circle, you won't be utilising the full area of the prism. And you also have to make sure that the prism is clear of the sensor area, or you will see the edge in all your images. Finally, a larger prism means a thicker stem and therefore a thicker oag. This adds to the "backfocus"

Thanks for your response! My DSLR is a crop sensor with a diagnonal of 27mm, which is quite a bit larger than a dedicated astro cam. So would this mean I have far less room to play with and would be better off purchasing a standard zwo OAG with the 8mm prism? Pairing that with a asi290mm?

[wimvb]

The OAG-L has a "window" of 11.7 x 8.3 mm. Both the ASI120 and ASI290 have smaller sensors than this. Only the ASI174 has a sensor that covers (most of) the oag's light passing aperture. If you were to pair this oag with the ASI290, you are not using its potential. In my experience, the ASI290 is sensitive enough to give you stars to guide on with the smaller oag. Even this oag has a larger window than the sensor size of the 290. So, unless you need the larger pixels of the ASI175 (5.86 um), I don't think you need the larger oag.