Ok, so I am working on a permanent observatory that will be based around my EdgeHD 11 fork mount on wedge. Ideally I would like to run at f2 with hyperstar, giving a 540mm focal length. I have not yet purchased a camera yet, but was leaning towards the ASI2600MM Pro with 3.76um pixels which seems to match reasonably well with an image scale of ~1.5 arcsec/pixel at 1x1 binning. The challenge with this mono camera is that I would ideally need 2" filters and the filter wheel would create a significant (insurmountable?) obstruction with hyperstar.
So I’m thinking there might be a number of potential solutions, none of which are ideal:
The hyperstar has room for a single 2" filter, so use that (primarily V, but manually swapping out other bands for special situations) - not ideal for an automated observatory
Use the ZWO mini-filter wheel with 1.25" filters, and deal with the vignetting and smaller usable FOV
Go with the ASI2600MC color version of the camera and debayer/transform to the standard system
Run at F7 with the EdgeHD 0.7 FR and a camera in back. I’m not thrilled with running at ~2000mm focal length and this would likely require a camera with larger pixels (e.g., 9um)
Any thoughts, experiences, other suggestions? My primary focus is VSO and I want to make sure I get this right for my permanent setup.
2000 mm F.L. reduced by .7 is 1400 mm, right?
That is a nice all-purpose F.L. and FOV.
Good for the image solver too.
I sometimes wish I had a long refractor to narrow the field of view to avoid bright stars that saturate the CCD.
Then you can contribute to the standard photometry with BVRI filters in a filter wheel.
I own such an optical configuration (c11 edge + AP CCDT67focal reducer + zwo2600mono).
I do not think another camera would do a better job, even with 9u pixels, than the 2600.
I ve got something like 42’ field at fd 6.7.
You probably can expect about 40’ field at f7.
The limiting mag (no filter) is about 20-20.3 in 180 sec in a suburban/countryside sky. You can work on mag 17-18+ targets with sufficient SNR, I think.
The 3.76u pixels allow for longer exposure, than 9u pixels, for the stellar light is divided in a larger number of pixels, so as you don’t always need to defocuse to avoid saturation.
I mean at such focal lenght (1800-2000mm), the stars are not pin point on the chipset. That’s not too good for any astrophotographer seeking for the perfect nebula image … but as a photometrist and variable star observer, this is somewhat perfect
Thank you! This is exactly the kind of experience and advice I was hoping to get prior to finalizing my setup. Do you software bin your images or just do photometry on the unbinned (oversampled) images? I did CCD photometry years ago, but understand that using CMOS cameras is a bit of a different ball game.
I never had a CCD, so I can t compare both types of camera.
With cmos, you cannot do binning, just crop when imaging.
I perform photometry on the primary images; it works well.
I must admit I m not extremely experimented with this cam, coz I own it only since a couple of month. Nevertheless, it s a good value.
I do astrometry on the fits
with Astap, and extract photometry with astroImageJ, as I do with images from my DSLR.
Hi, I have a QHY600M CMOS camera with 36x24mm^2 chip size and 3.76 microns pixel. I use it on a C14 Edge HD at f/7 and use it 4x4 binned due to reducing the storage of 800+ images a night (7.5 MB compared to 120 MB in 1x1 bin). I would go for binning using either NINA or MAXIM DL. My local seeing is between 2 and 3 arcsec. I submit my photometry the AAVSO database.
Regards,
F.-J. (Josch) Hambsch (HMB)
What are your major objectives? If photometry, then in my opinion a Hyperstar setup at F2 is exactly what you do not want. People will tell you that they can do photometry with a Hyperstar, but there are complications when you have f-ratios much under F4. My recommendation is to (1) go with the reducer and camera in the back and bin, (2) figure out a camera that have an FOV that covers no more than a one degree FOV (mine was about 28x18 arcminutes). A n ASI2600mm or comparable camera has an FOV of 41x27.5 arcminutes (0.69x0.46 degrees)which is just about perfect IMO. (3) Do not contemplate a color camera, get a mono camera without amp glow and some photometric filters.
