Jack of all trades ??

[picclock]

Jack of all trades ??, master of none - but perhaps good enough.

I am returning to the hobby after a number of years and am looking to put together some kit for observing and occasional photography. My old telescope was a Meade 8" LX10 sct.  I want to keep the size and number of parts down to a manageable level.

The idea is to put together a new "goto" kit but a bit smaller and more portable than the 8". I'm thinking a Meade 6" LX65 sct. Not sure its worth it for the ACF version.  Quite like the idea of being able to mount another scope next to the primary one.  I'm wondering if I could get reasonable pictures using one of the sensitive ZWO cooled pro one shot cmos cameras. Because the exposure times are quite short I am hoping that I will be able to achieve good results despite the lx65 being an alt azimuth mount..

Any thoughts or comments on this odd marriage much appreciated.

Best Regards

picclock

 

 

 

[picclock]

I was wondering if there is any performance difference between a normal equatorial mount and an alt azimuth one with a wedge (apart from not having to lug weights around 🤪).  I remember having pretty good tracking with my old meade lx10, but that was ~20 years ago.

Re the cmos camera.  I've been looking into the Canon Eos Rp (35.9 x 24 mm CMOS sensor , 26.2Mp

)  rather than go for the dedicated Astro cam. Conveniently this already has a filter to allow hydrogen reds through, so no modification necessary, leaving it available for normal use.  My info (may be suspect) says 68% QE for the canon Rp.

Does anyone have both for a comparison ?

Any help or suggestions much appreciated

Best Regards

picclock

[vlaiv]

You really need to manage your expectations of astrophotography you are going to achieve with that scope / mount.

That setup will be very nice in doing lunar / white light solar (with appropriate full aperture filter - check out baader solar foil) / planetary imaging scope if you purchase modern dedicated astro CMOS camera. Planetary imaging / lucky imaging utilizes very short exposures and AltAz mount or mount that is not tracking good enough - simply does not matter there.

I would also advocate going for dedicated astro camera with cooling for DSO AP - but, I highly doubt that you will have easy time doing either short exposure or long exposure with wedge. Probably best thing to do is EEVA - which is very similar to regular astrophotography except it is real time / live stacking approach (check EEVA section here on SGL for ideas). Again, subs are kept reasonably short - like a few seconds and image is created in real time on computer screen. Helps in light polluted areas instead of regular observing but it can also be a sort of astrophotography thing as you can save live stacked image for further processing.

In any case - dedicated astro camera, if it has set point cooling, enables you to do proper calibration of your subs - bias / dark current removal and flats application. That is advantage over DSLR. Another advantage is that you can see in real time on computer screen what you are capturing (for focus purposes or framing) and you can do high frame rate without any distortion for planetary imaging (DSLR can shoot movie but it utilizes compression which creates certain artifacts).

Another reason why you should go for dedicated astro camera is - it is very unlikely that 6" SCT will illuminate and have fully corrected circle the size of full frame DSLR. It will be APS-C size at best (a bit shy of 30mm), so full frame DSLR will be effectively "wasted" on that scope (mind you - not many scopes do have such a large usable field).

 

[picclock]

@Main Sequence

Thanks for that information. I have looked into this a bit more and have revised my list of purchases in view of your comments and further research.

The mount is really important. I found with my old wedge alt azimuth LX10, that I could use the cooldown time to get the tracking sorted, and once done it was pretty good, just a case of getting the habit. I think I will be going for  iOptron CEM25P. It has reasonable load carrying and is allegedy pretty accurate. Nice low power requirements. Most importantly its affordable.

Re the OTA, Ive been looking at an iOptron Photon RC8 8" F8 Ritchey-Chretien. This seems a pretty good buy and with lots of aperture for the deep space objects. Only snag with this is it does not come with eyepieces, finder scope etc, which is a bit tacky (wish I'd kept my old Meade ones 🥴). Plus points are active fan cooldown, no dewshield needed, no coma, spherical or chromatic aberration (so it says), although its not a flat field.

Re the sensor, Ive looked into it and think the Canon RP will be pretty close in performance to the ZWO ASI294MC-PRO and likely better than the uncooled ASI294MC. It has a larger sensor (35.9 x 24 mm vs  9x13mm) and larger pixel size (5.73um vs 4.63um). Without active cooling its power requirements will be minimal. It may be a bit noisier but should still give pretty good results., especially after a few reference frames.

With a bit of luck and using zero drift alignment the exposure times will be relatively short, <5mins, so hopefully no guiding needed.

I'm just about ready to go with these so if anyone can see anything wildly out of line I'd value your comments.

Best Regards

picclock

[vlaiv]

I guess CEM25p is a good mount. I've not seen or operated it, but what I've heard of iOptron mounts - it should be ok. It is eq5 class mount, but probably much better in performance to SkyWatcher EQ5.

As for OTA - I think it is very good instrument - I own one - TS RC (GSO rebranded - I guess iOptron model is as well because they look identical) and have paired it with ASI1600 camera.

A few pointers on the scope - I replaced stock focuser to a better 2.5" one because stock focuser (2" monorail) does not have threaded connection. I use OAG for guiding with it rather than guide scope.

Don't think that you will be able to get decent results with such scope unguided. It is very long focal length scope - 1600mm FL and FOV will be small. Corrected and flat field is rather limited - less than APS-C sized chip - I think I can start to see field curvature at 22mm diagonal of ASI1600 in the far corners. With suitable corrector / flattener (maybe even reducing one) - you should be able to do APS-C sized field.

Again, I believe that full size sensor is going to be wasted on this scope as I don't think it will be illuminated or corrected passed APS-C size (or about 30mm, so almost 1/3 of diagonal wasted).

Some people have issues with collimation of this scope - I found it rather easy to align properly.

RC8" can also be considered Jack of all trades, but it actually is master of astrophotography with planetary observing and imaging quite lagging behind due to large central obstruction. My advice would be to go for better mount to carry that scope. Something in EQ6 class - like this one:

https://www.firstlightoptics.com/ioptron-mounts/ioptron-cem40-center-balanced-equatorial-goto-mount.html

[picclock]

@vlaiv

I think the CEM 40 is beyond my budget ATM. I like the idea of an OAG, but I dont like the extra added length or the partial obstruction to the light path, so I think I'll use a conventional finderscope type guider if I need it.

Just a couple of questions. If you have this scope what are the cool down times like?.  The rp camera is very lightweight for a canon DSLR, @485g inc battery. How sensitive is the OTA to weight hung of the back, and does it affect the focus much.

Best Regards

picclock

[vlaiv]

4 minutes ago, picclock said:

I think the CEM 40 is beyond my budget ATM. I like the idea of an OAG, but I dont like the extra added length or the partial obstruction to the light path, so I think I'll use a conventional finderscope type guider if I need it.

I use this OAG:

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p8319_TS-Optics-Off-Axis-Guider-TSOAG16---stabil---Baulaenge-16-mm.html

It's even less expensive than guide scope. It adds only 16mm to optical path, or rather uses up only 16mm of optical path. RC8" has very generous back focus distance - 20cm or so. I use rather long focuser and another 5cm extension M90 before focuser. I have OAG, spacers and filter slider as well as rotator.

11 minutes ago, picclock said:

Just a couple of questions. If you have this scope what are the cool down times like?.  The rp camera is very lightweight for a canon DSLR, @485g inc battery. How sensitive is the OTA to weight hung of the back, and does it affect the focus much.

As for cool down time - I honestly have no idea. It is open design and it should not dew up, but I actually had it dew up once (very humid night). It was slow build up on secondary mirror. I keep it in basement, so it is close to ambient temperature and by the time I get everything sorted out (camera focus, everything connected, plate solving + alignment point, etc ...) it is ready to image. I never used it as visual scope (I did look once thru it - Jupiter was the target but image was rather pale / lacking contrast at such high magnification - about x250 or so - I believe it was 6 or 7mm eyepiece at 1624mm FL, due to large secondary obstruction).

I don't think there will be a problem with half a kilo of weight on the scope. I hang following: Two cameras, one cooled, one not - 410g + 120g so 0.5Kg only in cameras, I have extension tubes, filter drawer, OAG, rotator - probably close to 1Kg of gear. Replacement focuser is 1.2Kg, so there is about 2+Kg hanging off the back end of the scope - there is usually no problem there - except that I need to add 1Kg weight on front side of the scope to balance it properly.

This scope does not have a moving mirror like SCT scopes, so you don't have to worry about tilt as much.

[Shimrod]

36 minutes ago, picclock said:

@vlaiv

I think the CEM 40 is beyond my budget ATM. I like the idea of an OAG, but I dont like the extra added length or the partial obstruction to the light path, so I think I'll use a conventional finderscope type guider if I need it.

Just a couple of questions. If you have this scope what are the cool down times like?.  The rp camera is very lightweight for a canon DSLR, @485g inc battery. How sensitive is the OTA to weight hung of the back, and does it affect the focus much.

Best Regards

picclock

You seem to have deviated from your original goal of a smaller and more portable version of your current scope! You haven't said what you want to observe or image, which may affect the overall scope selection.

Using a DSLR is handy if you don't want to image using a laptop, but I have found that I have ended up using my laptop anyway so check focus. If you are going to be using a laptop for guiding, then I would consider getting a dedicated astro camera from the start - you are only going to end up wanting one anyway! The only reason I can see for getting the RP would be if you also wanted to use the camera for 'normal' photography as well - it's not like you're saving money over the purchase of a dedicated Astro camera.

[picclock]

@shimrod

>> your current scope!

Currently I have no scope 😪.  A couple of decades ago I had an 8" sct, a meade lx10. This got written off by accident by one of my kids, resulting in the mirror and corrector plate being broken, beyond redemption.  So kids, no time, no finance meant I had to give up the hobby. 

Now am retired and have a little cash, and time.

>>You seem to have deviated from your original goal of a smaller and more portable version

Smaller and more portable. Well it will be a lot lighter than the LX10., 13.1kgs vs 22.2kgs. So maybe more portable, smaller not so much. Will likely fix a carrying handle to the tube. Its difficult for me to downsize when I remember the really good seeing I had with my old one.

>> The only reason I can see for getting the RP would be if you also wanted to use the camera for 'normal' photography as well - it's not like you're saving money over the purchase of a dedicated Astro camera.

Having a 'normal' camera gives massive versatility. When 'she who must be obeyed' sees it, it will be recognised as a family asset, and I can even take pictures with it (of her of course). If I show her a small red cylinder and tell her I bought it for £900 there will be much weeping and gnashing of teeth, likely from me.

It will be interesting to see how it stacks up against a ZWO ASI294MC-PRO, because after you subtract the dark frames I suspect there is not a lot in it. For a camera to have a QE of 68% is quite simply amazing. Should be a fun toy.

@vlaiv

Your quite correct about the off axis guider. That one you linked to seems really short. I think that must be the way to go.

Many thanks for your comments and assistance.

Best Regards

picclock

 

 

 

 

Edited June 16, 2020 by picclock