Why the 1-degree FOV for photometry? In doing differential photometry we assume that the target and comps are in the same airmass so that we do not have to consider 1-st order extinction. Much beyond that you cannot make that conclusion, and you have violated the assumption. Yes, there are ways of dealing with that, but for normal operations it’s better not to go there unless you know what you are doing.
You will find tracking performance better if you have a good off-axis guider and not a guide scope while imaging at long focal lengths. Further, the quality of your mount becomes more important. I don’t know much about fork mounts, so I cannot comment on the quality of yours – but give it every opportunity to perform with the correct setup in terms of guiding.
I used your (4.) set up for several years with an off-axis guider but on an AP900. You probably could find a CCD with 9um on the market, but you can also bin a CMOS.
I forgot something. I have no idea what the clearance is in your fork mount. If I were you I would do some careful measures to make sure that whatever configuration you have clears the bottom.
Thank you all for your input! I indeed decided to go with the f/7 setup, and my 2600MM, reducer and filter wheel just arrived today (still waiting for the filters).
@Ed_Wiley_WEY, the fork mounted 11 was what I had so we’ll see how it performs on the wedge. If it tracks well enough to give me 3-5 minute exposures then I’ll be happy. If not, then I’m willing to invest in an EQ mount. But I want to see what the fork and wedge can do first.
For now, we’re looking at a couple weeks of pretty clear weather here in Michigan so I want to get it put together and running while that lasts. On a tripod for now, will be putting in an observatory with a permanent pier in the spring.
Again, thanks all (and Arne @HQA who also provided input via email)!
@chrismlt would you mind sharing with me your image train? I’m trying to figure that out, I know 17.5mm to the camera sensor, 20mm for the filter wheel, but then how best to get to the required 146mm of the EdgeHD 11?
As you may know (or not), the AP CCDT67 is a variable focal reducer, so I’m not fixed on 146 mm as you are, or 55-65 mm as most configurations are (the farther you are from the reducer lense, the higher focal reduction you’ve got).
Then after the focal reducer, I’ve got two M42 extensions, then comes the autoguider, then the EWF, and finally the camera.
You should ask your camera dealer for more informations about this - I’m sure he will be pleased to inform you. I’m not sure how you can get those famous 146mm.
Don’t hesitate to ask, if more help are needed - I’ll keep you informed as I can.
By the way, I’m interested, if you would like,
to know what will be your different limiting mags (no filter, U B V R I), once you completed the first tests. Thanks in advance, Brian.
@Ed_Wiley_WEY could you elaborate on the complications that might appear when one uses fast optical systems (say F/2-F/4) for photometry? For the record, AAVSOnet has several F/2.8 Takahashi E180 systems for their bright star monitors. Likewise, I’m using a F/2.0 for my work. Both systems work well, but the lower F-ratio systems require some patience.
The issues I’ve encountered with my F/2 system are as follows:
Backfocus and tilt - lower f-ratio systems are incredibly sensitive to backfocus and tilt. This is a solvable problem, but requires a lot of patience.
No filter wheels - With both hyperstar and RASA configurations, you will block 40% or more of the aperture if you use a filter wheel. Most people use
Narrowband filter interference - fast optical systems don’t play well with narrowband interference filters. Fortunately, all filter manufacturers now make filters for fast optical systems.
Sensor size - If you stray outside of the design limitations of the telescope, i.e. outside the true or effective image circle, various optical aberrations / blockages start to appear. This messes with the photometry quite badly.
All of these issues are solvable.
In terms of photometry, I always use full transformation equations to reduce my data (i.e. including zero point, color, and airmass). I determine the coefficients on a nightly basis using all of the data, not just a single image, so I haven’t had much of an issue calibrating my data.
